Understanding the behavior of surfactants at interfaces is crucial for many applications in materials science and chemistry. Optical tweezers combined with trajectory analysis can become a powerful tool for investigating surfactant characteristics. In this study, we perform trap-and-track analysis to compare the behavior of cetyltrimethylammonium bromide (CTAB) and cetyltrimethy-lammonium chloride (CTAC) at water–glass interfaces. We use optical tweezers to trap a gold nanoparticle and statistically analyze the particle’s movement in response to various surfactant concentrations, evidencing the rearrangement of surfactants adsorbed on glass surfaces. Our results show that counterions have a significant effect on surfactant behavior at the interface. The greater binding affinity of bromide ions to CTA+ micelle surfaces reduces the repulsion among surfactant head groups and enhances the mobility of micelles adsorbed on the interface. Our study provides valuable insights into the behavior of surfactants at interfaces and highlights the potential of op-tical tweezers for surfactant research. The development of this trap-and-track approach can have important implications for various applications, including drug delivery and nanomaterials.
©(2023) The Authors

We study the plasmon modes of gold nanorods (as short as ∼100 nm) on a nonmetallic conductive substrate using scanning tunneling microscope-induced light emission (STM-LE) with a nonplasmonic tungsten tip at room temperature in high vacuum (10–7 mbar). The far-field light is identified as the radiative decay of plasmon modes on the nanorods excited by inelastic electron tunneling. The spatial intensity distributions of the first three longitudinal multipolar modes on nanorods are spatially resolved on the order of 10–20 nm. These intensity distributions are related to the radiative electromagnetic local density of states and agree very well with numerical simulations. We discover that the presence of the tungsten tip with a high-dielectric constant influences the line shapes of the plasmon spectra and enhances the strength of the plasmon peaks.
©(2023) The Authors

Electromagnetic forces and torques enable many key technologies, including optical tweezers or dielectrophoresis. Interestingly, both techniques rely on the same physical process: the interaction of an oscillating electric field with a particle of matter. This work provides a unified framework to understand this interaction both when considering fields oscillating at low frequencies─dielectrophoresis─and high frequencies─optical tweezers. We draw useful parallels between these two techniques, discuss the different and often unstated assumptions they are based upon, and illustrate key applications in the fields of physical and analytical chemistry, biosensing, and colloidal science.
©(2023) The Authors

The response of simple plasmonic nanorods to polarized illumination is studied in detail. Depending on the orientation of that polarization with respect to the symmetry axes of the nanostructure, a chiral response can occur, which can be analyzed through a second polarizer, in order to control the spectral response of the system. Specifically, for the Ag nanorods fabricated here, a broad variety of colors can be produced that cover half of the chromaticity diagram. Depending on the illumination and detection polarizations, these colors range from white to vivid colors or even black, in spite of the fact that the material at hand does not absorb much light. By exploiting two additional degrees of freedom, namely the nanorod length and its orientation within the unit cell, it is possible to produce a very rich palette of optical effects that are controlled by the polarization of light. Their utilization to reproduce artworks is demonstrated, together with their operation as encrypting system, where the polarizations are used as keys and the message is encrypted in a quaternary color subset.
©(2023) The Authors

Dielectrophoresis (DEP) is a versatile tool for the precise microscale manipulation of a broad range of substances. To unleash the full potential of DEP for the manipulation of complex molecular-sized particulates such as proteins requires the development of appropriate theoretical models and their comprehensive experimental verification. Here, we construct an original DEP platform and test the Hölzel–Pethig empirical model for protein DEP. Three different proteins are studied: lysozyme, BSA, and lactoferrin. Their molecular Clausius–Mossotti function is obtained by detecting their trapping event via the measurement of the fluorescence intensity to identify the minimum electric field gradient required to overcome dispersive forces. We observe a significant discrepancy with published theoretical data and, after a very careful analysis to rule out experimental errors, conclude that more sophisticated theoretical models are required for the response of molecular entities in DEP fields. The developed experimental platform, which includes arrays of sawtooth metal electrode pairs with varying gaps and produces variations of the electric field gradient, provides a versatile tool that can broaden the utilization of DEP for molecular entities.
©(2023) The Authors

Single-particle tracking and optical tweezers are powerful techniques for studying diverse processes at the microscopic scale. The stochastic behavior of a microscopic particle contains information about its interaction with surrounding molecules, and an optical tweezer can further facilitate this observation with its ability to constrain the particle to an area of interest. Although these techniques found their initial applications in biology, they can also shed new light on microscopic interface phenomena by unveiling nanoscale morphologies and molecular-level interactions in real time, which are obscured in traditional ensemble analysis. Here, the application of single-particle tracking and optical tweezers are demonstrated for studying molecular interactions at solid–liquid interfaces. Specifically, the surfactant behaviors at the water–glass interface are investigated by tracing gold nanoparticles that are optically trapped on these molecules. The underlying mechanisms governing the particle motion, which can be explained by hydrophobic interactions, disruptions, and rearrangements among surfactant monomers at the interfaces, are discovered. These interpretations are further supported by statistical analysis of an individual trajectory and comparison with theoretical predictions. The findings provide new insights into the surfactant dynamics and also illustrate the promise of single-particle tracking and optical manipulation for studying nanoscale physics and chemistry of surfaces and interfaces.
©(2023) The Authors

Among the two materials families used in nanophotonics, the fundamental mode for metal nanostructures is electric, while that for dielectric nanostructures is magnetic. Consequently, the optical properties of hybrid dimers that incorporate both materials have prompted significant research. Here, we study the optical response of such hybrid dimers with the coupled electric and magnetic dipoles method, and demonstrate how their electromagnetic interactions can be used to modulate the electric and magnetic responses. The conditions for the complete suppression of the magnetic dipole in the visible range for high refractive index dielectric nanoparticles are described. The electric and magnetic responses can be enhanced or reduced by positioning the dielectric particle in the nodes of the standing wave formed by the metallic particle. This controlled near-field interaction provides a handle on the near- and far-field responses of the system, with possible applications such as sensing and optical switches.
©(2022) American Physical Society

We review three different approaches for the calculation of electromagnetic multipoles, namely, the Cartesian primitive multipoles, the Cartesian irreducible multipoles, and the spherical multipoles. We identify the latter as the best suited to describe the scattering of electromagnetic radiation, as exemplified for an amorphous silicon sphere. These multipoles are then used to calculate the optical force acting on semiconductor, dielectric or metallic particles in a wide wavelength range, from the dipolar regime down to the Mie regime.
©(2022) The Authors

The optical characterization of metasurfaces and nanostructures that alter the polarization of light is tricky and can lead to unphysical results, such as reflectance beyond unity. We track the origin of such pitfalls to the response of some typical optical components used in a commercial microscope or a custom-made setup. In particular, the beam splitter and some mirrors have different responses for both polarizations and can produce wrong results. A simple procedure is described to correct these erroneous results, based on the optical characterization of the different components in the optical setup. With this procedure, the experimental results match the numerical simulations perfectly. The methodology described here is simple and will enable the accurate spectral measurements of nanostructures and metasurfaces that alter the polarization of the incoming light.
©(2022) Optica Publishing Group (open access)

We look beyond the standard time-average approach and investigate optical forces in the time domain. The formalism is developed for both the Abraham and Minkowski momenta, which appear to converge in the time domain. We unveil an extremely rich – and by far unexplored – physics associated with the dynamics of the optical forces, which can even attain negative values over short time intervals or produce low frequency dynamics that can excite mechanical oscillations in macroscopic objects under polychromatic illumination. The magnitude of this beating force is tightly linked to the average one. Implications of this work for transient optomechanics are discussed.
©(2022) Optica Publishing Group (open access)

Nanofabrication is key to many technological advances, especially the challenge of merging nanophotonics with electronics. Here, we investigate the fabrication process of plasmonic interdigitated gold electrodes having a very high aspect ratio (i.e. long and thin geometries) and a large surface area. Stringent stability issues that arise when these structures are fabricated using inorganic adhesion layers, such as titanium or chromium, on silica substrates are highlighted. We ascribe these problems to thermodynamical non-equilibrium states of freshly deposited gold and, in particular, discuss the role of surface energy in determining the structural properties of high aspect ratio gold nanostructures. We then show that the use of organic silane self-assembled monolayers improves the long term stability of these structures and, finally, characterize the fabricated electrodes. This technology can unleash the potential of hybrid optoelectronic circuits where current and light are manipulated with the same component.
©(2022) The Authors

Optical metasurfaces rely on subwavelength scale nanostructures, which puts significant constraints on nanofabrication accuracies. These constraints are becoming increasingly important, as metasurfaces are maturing toward real applications that require the fabrication of very large area samples. Here, we focus on beam steering gradient metasurfaces and show that perfect nanofabrication does not necessarily equate with best performances: metasurfaces with missing elements can actually be more efficient than intact metasurfaces. Both plasmonic metasurfaces in reflection and dielectric metasurfaces in transmission are investigated. These findings are substantiated by experiments on purposely misfabricated metasurfaces and full-wave calculations. A very efficient quasi-analytical model is also introduced for the design and simulations of metasurfaces; it agrees very well with full-wave calculations. Our findings indicate that the substrate properties play a key role in the robustness of a metasurface and the smoothness of the approximated phase gradient controls the device efficiency.
©(2022) American Chemical Society

Plasmonic effects including near-field coupling, light scattering, guided mode through surface plasmon polaritons (SPPs), Förster resonant energy transfer (FRET), and thermoplasmonics are extensively used for harnessing inexhaustible solar energy for photovoltaics and photocatalysis. Recently, plasmonic hot carrier-driven photocatalysis has received additional attention thanks to its specific selectivity in the catalytic conversion of gas molecules and organic compounds, resulting from the direct injection of hot carriers into the lowest unoccupied molecular orbital of the adsorbate molecule. The excellent light trapping property and high efficiency of hot charge-carrier generation through electromagnetic surface plasmon decay have been identified as the dominant mechanisms that promote energy-intensive chemical reactions at room temperature and atmospheric pressure. However, understanding the electromagnetic effects of plasmonics and distinguishing them from chemical effects in photocatalysis is challenging. While there exist several reviews underlining the experimental observations of plasmonic effects, this critical review addresses the physical origin of the various plasmon-related phenomena and how they can promote photocatalysis. The conditions under which each plasmonic effect dominates and how to distinguish one from another is also discussed. Finally, future research directions are proposed with the aim to accelerate progress in this field at the interface between chemistry and physics.
©(2022) The Authors

We present a frequency-domain modeling technique for second-order nonlinear metasurfaces. The technique is derived from the generalized sheet transition conditions (GSTCs), which have been so far mostly used for modeling linear metasurfaces. In this work, we extend the GSTCs to include effective nonlinear polarizations. This allows retrieving the effective nonlinear susceptibilities of a given metasurface and predict its nonlinear scattering responses under arbitrary illumination conditions. We apply this modeling technique to the case of metasurfaces made of a periodic arrangement of T-shaped gold nanoparticles. For verification, several metasurfaces are fabricated and a fair agreement is found when comparing simulated data and experimental results. The proposed model may thus serve as a design platform to implement complex nonlinear metasurface based applications.
©(2022) The Authors

The material and exact shape of a nanostructure determine its optical response, which is especially strong for plasmonic metals. Unfortunately, only a few plasmonic metals are available, which limits the spectral range where these strong optical effects can be utilized. Alloying different plasmonic metals can overcome this limitation, at the expense of using a high-temperature alloying process, which adversely destroys the nanostructure shape. Here, a low-temperature alloying process is developed where the sample is heated at only 300 °C for 8 h followed by 30 min at 450 °C and Au–Ag nanostructures with a broad diversity of shapes, aspect ratios, and stoichiometries are fabricated. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses confirm the homogeneous alloying through the entire sample. Varying the alloy stoichiometry tunes the optical response and controls spectral features, such as Fano resonances. Binary metasurfaces that combine nanostructures with different stoichiometries are fabricated using multiple-step electron-beam lithography, and their optical function as a hologram or a Fresnel zone plate is demonstrated at the visible wavelength of λ = 532 nm. This low-temperature annealing technique provides a versatile and cost-effective way of fabricating complex Au–Ag nanostructures with arbitrary stoichiometry.
©(2022) The Authors

Capillary-assisted particle assembly (CAPA) in predefined topographical templates is a scalable method for the precise positioning of nanoscale objects on various surfaces. High-resolution CAPA templates are typically fabricated by expensive electron-beam lithography and are used for a single assembly process. To increase the scalability and reduce the costs of the CAPA technique, the fabrication and characterization of reusable templates with nanoscale funnel-shaped traps for repetitive precise nanoparticle placement are demonstrated. The yield of the first assembly of 100 nm gold nanoparticles (AuNPs) is as high as 94% with a median position offset of about 10 nm. The subsequent transfer process of the AuNPs from the silicon assembly template onto polymer surfaces, such as the elastic polydimethylsiloxane or the inelastic OrmoComp, shows a transfer yield larger than 99%. After the first transfer process, the assembly template is reused, resulting in a position offset and an assembly and transfer yield of this second assembly/transfer step that are comparable to the first ones. The obtained results demonstrate that the nanotemplates made by electron-beam lithography can be reused for repeatable CAPA processes and thereby eliminate the need for recurring lithography steps for each assembly and thus make the CAPA technique more cost-efficient.
©(2022) The Authors

The visual appearance of any colorized object is usually determined by the spectral absorption of either pigments or dyes. One can however also colorize objects with structural colors that typically have an iridescent appearance, as seen on many beetles. The former colorization mechanism is based on the chemical absorption of the used pigments or dye, while the latter mechanism exploits optical/physical effects such as interference, diffraction, or scattering. Both mechanisms are usually engineered separately and by different specialists. Actually, they can be combined synergistically and it is shown that absorption of three different dye lacquers is significantly enhanced when applied on a corrugated metal surface. Inspired by works on emission enhancement for molecules on top of grating surfaces, these dye lacquers are applied conformally on metallic crossed gratings. We vary thickness and extinction coefficient of the dye lacquers and find that absorption can be enhanced locally by more than a factor of 20. Simulations reveal the underlying mechanisms of this enhancement and the huge color gamut that can be realized by combining otherwise colorless dye lacquers with a corrugated surface is revealed. This approach has potential for applications in sensing with focus on visual inspection like the pregnancy test.
©(2022) The Authors

Beyond their original capability to grab and hold tiny objects, optical tweezers have emerged as a powerful tool to investigate fundamental physics at microscopic scales. A precise characterization of the optical trap is one of the key requirements in such applications. A typical trapping system often involves a colloidal particle, stabilized in a fluid as an optical probe. Surfactants are commonly added to provide colloidal stability, but their incidental effects on the tweezer–particle interactions have been overlooked despite their prevalent use. Here, we study the interplay among the tweezer, the particle, and the surfactants adsorbed on the interfaces, including a nearby glass wall. In trapped particles’ motion analysis, we find that the surfactants can affect the motion of the particle through the interactions between them. We discuss the effect of the surfactants’ assembly structures on the particles’ statistical behaviors. In particular, we investigate the thermal effect on the particle surroundings induced by the optically heated particle by analyzing the difference between metallic and dielectric probes. Our results explain how, under nanoscale confinement, the adsorbed surfactants can affect the particle behavior in an optical trap and propose a possible strategy of using an optically heated particle for localized surface modulation.
©(2022) American Chemical Society

Materials are, in general, either transparent or not. In principle, it is impossible to switch a material from a reflective optical state to a fully transparent one. Transparency has received relatively less research attention compared to other optical properties such as absorption, research on which have successfully produced perfect blacks, that is, highly absorbing materials. The ability to change optical transparency, especially locally and on demand, can enable several applications. Here, we present an absorbing three-layer system whose transparency can be altered by pulsed laser processing to realize different states, ranging from full transparency to mirror, black, and combinations thereof. An initially black surface can be made highly reflective or transparent by changing the laser pulse energy. The corresponding process window, including the influence of the substrate material, was investigated in detail.
©(2021) The Authors

We provide a self-consistent extension of the Lorentz reciprocity theorem and the Poynting theorem for media possessing electric and magnetic dipolar and quadrupolar responses related to electric and magnetic fields and field gradients thus corresponding to weak spatial dispersion. Using these two theorems, we respectively deduce the conditions of reciprocity and gainlessness and losslessness that apply to the various tensors mediating the interactions of these multipole moments and the associated fields and field gradients. We expect that these conditions will play an essential role in developing advanced metamaterial modeling techniques that include quadrupolar and spatially dispersive responses.
©(2021) American Pysical Society

We provide a detailed discussion on the electromagnetic modeling and classification of polarization converting bianisotropic metasurfaces. To do so, we first present a general approach to compute the scattering response of suchmetasurfaces, which relies on a generalized sheet transition conditions based susceptibility model. Then, we review how the fundamental properties of reciprocity, energy conservation, rotation invariance and matching may be expressed in terms of metasurface susceptibilities and scattering parameters, and show how these properties may affect and limit the polarization effects of metasurfaces. Finally, we connect together the metasurface susceptibility model to the structural symmetries of scattering particles and their associated polarization effects. This work thus provides a detailed understanding of the polarization conversion properties of metasurfaces and may prove to be of particular interest for their practical implementation.
©(2021) IEEE

After providing a detailed overview of nanofabrication techniques for plasmonics, we discuss in detail two different approaches for the fabrication of metallic nanostructures based on e-beam lithography. The first approach relies on a negative e-beam resist, followed by ion beam milling, while the second uses a positive e-beam resist and lift-off. Overall, ion beam etching provides smaller and more regular features including tiny gaps between sub-parts, that can be controlled down to about 10 nm. In the lift-off process, the metal atoms are deposited within the resist mask and can diffuse on the substrate, giving rise to the formation of nanoclusters that render the nanostructure outline slightly fuzzy. Scattering cross sections computed for both approaches highlight some spectral differences, which are especially visible for structures that support complex resonances, such as Fano resonances. Both techniques can produce useful nanostructures and the results reported therein should guide the researcher to choose the best suited approach for a given application, depending on the available technology.
©(2021) The Authors

Dielectric metasurfaces have shown prominent applications in nonlinear optics due to strong field enhancement and low dissipation losses at the nanoscale. Chalcogenide glasses are one of the promising materials for the observation of nonlinear effects thanks to their high intrinsic nonlinearities. Here, we demonstrate, experimentally and theoretically, that significant second harmonic generation (SHG) can be obtained within amorphous Selenium (Se)-based chalcogenide metasurfaces by exploiting the coupling between lattice and particle resonances. We further show that the highquality factor resonance at the origin of the SHG can be tuned over a wide wavelength range using a simple and versatile fabrication approach. The measured second harmonic intensity is orders of magnitude higher than that from a dewetted Se film consisting of random Se nanoparticles. The achieved conversion efficiency in the resonance region is of the order of 10−6 which is comparable with direct bandgap materials and at least two orders of magnitude higher than that of conventional plasmonics- and Si-based structures. Fabricated via a simple and scalable technique, these all-dielectric architectures are ideal candidates for the design of flat nonlinear optical components on flexible substrates.
©(2021) The Authors

In this paper, we aim at unveiling the underlying physical mechanism for transversal optical forces, appearing due to the simultaneous illumination of a spherical object with two plane waves possessing different polarizations. The appearance of such a transversal force is quite counterintuitive since it seems to contradict the law of momentum conservation. We consider the cases of perfect electric conductor (PEC) and silver spheres illuminated by two orthogonally polarized plane waves propagating obliquely with respect to each other. Interestingly, the Poynting vector in these cases acquires a nonzero component transverse to the plane of propagation. Since the momentum transfer is related to the energy transfer, or equivalently, to non-negligible Poynting vector pointed in a particular direction, an arbitrary object placed in such external field is expected to experience a transversal force. To cast light upon this peculiar effect, we use a surface integral equation method and, along with the Maxwell stress tensor formalism, find the optical force acting on various spheres. We observe this effect for PEC spheres of different sizes and find that they are indeed subject to such transversal force. We find an explanation for this phenomenon via interference effects between selected multipoles excited in the structure. With recently developed methods, we expand the optical force into contributing pairs of selected multipoles and show that, depending on the phase between each multipole pair, the sign and direction of the force can be controlled. We also compare the results for silver and PEC spheres and find that the transversal force magnitude in silver has higher values for more limited range of sphere radii, as compared to PEC.
©(2021) SPIE

Hybrid structures that combine dielectric resonators with plasmonic structures hold great promises due to the diversity of optical modes they possess. Here, we explore the physics underlying the scattering response of a hybrid nanoantenna made of a metal disk placed on top of a dielectric cylinder and study the hybridization of the different modes excited in the dielectric and metallic parts. Surprisingly, we note that the signature of an anapole state – usually only seen in high refractive index dielectrics – can be observed in the metallic part of the system. The Cartesian multipoles excited in the dielectric and metal interfere in a complex manner, leading to an unexpected high-order vector spherical multipolar response in the far-field. These effects are thoroughly studied in terms of the near-field and absorption enhancements. We also show that very fine control over the multipoles’ resonant positions can be achieved by varying the geometry of the structure. This flexibility renders this system very promising for sensing applications. Based on these developments, we have designed and fabricated such hybrid nanoantennas using silicon and aluminum and measured a preliminary sensitivity of 160 nm/RIU, which is competing with conventional sensors based on localized surface plasmon resonances.
©(2021) SPIE

We perform a systematic study showing the evolution of the multipoles along with the spectra for a hybrid metal-dielectric nanoantenna, a Si cylinder and an Ag disk stacked one on top of another, as its dimensions are varied one by one. We broaden our analysis to demonstrate the "magnetic light" at energies above 1 eV by varying the height of the Ag on the Si cylinder and below 1 eV by introducing insulating spacing between them. We also explore the appearance of the anapole state along with some exceptionally narrow spectral features by varying the radius of the Ag disk.
©(2021) American Optical Society

Light coupling in waveguides has been extensively investigated in a variety of contexts, from photonic integrated circuits to biosensing and near-eye displays for augmented reality. Here, narrowband diffraction is reported using a Fano interference effect in hybrid nanostructures. The excitation of hybrid plasmonic and bulk waveguides allows for a selectivity of 10 nm bandwidth in the first order and strong reduction of the entire zeroth order. A Fano formalism is used to predict the maximal diffraction efficiency at critical coupling, when external mode coupling balances intrinsic losses. It is found that the first order and zeroth order are related by a Fano-like spectral profile with similar spectral widths, resonance wavelengths, and modulation depths and differ only in the asymmetry parameter. The diffraction efficiency, angle, and wavelength can be solely tuned by the thin film thickness. A semianalytical dispersion model of the hybrid system is introduced and validated experimentally. Applications are foreseen in many optical devices that require color-selective coupling or dispersive properties such as optical document security or near-eye displays. The dispersion behavior under a divergent light source can also be utilized to design inexpensive, compact, and robust spectrometers or biosensors.
©(2021) American Chemical Society

In this work, we use finite elements simulations to study the far field properties of two plasmonic structures, namely a dipole antenna and a cylinder dimer, connected to a pair of nanorods. We show that electrical, rather than near field, coupling between the modes of these structures results in a characteristic Fano lineshape in the far field spectra. This insight provides a way of tailoring the far field properties of such systems to fit specific applications, especially maintaining the optical properties of plasmonic antennas once they are connected to nanoelectrodes. This work extends the previous understanding of Fano resonances as generated by a simple near field coupling and provides a route to an efficient design of functional plasmonic electrodes.
©(2021) American Optical Society

Avalanche multiphoton photoluminescence (AMPL) is observed from coupled Au–Al nanoantennas under intense laser pumping, which shows more than one order of magnitude emission intensity enhancement and distinct spectral features compared with ordinary metallic photoluminescence. The experiments are conducted by altering the incident laser intensity and polarization using a home-built scanning confocal optical microscope. The results show that AMPL originates from the recombination of avalanche hot carriers that are seeded by multiphoton ionization. Notably, at the excitation stage, multiphoton ionization is shown to be assisted by the local electromagnetic field enhancement produced by coupled plasmonic modes. At the emission step, the giant AMPL intensity can be evaluated as a function of the local field environment and the thermal factor for hot carriers, in accordance with a linear relationship between the power law exponent coefficient and the emitted photon energy. The dramatic change in the spectral profile is explained by spectral linewidth broadening mechanisms. This study offers nanospectroscopic evidence of both the potential optical damages for plasmonic nanostructures and the underlying physical nature of light–matter interactions under a strong laser field; it illustrates the significance of the emerging topics of plasmonic-enhanced spectroscopy and laser-induced breakdown spectroscopy.
©(2021) American Institute of Physics

At the nanoscale level, optical properties of materials depend greatly on their shape. Finding the right geometry for a specific property remains a fastidious and long task, even with the help of modelling tools. In this work, we overcome this challenge by using artificial intelligence to guide a reverse engineering method. We present an optimization algorithm based on a deep convolution generative adversarial network for the design a 2-dimensional optical cloak. The optical cloak consists in a shell of uniform and isotropical dielectric material, and the cloaking is achieved via the geometry of this shell. We use a feedback loop from the solutions of this generative network to successively retrain it and improve its ability to predict and find optimal geometries. This generative method allows to find a global solution to the optimization problem without any prior knowledge of good cloaking geometries.
©(2021) American Optical Society

Hybrid metal-dielectric nanostructures have recently gained prominence because they combine strong field enhancement of plasmonic metals and the several low-loss radiation channels of dielectric resonators, which are qualities pertaining to the best of both worlds. In this work, an array of such hybrid nanoantennas is successfully fabricated over a large area and utilized for bulk refractive index sensing with a sensitivity of 208 nm/RIU. Each nanoantenna combines a Si cylinder with an Al disk, separated by a SiO2 spacer. Its optical response is analyzed in detail using the multipoles supported by its subparts and their mutual coupling. The nanoantenna is further modified experimentally with an undercut in the SiO2 region to increase the interaction of the electric field with the background medium, which augments the sensitivity to 245 nm/RIU. A detailed multipole analysis of the hybrid nanoantenna supports our experimental findings.
©(2020) American Chemical Society

We analyze the superposition of Cartesian multipoles to reveal the mechanisms underlying the origin of optical forces. We show that a multipolar decomposition approach significantly simplifies the analysis of this problem and leads to a very intuitive explanation of optical forces based on the interference between multipoles. We provide an in-depth analysis of the radiation coming from the object, starting from low-order multipole interactions up to quadrupolar terms. Interestingly, by varying the phase difference between multipoles, the optical force as well as the total radiation directivity can be well controlled. The theory developed in this paper may also serve as a reference for ultra-directional light steering applications.
©(2020) Optical Society of America

In this article we review some machine learning methods for the design of nanomaterials. The first part will discuss how to use neural network to build a predictive model of the optical properties of a certain material or structure. The second part is dedicated to the optimization and reverse engineering of an optical material using generative networks.
©(2020) SPIE

A large amount of experimental and theoretical works deals with the second harmonic generation from different plasmonic geometries. Since they often consider relatively long optical pulses, many of these studies are focused on the investigation of a quasi-monochromatic response of the system and can be understood through the excitation of one, possibly two, optical modes. On the other hand, when the excitation pulse duration is short (say, below several tens of fs), the excitation spectrum becomes broader and a very interesting dynamics emerges from the interplay between several optical modes. In this work, the dynamics of modes at the second harmonic frequency for two silver spheres of different diameters and a nanorod is investigated numerically and shown to be quite different. For the pulsed illumination with length close to the modes lifetime, apart from different relative contributions of dipolar and quadrupolar multipoles in the far-field, we have been able to observe and explain non constant phase difference between multipoles, which is not accessible in continuous wave regime. Short pulse durations also allow us to observe only one mode, while another one has already decayed. For the case of the nanorod we also perform an eigenmode analysis, which allows to understand the modes interplay that explains the observed spectra. In the paper, we also show a method allowing a significant reduction of required computational steps to find the response of a plasmonic nanostructure to a pulsed illumination with arbitrary frequency-domain method.
©(2020) SPIE

A novel approach is introduced to determine the time evolution of optical forces and torques on arbitrary shape nanostructures by combining Maxwell's stress tensor with the surface integral equation method (SIE). Conventional time averaging of Maxwell’s stress tensor allows obtaining an elegant form in terms of surface currents for the force exerted on nanostructures. Unfortunately, the information about the time dependence of the force – which can be very important in ultrafast photonics experiments and in nano-manipulation applications – is lost in such an approach. To overcome this, we have developed a time-domain method based on the inverse Fourier transform of the frequency-domain SIE. The calculations in the frequency domain allow accurately taking into account the dispersion of the permittivity function of the system and the use of surface currents enables the rigorous treatment of intricate geometries for the scatterer. Furthermore, the integration of Maxwell’s stress tensor directly on the scatterer’s boundary significantly reduces the required computation time and increases the accuracy of the method. We show quite unusual sum frequency-like terms in the dynamics of the force appearing in Maxwell’s stress tensor, which normally vanish for the time-averaged force. To illustrate this effect, we study how the pulse duration influences the dynamics of optical force in the case of a rectangular shape and Gaussian pulses illuminating thin film at normal incidence. In the framework of the developed numerical method, we study the influence of the sum-frequency-like terms on the dynamics of optical forces in the case of a spherical scatterer.
©(2020) SPIE

We propose a theoretical study on the electromagnetic forces resulting from the superposition of TE and TM plane waves interacting with a sphere. Specifically, we first show that, under such an illumination condition, the sphere is subjected to a force transverse to the propagation direction of the waves. We then analyze the physical origin of this counterintuitive behavior using a multipolar decomposition of the electromagnetic modes involved in that scattering process. This analysis reveals that interference effects, due to the two-wave illumination, lead to a Kerker-like asymmetric scattering behavior resulting in this peculiar transverse force.
©(2020) Amercian Physical Society

The fabrication of highly ordered nanostructures over large areas is key for many technologies and colloidal lithography using the Langmuir Blodgett technique appears a simple and straightforward way of reaching that goal. While this technique has been widely reported in the literature, its straightforward implementation to obtain well-ordered nanostructures over very large areas is far from obvious, since many key technical subtleties are rarely documented. Here, we describe an easily and highly reproducible recipe and detail aspects such as beads preparation, composition of the subphase, beads transfer method, influence of the spreading agent and the barrier compression rate, as well as monolayer transfer to the substrate. A drastic improvement in the polystyrene self-assembly at the air-water interface is observed after removing the common salt and surfactant molecules from commercial polystyrene beads suspensions. Similarly, an electrolyte free water subphase enhances the hexagonal arrangement of the beads and the long-range order. The beads sinking into the bulk of the water is reduced by dispensing the beads using a glass slide and the polystyrene suspension prepared using water and ethanol at 1:1 mitigates repulsive and attractive forces, leading to excellent hexagonal close packed arrangement. By following the recipe shown here, the reader should easily fabricate lattice-like colloidal masks for producing nanostructures over larger areas.
©2020 Elsevier B.V.

In this Letter, we demonstrate how harmonic oscillator equations can be integrated in a neural network to improve the spectral response prediction for an optical system. We use the optical properties of a one-dimensional nanoslit array for a practical implementation of the study. This method allows to build more generalizable relations between the input parameters of the array and its optical properties, showing a 20-fold improvement for parameters outside the range used for the training. We also show how this model generates the output spectrum from phenomenological relationships between the input parameters and the output spectrum, indicating how it grasps the physical mechanisms of the optical response of the structure.
©(2020) Optical Society of America

This article aims at studying the angular scattering properties of bianisotropic metasurfaces and clarifying the different roles played by tangential and normal polarization densities. Different types of metasurfaces are considered for this study and are classified according to their symmetrical/asymmetrical and reciprocal/nonreciprocal angular scattering behavior. Finally, the article presents the relationships between the symmetrical angular scattering properties of reciprocal metasurfaces and the structural symmetries of their scattering particles. This may prove to be practically useful for the implementation of metasurfaces with complex angular scattering characteristics.
©(2020) IEEE

The full wave surface integral equation computation of the second harmonic generation (SHG) dynamics for metal spheres and nanorods - presented as multimedia files - is performed to reveal the dynamics of the modes supported by the nanostructure. We demonstrate that the interplay between different modes controls the nonlinear response and that the size-induced redshift of the eigenmodes can be manipulated by adjusting the nanostructure geometry, so that the SHG signal can be boosted at specified frequencies. We show that the SHG radiation is not necessarily quadrupolar in spherical nanoparticles, as it is often assumed. Finally, we introduce an efficient way to reduce the SHG calculation time.
©(2019) Optical Society of America

Plasmonic antennas improve the stiffness and resolution of optical tweezers by producing a strong near-field. When the antenna traps metallic objects, the optically-resonant object affects the near-field trap, and this interaction should be examined to estimate the optical force accurately. We study this effect in detail by evaluating the force using both Maxwell’s stress tensor and the dipole approximation. In spite of the strong optical interaction between the particle and the antenna, the results show that the dipole approximation remains accurate for calculating forces on Rayleigh particles. For particles whose sizes exceed the dipole limit, we observe different coupling regimes where the force becomes either attractive or repulsive. The distributions of field amplitudes and polarization charges explain such a behavior.
©(2019) Optical Society of America

Second-harmonic generation (SHG) is investigated from three kinds of lithographically fabricated plasmonic systems: Al monomers, Au monomers and Au–Al heterodimers with nanogaps of 20 nm. Spectrally integrated SHG intensities and the linear optical responses are recorded and compared. The results show that for the monomer nanoantennas, the SHG signal depends sensitively on the linear excitation of the plasmon resonance by the fundamental wavelength. For Au–Al heterodimer nanoantennas, apart from fundamental resonant excitation, nonlinear optical factors such as SH driving fields and phase interferences need to be taken into account, which play significant roles at the excitation and scattering stages of SHG radiation. It is interesting to note that a possible energy transfer process could take place between the two constituting nanoparticles (NPs) in the Au–Al heterodimers. Excited at the linear plasmon resonance, the Au NP transfers the absorbed energy from the fundamental field to the nearby Al NP, which efficiently scatters SHG to the far-field, giving rise to an enhanced SHG intensity. The mechanisms reported here provide new approaches to boost the far-field SHG radiation by taking full advantage of strongly coupled plasmonic oscillations and the synergism from materials of different compositions.
©(2019) Royal Society of Chemistry

We are all familiar with crystal structures built from the periodic arrangement in space of atoms, molecules or even scatterers. Their periodicity is at the origin of a wealth of optical phenomena ranging from birefringence—first observed by Christiaan Huygens in the 17th century1—to optical bandgaps in photonic crystals and their promise to slow down light in the 21st century.2 This modulation of the dielectric function is not limited to one, two or three space dimensions, but can also be forced upon the fourth dimension: time! Actually, as early as the beginning of the 1960s, the idea of modulating the dielectric function in time emerged,3 and was later elaborated to control parametric nonlinear processes by virtue of travelling waves,4 paving the way for extremely efficient devices, such as traveling-wave photodetectors.5 Exploring periodic variations of the dielectric function in all four dimensions of space and time unleashes fascinating physics and provides opportunities for exciting experiments.
©(2019) The Author

We demonstrate a novel method for fabricating single crystal diamond diffraction gratings based on crystallographic etching that yields high-quality diffraction gratings from commercially available <100> diamond plates. Both V-groove and rectangular gratings were fabricated and characterised using scanning electron microscopy and atomic force microscopy, revealing angles of 57° and 87° depending on the crystal orientation, with mean roughness below Ra = 5 nm on the sidewalls. The gratings were also optically characterised, showing good agreement with simulated results. The fabrication method demonstrated in this contribution shows the way for manufacturing high-quality diamond diffractive components that surpass existing devices both in quality and manufacturability.
©(2019) Optical Society of America

We study the angular scattering behavior of bianisotropic metasurfaces and de-duce relationships between the corresponding symmetrical angular scattering properties and the structural symmetries of their scattering particles. This may be of practical interest for the realization of metasurfaces with complex angular scattering characteristics.
©(2019) IEEE

The relationship between composition and plasmonic properties in noble metal nanoalloys is still largely unexplored. Yet, nanoalloys of noble metals, such as gold, with transition elements, such as iron, have unique properties and a number of potential applications, ranging from nanomedicine to magneto-plasmonics and plasmon-enhanced catalysis. Here, we investigate the localized surface plasmon resonance at the level of the single Au–Fe nanoparticle by applying a strategy that combines experimental measurements using near field electron energy loss spectroscopy with theoretical studies via a full wave numerical analysis and density functional theory calculations of electronic structure. We show that, as the iron fraction increases, the plasmon resonance is blue-shifted and significantly damped, as a consequence of the changes in the electronic band structure of the alloy. This allows the identification of three relevant phenomena to be considered in the design and realization of any plasmonic nanoalloy, specifically: the appearance of new states around the Fermi level; the change in the free electron density of the metal; and the blue shift of interband transitions. Overall, this study provides new opportunities for the control of the optical response in Au–Fe and other plasmonic nanoalloys, which are useful for the realization of magneto-plasmonic devices for molecular sensing, thermo-plasmonics, bioimaging, photocatalysis, and the amplification of spectroscopic signals by local field enhancement.
©2019 American Chemical Society

Fano resonances in metamaterials can be explained by the coupling between different modes sustained by the meta-atom. While the associated spectral features are usually quite easy to identify for top-down metamaterials, thanks to the deterministic arrangement of the meta-atom relative to the illumination, the identification of spectral features due to Fano resonances is much more challenging for bottom-up metamaterials. There, the response is spurred by the random arrangement of the meta-atoms relative to the illumination. To improve the situation, we introduce a quantity that measures the nonorthogonality of the modes sustained by the meta-atom. The measure allows us to disclose whether specific details in the spectral response emerge from Fano features or whether they are only due to the incoherent superposition of different modes. We strengthen our argumentation while discussing the multipolar decomposition of the different modes that contribute to the Fano features for a representative meta-atom.
©2019 American Physical Society

We propose a surface-wave dispersion retrieval method and synthesis technique that applies to bianisotropic metasurfaces that are embedded in symmetric or asymmetric environments. Specifically, we use general zero-thickness sheet transition conditions to relate the propagation constants of surface-wave modes to the bianisotropic susceptibility components of the metasurface, which can themselves be directly related to its scattering parameters. It is then possible to either obtain the metasurface dispersion diagram from its known susceptibilities or, alternatively, compute the susceptibilities required to achieve a desired surface-wave propagation. The validity of the method is demonstrated by comparing its results to those obtained with exact dispersion relations of well known structures such as the propagation of surface plasmons on thin metallic films. In particular, this work reveals that it is possible to achieve surface-wave propagation only on one side of the metasurface either by superposition of symmetric and asymmetric modes in the case of anisotropic metasurfaces or by completely forbidding the existence of the surface wave on one side of the structure using bianisotropic metasurfaces.
©2019 Amercan Physical Society

Ammonia production at room temperature and atmospheric pressure is in high demand to assist in energy saving and the protection of the environment worldwide, as well as to help reduce CO2 emissions. Recently, plasmonic nanomaterials have been frequently used for solar to chemical energy conversion, which has the potential to replace existing energy-intensive industrial processes. In our approach, plasmonic aluminium nanotriangles (AlNTs) were used to investigate the impact of plasmonic effects on photocatalytic ammonia production. Plasmonic near-field coupling to a semiconductor and hot electron generation from AlNTs were studied in detail through the use of electrochemical photocurrent measurements. A narrowband LED beam with a central wavelength at 365 nm was used to illuminate the AlNTs and their hot electron generation efficiency was estimated to be 2 × 10−4%, resulting in an ammonia production rate of 4 × 10−5 μM h−1 mW−1 cm−2, which corresponds to a quantum efficiency of 2.5 × 10−5%. In the case of plasmonic near-field coupling, AlNTs-embedded TiO2 demonstrates a charge-carrier generation efficiency of 2.7%, which is ∼2.3 times higher than that of bare TiO2. The ammonia production rate of AlNTs–TiO2 is 0.1 μM h−1 mW−1 cm−2 with a quantum efficiency of ∼0.06%, which corresponds to ∼2.4 times that of the rate demonstrated by bare TiO2 (0.04 μM h−1 mW−1 cm−2, quantum efficiency ∼ 0.025%). The obtained results confirm successful ammonia production through nitrogen splitting at room temperature and under atmospheric pressure. Moreover, according to the presented results, the use of plasmonic aluminium structures remarkably improves the ammonia production rate.
©2019 Royal Society of Chemistry

We investigate optical second-harmonic generation (SHG) from metasurfaces where noncentrosymmetric Vshaped gold nanoparticles are ordered into regular array configurations. In contrast to expectations, a substantial enhancement of the SHG signal is observed when the number density of the particles in the array is reduced. More specifically, by halving the number density, we obtain over 5-fold enhancement in SHG intensity. This striking result is attributed to favorable interparticle interactions mediated by the lattice, where surfacelattice resonances lead to spectral narrowing of the plasmon resonances. Importantly, however, the results cannot be explained by the improved quality of the plasmon resonance alone. Instead, the lattice interactions also lead to further enhancement of the local fields at the particles. The experimental observations agree very well with results obtained from numerical simulations including lattice interactions.
©2018 American Chemical Society

The silencing of the second harmonic generation process from plasmonic nanostructures corresponds to the limited far-field second harmonic radiation despite the huge fundamental electric field enhancement in the interstice between two plasmonic nanoparticles forming a nanodimer. In this article, we report a comprehensive investigation of this effect using a surface integral equation method. Various geometries are considered, including nanoantennas with cylindrical and rectangular arms as well as nanodimers with surface defects. The existence of the silencing of the second harmonic generation from plasmonic nanogaps is first confirmed, and the problem of the origin of the second harmonic light from these plasmonic nanostructures is addressed in detail. Our results show that the distribution of the second harmonic sources, especially on the arm sides, plays a non-negligible role in the overall second harmonic emission. This contribution is induced by retardation effects at the pump wavelength and results in a dipolar second harmonic emission.
©2018 Butet et al.; licensee Beilstein-Institut.

Recently, it was noted that losses in plasmonics can also enable several useful optical functionalities. One class of structures that can maximize absorption are metal insulator metal systems. Here, we study 3-layer systems with a nano-composite metal layer as top layer. These systems can absorb almost 100% of light at visible frequencies, even though they contain only dielectrics and highly reflecting metals. We elucidate the underlying physical phenomenon that leads to this extraordinary high and broadband absorption. A comprehensive study of the particle material and shape, mirror material and dielectric spacer thickness is provided to identify their influence on the overall absorption. Thus, we can provide detailed design guidelines for realizing optical functionalities that require near-perfect absorption over specific wavelength bands. Our results reveal the strong role of lossy Fabry-Perot interference within these systems despite their thickness being well below half a wavelength.
©2018 Optical Society of America

Due to its symmetry properties, second-harmonic generation in plasmonic nanostructures enables the observation of even-parity modes that couple weakly to the far field. Consequentially, those modes radiate less and thus have a longer lifetime. Using a full-wave numerical method, we study the linear and second harmonic dynamical responses of a silver nanorod under plane-wave femtosecond pulse illumination. Depending on the spectral position and duration of the pulse, the decaying field of the different modes can be separated, and the free oscillations of each mode are well fitted by a damped harmonic oscillator model, both in the linear and nonlinear regimes. Additionally, interference effects between different modes excited at the second harmonic are observed.
©(2018) SPIE

We present an extensive discussion on the homogenization and scattering analysis of second-order nonlinear metasurfaces. We use the generalized sheet transition conditions (GSTCs) in the frequency-domain to model the electromagnetic responses of nonlinear meta-surfaces. We present the general second-harmonic scattering relations, in the undepleted pump regime approximation, and the resulting reflectionless, transmissionless and asymmetric reflec-tion and transmission conditions. Finally, to clarify certain misconceptions, we also discuss the concept of nonreciprocal scattering in nonlinear optics.
©(2018) IEEE

We propose to control the motion of nanoparticles using phase-gradient metasur-faces. The latter are used to generate surface waves, which put the particles into motion, when illuminated by a normally incident plane wave. We present an initial study of the force and acceleration acting on these particles due to their interactions with the surface wave.
©(2018) IEEE

We study electromagnetic trapping in an optical lattice formed by three equiangular in-plane beams. We demonstrate analytically that this optical lattice offers stable trapping sites for particles satisfying specific symmetries, irrespective of the exact nature of their electromagnetic response. Under small displacements, the particles are shown to be subject to equal restoring forces along all directions and the trap is isotropic. Though the intensity distribution of the trap forms a perfect hexagonal lattice, differences in phase variation along opposite directions cause the restoring force to be asymmetric for large displacements, resulting in a force landscape possessing only threefold symmetry. We then show numerically that this asymmetry affects the optical binding force between particles in adjacent trap positions and results in unequal shifts of their equilibrium positions. Universal trapping in the optical lattice combined with this asymmetric mechanical response of trapped particle pairs promises rich optomechanical effects.
©2018 American Physical Society

We propose an extensive discussion on the homogenization and scattering analysis of second-order nonlinear metasurfaces. Our developments are based on the generalized sheet transition conditions (GSTCs) which are used to model the electromagnetic responses of nonlinear metasurfaces. The GSTCs are solved both in the frequency domain, assuming an undepleted pump regime, and in the time-domain, assuming dispersionless material properties but a possible depleted pump regime. Based on these two modeling approaches, we derive the general secondharmonic reflectionless and transmissionless conditions as well as the conditions of asymmetric reflection and transmission. We also discuss and clarify the concept of nonreciprocal scattering pertaining to nonlinear metasurfaces.
©2018 IEEE

Resonant waveguide gratings (RWGs), also known as guided mode resonant (GMR) gratings or waveguide‐mode resonant gratings, are dielectric structures where these resonant diffractive elements benefit from lateral leaky guided modes from UV to microwave frequencies in many different configurations. A broad range of optical effects are obtained using RWGs such as waveguide coupling, filtering, focusing, field enhancement and nonlinear effects, magneto‐optical Kerr effect, or electromagnetically induced transparency. Thanks to their high degree of optical tunability (wavelength, phase, polarization, intensity) and the variety of fabrication processes and materials available, RWGs have been implemented in a broad scope of applications in research and industry: refractive index and fluorescence biosensors, solar cells and photodetectors, signal processing, polarizers and wave plates, spectrometers, active tunable filters, mirrors for lasers and optical security features. The aim of this review is to discuss the latest developments in the field including numerical modeling, manufacturing, the physics, and applications of RWGs. Scientists and engineers interested in using RWGs for their application will also find links to the standard tools and references in modeling and fabrication according to their needs.
©2018 Wiley

Second-harmonic generation in plasmonic nanostructures is known to enable the observation of modes with vanishing dipolar moments, i.e., having small radiation losses and thus long lifetimes. With the aid of a full wave numerical method, we study the far-field temporal dynamics of the linear and nonlinear responses of a silver nanorod driven by femtosecond pulses. The results show that the plasmons lifetime is observable in the decaying field oscillations surviving after the exciting pulse, for both processes, and fits with the damped harmonic oscillator model. In addition, using a detailed mode analysis, we find that the multipolar characteristic of the nonlinear radiation is strongly influenced by both the pulse central frequency and width. Implications for the accurate measurement of plasmon lifetime with the help of nonlinear optics are discussed, especially the need to carefully disentangle the linear and nonlinear plasmon dynamics.
©2018 American Chemical Society

C-reactive protein (CRP) is one of the most expressed proteins in blood during acute phase inflammation, and its minute level increase has also been recognized for the clinical diagnosis of cardio vascular diseases. Unfortunately, the available commercial immunoassays are labour intensive, require large sample volumes, and have practical limitations, such as low stability and high production costs. Hence, we have developed a simple, cost effective, and label-free electrochemical immunoassay for the measurement of CRP in a drop of serum sample using an immunosensor strip made up of a screen printed carbon electrode (SPE) modified with anti-CRP functionalized gold nanoparticles (AuNPs). The measurement relies on the decrease of the oxidation current of the redox indicator Fe3+/Fe2+, resulting from the immunoreaction between CRP and anti-CRP. Under optimal conditions, the present immunoassay measures CRP in a linear range from 0.4–200 nM (0.047–23.6 µg mL−1), with a detection limit of 0.15 nM (17 ng mL−1, S/N = 3) and sensitivity of 90.7 nA nM−1, in addition to a good reproducibility and storage stability. The analytical applicability of the presented immunoassay is verified by CRP measurements in human blood serum samples. This work provides the basis for a low-priced, safe, and easy-to-use point-of-care immunosensor assay to measure CRP at clinically relevant concentrations.
©2018 by the authors

Practice oriented point-of-care diagnostics require easy-to-handle, miniaturized, and low-cost analytical tools. In a novel approach, screen printed carbon electrodes (SPEs), which were functionalized with nanomaterials, are employed for selective measurements of bilirubin, which is an important biomarker for jaundice. Multi-walled carbon nanotubes (MWCNT) and graphene separately deposited on SPEs provide the core of an electrochemical sensor for bilirubin. The electrocatalytic activity towards bilirubin oxidation (bilirubin to biliverdin) was observed at +0.25 V. In addition, a further peak corresponding to the electrochemical conversion of biliverdin into purpurin appeared at +0.48 V. When compared to MWCNT, the graphene type shows a 3-fold lower detection limit (0.3 +/- 0.022 nM and 0.1+/-0.018 nM, respectively),moreover, the graphene type exhibits a larger linear range (0.1–600 μM) than MWCNT (0.5–500 μM) with a two-fold better sensitivity, i.e., 30 nA μM(-1)cm(-2)2, and 15 nA μM(-1)cm(-2), respectively. The viability is validated through measurements of bilirubin in blood serum samples and the selectivity is ensured by inhibiting common interfering biological substrates using an ionic nafion membrane. The presented approach enables the design and implementation of low cost andminiaturized electrochemical sensors.
©2018 by the authors

The D- and G-band Raman signals from random arrays of vertically aligned, multi-walled carbon nanotubes are significantly enhanced (up to B14) while the signal from the underlying Si substrate is simultaneously attenuated (up to B6x) when the nanotubes are dressed, either capped or coated, with Ag. These Ag-induced counter-changes originate with the difference in geometry of the nanotubes and planar Si substrate and contrast in the Ag depositions on the substrate (essentially thin film) and the nanotube (nano-particulate). The surface integral equation technique is used to perform detailed modelling of the electromagnetic response of the system in a computationally efficient manner. Within the modelling the overall antenna response of the Ag-dressed nanotubes is shown to underpin the main contribution to enhancement of the nanotube Raman signal with hot-spots between the Ag nanoparticles making a subsidiary contribution on account of their relatively weak penetration into the nanotube walls. Although additional hot-spot activity likely accounts for a shortfall in modelling relative to experiment it is nonetheless the case that the significant antenna-driven enhancement stands in marked contrast to the hot-spot dominated enhancement of the Raman spectra from molecules adsorbed on the same Ag-dressed structures. The Ag-dressing procedure for amplifying the nanotube Raman output not only allows for ready characterisation of individual nanotubes, but also evidences a small peak at B1150 cm-1 (not visible for the bare, undressed nanotube) which is suggested to be due to the presence of trans-polyacetylene in the structures.
©2018 Royal Society of Chemistry

Resonant waveguide gratings (RWGs) are thin-film structures, where coupled modes interfere with the diffracted incoming wave and produce strong angular and spectral filtering. The combination of two finite-length and impedance matched RWGs allows the creation of a passive beam steering element, which is compatible with up-scalable fabrication processes. Here, we propose a design method to create large patterns of such elements able to filter, steer, and focus the light from one point source to another. The method is based on ellipsoidal mirrors to choose a system of confocal prolate spheroids where the two focal points are the source point and observation point, respectively. It allows finding the proper orientation and position of each RWG element of the pattern, such that the phase is constructively preserved at the observation point. The design techniques presented here could be implemented in a variety of systems, where large-scale patterns are needed, such as optical security, multifocal or monochromatic lenses, biosensors, and see-through optical combiners for near-eye displays.
©2018 SPIE

Optical second harmonic generation (SHG) from nanostructured graphene has been studied in the framework of classical electromagnetism using a surface integral equation method. Single disks and dimers are considered, demonstrating that the nonlinear conversion is enhanced when a localized surface plasmon resonance is excited at either the fundamental or second harmonic frequency. The proposed approach, beyond the electric dipole approximation used in the quantum description, reveals that SHG from graphene nanostructures with centrosymmetric shapes is possible when retardation effects and the excitation of high plasmonic modes at the second harmonic frequency are taken into account. Several SHG effects similar to those arising in metallic nanostructures, such as the silencing of the nonlinear emission and the design of double resonant nanostructures, are also reported. Finally, it is shown that the SHG from graphene disk dimers is very sensitive to a relative vertical displacement of the disks, opening new possibilities for the design of nonlinear plasmonic nanorulers.
©Â© 2017 Optical Society of America

We investigate the plasmonic behavior of Koch snowflake fractal geometries and their possible application as broadband optical antennas. Lithographically defined planar silver Koch fractal antennas were fabricated and characterized with high spatial and spectral resolution using electron energy loss spectroscopy. The experimental data are supported by numerical calculations carried out with a surface integral equation method. Multiple surface plasmon edge modes supported by the fractal structures have been imaged and analyzed. Furthermore, by isolating and reproducing self-similar features in long silver strip antennas, the edge modes present in the Koch snowflake fractals are identified. We demonstrate that the fractal response can be obtained by the sum of basic self-similar segments called characteristic edge units. Interestingly, the plasmon edge modes follow a fractal-scaling rule that depends on these selfsimilar segments formed in the structure after a fractal iteration. As the size of a fractal structure is reduced, coupling of the modes in the characteristic edge units becomes relevant, and the symmetry of the fractal affects the formation of hybrid modes. This analysis can be utilized not only to understand the edge modes in other planar structures but also in the design and fabrication of fractal structures for nanophotonic applications.
©2017 American Chemical Society

Surface plasmons are excited at a metal/dielectric interface, through the coupling between conduction electrons and incident photons. The surface plasmon generation is therefore strongly determined by the accessibility of the surface to the incoming electromagnetic field. We demonstrate the role of this surface for plasmonic nanoantennas with identical volumes and resonant lengths. An antenna is stratified parallel to the plane of its main dipolar resonance axis and the influence of the number of layers and the spacing between them on the optical properties of the antenna are investigated experimentally. We show that increasing the number of layers and, hence, increasing the total accessible surface of the antenna, results in an enhanced scattering cross section and a redshift which indicates that lower energy photons are required to couple to the metal electrons. In particular, the far-field enhancement observed for double-layer nanostructures suggests that standard single-layer metal deposition can be easily and advantageously substituted with metal/dielectric/metal deposition to boost light scattered by a plasmonic antenna.
©2017 SPIE

Junctions between n-type semiconductors of different electron affinity show rectification if the junction is abrupt enough. With the advent of 2D materials, we are able to realize thin van der Waals (vdW) heterostructures based on a large diversity of materials. In parallel, strongly correlated functional oxides have emerged, having the ability to show reversible insulator-to-metal (IMT) phase transition by collapsing their electronic bandgap under a certain external stimulus. Here, we report for the first time the electronic and optoelectronic characterization of ultra-thin n-n heterojunctions fabricated using deterministic assembly of multilayer molybdenum disulphide (MoS2) on a phase transition material, vanadium dioxide (VO2). The vdW MoS2/VO2 heterojunction combines the excellent blocking capability of an n-n junction with a high conductivity in on-state, and it can be turned into a Schottky rectifier at high applied voltage or at temperatures higher than 68 °C, exploiting the metal state of VO2. We report tunable diode-like current rectification with a good diode ideality factor of 1.75 and excellent conductance swing of 120 mV/dec. Finally, we demonstrate unique tunable photosensitivity and excellent junction photoresponse in the 500/650 nm wavelength range.
©2017 The Authors

In this work, we investigate the generation of second-harmonic light by gold nanorods and demonstrate that the collected nonlinear intensity depends upon a phase interplay between different modes available in the nanostructure. By recording the backward and forward emitted second-harmonic signals from nanorods with various lengths, we find that the maximum nonlinear signal emitted in the forward and backward directions is not obtained for the same nanorod length. We confirm the experimental results with the help of full-wave computations done with a surface integral equation method. These observations are explained by the multipolar nature of the second-harmonic emission, which emphasizes the role played by the relative phase between the second-harmonic modes. Our findings are of particular importance for the design of plasmonic nanostructures with controllable nonlinear emission and nonlinear plasmonic sensors as well as for the coherent control of harmonic generations in plasmonic nanostructures.
©Â© 2017 American Chemical Society

A novel thin-film single-layer structure based on resonant waveguide gratings (RWGs) allows to engineer selective color filtering and steering of white light. The unit cell of the structure consists of two adjacent finite-length and cross-talking RWGs, where the former acts as in-coupler and the latter acts as out-coupler. The structure is made by only one nano-imprint lithography replication and one thin film layer deposition, making it fully compatible with up-scalable fabrication processes. We characterize a fabricated optical security element designed to work with the flash and the camera of a smartphone in off-axis light steering configuration, where the pattern is revealed only by placing the smartphone in the proper position. Widespread applications are foreseen in a variety of fields, such as multifocal or monochromatic lenses, solar cells, biosensors, security devices and seethrough optical combiners for near-eye displays.
©2017 SPIE

Electromagnetic metasurfaces with strong nonlinear responses and angular selectivity could offer many new avenues for designing ultrathin optics components. We investigated the optical second harmonic generation from plasmonic metasurfaces composed of aligned gold nanopillars with pronounced out-of-plane tilt using a flexible nonlinear Fourier microscope. The experimental and computational results demonstrate that these samples function as wavevector-selective nonlinear metasurfaces, that is, the coherent second harmonic signal does not only depend on the polarization and wavelength of the excitation beam, but also of its direction of incidence, in spite of the subwavelength thickness of the active layer. Specifically, we observe that the nonlinear response can vary by almost two orders-of-magnitude when the incidence angle is changed from positive to negative values compared to the surface normal. Further, it is demonstrated that these metasurfaces act as a directional non-linear mirrors, paving the way for new design of directional meta-mirrors in the non-linear regime.
©2017 American Chemical Society

Controlling the phase of an electromagnetic field using plasmonic nanostructures provides a versatile way to manipulate light at the nanoscale. Broadband phase modulation has been demonstrated using inhomogeneous metasurfaces with different geometries; however, for many applications such as filtering, hyperspectral imaging and color holography, narrowband frequecy selectivity is a key functionality. In this work, we demonstrate, both theoretically and experimentally, a narrowband metasurface that relies on Fano resonances to control the propagation of light. By geometrically tuning the sub-radiant modes with respect to a fixed super-radiant resonance, we can create a phase modulation along the surface within a narrow spectral range. The resulting anomalous reflection measured for such a Fano-resonant metasurface exhibits a 100 nm bandwidth and a color routing efficiency of up to 81% at a central wavelength of λ=750 nm. The design flexibility provided by this Fano-assisted metasurface for color-selective light manipulation is further illustrated by demonstrating a highly directional color-routing effect between two channels, at λ=532 and 660 nm, without any crosstalk.
©2017 Nature Publishing Group

Silver (Ag) nanostructures and thin films are advantageous plasmonic materials as they have significantly lower losses than gold (Au). Unfortunately, Ag nanostructures suffer from poor chemical and thermal stability, which limit their applications. Here, the mechanisms leading to the deterioration of Ag nanostructures are clarified. It is first shown that oxygen alone cannot oxidize Ag nanostructures. Then, experiments using X-ray photoelectron spectroscopy reveal that the amount of sulfur in ambient air is too low for efficient tarnishing of the Ag surface. Finally, water is found to be the most critical factor for the degradation of Ag nanostructures and thin films. At high relative humidity, adsorbed water forms a thin film enabling the migration of Ag ions at the Ag/air interface, which deteriorates the Ag nanostructures. A dehydration treatment is developed which alters the morphology of the deposited silver, leading to an improved chemical and thermal stability of the Ag nanostructures and films, which then remain stable for more than 14 weeks under ambient laboratory conditions. In addition, dehydration also improves significantly the root-mean-square roughness for Ag thin films deposited on a glass substrate.
©2017 Wiley-VCH Verlag GmbH

Plasmonic antennas have enabled a wealth of applications that exploit tailored near-fields and radiative properties, further endowed by the bespoke interactions of multiple resonant building blocks. Specifically, when the interparticle distances are reduced to a few nanometers, coupling may be greatly enhanced leading to ultimate near-field intensities and confinement along with a large energy splitting of resonant modes. While this concept is well-known, the fabrication and characterization of suitable multimers with controlled geometries and few-nanometer gaps remains highly challenging. In this article, we present the topographically templated assembly of single-crystal colloidal gold nanorods into trimers, with a dolmen geometry. This fabrication method enables the precise positioning of high-quality nanorods, with gaps as small as 1.5 nm, which permits a gradual and controlled symmetry breaking by tuning the arrangement of these strongly coupled nanostructures. To characterize the fabricated structures, we perform electron energy loss spectroscopy (EELS) near-field hyperspectral imaging and geometrically accurate EELS, plane wave, and eigenmode full-wave computations to reveal the principles governing the electromagnetic response of such nanostructures that have been extensively studied under plane wave excitation for their Fano resonant properties. These experiments track the evolution of the multipolar interactions with high accuracy as the antenna geometry varies. Our results provide new insights in strongly coupled single-crystal building blocks and open news opportunities for the design and fabrication of plasmonic systems.
©2017 American Chemical Society

Multiresonant plasmonic nanoantennas have recently gained a lot of attention due to their ability to enhance nonlinear optical processes at the nanoscale. The first nanostructure designed for this purpose was an aluminum antenna composed of three arms, designed to be resonant at both the fundamental and the second harmonic frequencies. It was demonstrated that second harmonic generation induced by its resonances at both the fundamental and second harmonic wavelengths is higher than the one from a simple dipolar nanoantenna supporting a resonance at the fundamental wavelength only. However, the underlying mechanisms leading to this strong nonlinear signal are still unclear. In this study, both advanced simulations and experiments are combined to investigate in details the role of the mode coupling in the enhancement of second harmonic generation. By varying the length of the nanoantenna arms, it is clearly demonstrated that second harmonic generation is enhanced when the coupling between the quadrupole and the dipole modes at the second harmonic wavelength is significant. Indeed, using a numerical analysis based on the spatial selection of the second harmonic sources, it is shown that the second harmonic quadrupolar mode, which is directly excited by the fundamental pump, can resonantly transfer its energy to the second harmonic dipolar mode supported by another part of the nanoantenna due to near-field coupling. The study of the second harmonic generation mechanisms of double resonant plasmonic systems is important for the design of efficient second harmonic meta-devices such as coherent extreme-ultraviolet sources, ultrasensitive index, and chiral plasmonic sensors.
©2017 American Chemical Society

Plasmonic effects associated with metallic nanostructures have been widely studied for color generation. It became apparent that highly saturated and bright colors are hard to obtain, and very small nanostructures need to be fabricated. To address this issue, in this study, we employ metal−insulator−metal sandwich nanodisks that support enhanced in-phase electric dipole modes, which are blueshifted with respect to a single metal disk. The blue shift enables the generation of short wavelength colors with larger nanostructures. The radiation modes hybridize with the Wood's anomaly in periodic structures, creating narrow and high-resonance peaks in the reflection and deep valleys in the transmission spectra, thus producing vivid complementary colors in both cases. Full colors can be achieved by tuning the radius of the nanodisks and the periodicity of the arrays. Good agreement between simulations and experiments is demonstrated and analyzed in CIE1931, sRGB, and HSV color spaces. The presented method has potential for applications in imaging, data storage, ultrafine displays, and plasmon-based biosensors.
©2017 American Chemical Society

Plasmonic antennas and planar structures have been undergoing intensive developments in order to control the scattering and absorption of light. One specific class, extrinsic chiral surfaces, that does not possess 2-fold rotational symmetry exhibits strong asymmetric transmission for different circular polarizations under obliquely incident illumination. In this work, we show that the design of those surfaces can be optimized with complex multipolar resonances in order to twist the fluorescence emission from nearby molecules. While this emission is usually dipolar and linearly polarized, the interaction with these resonances twists it into a multipolar radiation pattern with opposite helicity in different directions. The proposed structure maximizes this effect and provides control over the polarization of light. Splitting of left- and right-handed circularly polarized light is experimentally obtained in the backward direction. These results highlight the intricate interplay between the near-field absorption and the far-field scattering of a plasmonic nanostructure and are further used for modifying the emission of incoherent quantum sources. Our finding can potentially lead to the development of polarization- and angle-resolved ultracompact optical devices.
©2017 American Chemical Society

While plasmonic antennas composed of building blocks made of the same material have been thoroughly studied, recent investigations have highlighted the unique opportunities enabled by making compositionally asymmetric plasmonic systems. So far, mainly heterostructures composed of nanospheres and nanodiscs have been investigated, revealing opportunities for the design of Fano resonant nanostructures, directional scattering, sensing and catalytic applications. In this article, an improved fabrication method is reported that enables precise tuning of the heterodimer geometry, with interparticle distances made down to a few nanometers between Au–Ag and Au–Al nanoparticles. A wide range of mode energy detuning and coupling conditions are observed by near field hyperspectral imaging performed with electron energy loss spectroscopy, supported by full wave analysis numerical simulations. These results provide direct insights into the mode hybridization of plasmonic heterodimers, pointing out the influence of each dimer constituent in the overall electromagnetic response. By relating the coupling of nondipolar modes and plasmon–interband interaction with the dimer geometry, this work facilitates the development of plasmonic heterostructures with tailored responses, beyond the possibilities offered by homodimers.
©2017 American Chemical Society

We introduce a general formalism combining the coupled oscillator model with the transfer matrix method to analyze and engineer the phase of the light reflected from a Fano-resonant metasurface. This method accounts for periodicity and the presence of substrates, and we demonstrate that these factors can be used to tune the reflected phase at will. Utilizing these effects and adjusting the coupling strength of the underlying unit cell, we achieve zero reflection at the dark resonance of the metasurface. We show that the resulting phase singularity can dramatically increase the sensitivity of phase-based detection schemes. The phase bifurcation unveiled in this work can be used to design plasmonic metasurfaces that explore the unusual phase behavior of light.
©2017 American Chemical Society

Engineered nanomaterials (ENMs) are key drivers for the development of highly sophisticated new technologies. As all new attainments, the rapidly increasing used of ENMs raise concerns about their safety for the environment and humans. There is growing evidence showing that if engineered nanomaterials are released into the environment, there is a possibility that they could cause harm to aquatic microorganisms. Among the divers effects triggering their toxicity the ability of ENMs to generate reactive oxygen species (ROS) capable of oxidizing biomolecules is currently considered a central mechanism of toxicity. Therefore, development of sensitive tools for quantification of the ROS generation and oxidative stress are highly sought. After briefly introducing ENMs-induced ROS generation and oxidative stress in the aquatic microorganisms (AMOs), this overview paper focuses on a new optical biosensor allowing sensitive and dynamic measurements of H2O2 in real-time using multiscattering enhanced absorption spectroscopy. Its principle is based on sensitive absorption measurements of the heme protein cytochrome c whose absorption spectrum alters with the oxidation state of constituent ferrous FeII and ferric FeIII. For biological applications cytochrome c was embedded in porous random media resulting in an extended optical path length through multiple scattering of light, which lowers the limit of detection to a few nM of H2O2. The sensor was also integrated in a microfluidic system containing micro-valves and sieves enabling more complex experimental conditions. To demonstrate its performance, abiotic absorption measurements of low concentrations of dye molecules and 10 nm gold particles were carried out achieving limits of detection in the low nM range. Other biologically relevant reactive oxygen species can be measured at sub-μM concentrations, which was shown for glucose and lactate through enzymatic reactions producing H2O2. In ecotoxicological investigations H2O2 excreted by aquatic microorganisms exposed to various stressors were measured. Pro-oxidant effects of nano-TiO2 and nano-CuO towards green alga Chlamydomonas reinhardtii were explored in various exposure media and under different light illuminations. Dynamics of Cd2+ induced effects on photosynthetic activity, sensitisation and recovery of cells of C. reinhardtii was also studied.
©2017 The Authors

The influence of Fano resonances on the nonlinear response of hybrid plasmonic nanostructures, i.e., nanoantennas loaded with a nonlinear optical material, is theoretically investigated using the combination of a surface integral equation method and an analytical model. The results demonstrate that a suitable design of the field enhancement enables the observation of optical bistability for incident conditions that would be impossible for a bare nanoantenna. This study provides new insights into the possibilities offered by Fano resonances to control the nonlinear response of hybrid plasmonic systems.
©2017 SPIE

The cyclic RGD (cRGD) peptide ligands of cells have become widely used for treating several cancers. We report a highly sensitive analysis of c(RGDfC) using surface enhanced Raman spectroscopy (SERS) using single dimer nanogap antennas in aqueous environment. Good agreement between characteristic peaks of the SERS and the Raman spectra of bulk c(RGDfC) with its peptide's constituents were observed. The exhibited blinking of the SERS spectra and synchronization of intensity fluctuations, suggest that the SERS spectra acquired from single dimer nanogap antennas was dominated by the spectrum of single to a few molecules. SERS spectra of c(RGDfC) could be used to detect at the nanoscale, the cells' transmembrane proteins binding to its ligand.
©2016 Wiley

Resonant waveguide gratings (RWGs) are subwavelength structures of great interest for biosensors, optical filters and optical security applications. We demonstrate and characterize a beam steering device, where the in-coupling and out-coupling processes make use of different RWGs that share the same ultrathin dielectric waveguide. This device enables selective color-filtering and redirection of a white light source (such as a white LED). Furthermore, this structure is compatible with up-scalable fabrication processes such as roll-to-roll replication, and is relevant for high-volume production. Because of its color selectivity and its use in low coherence illumination conditions, such a beam steering device could be implemented in a variety of optical applications such as optical security, multifocal or monochromatic lenses, biosensors, and see-through optical combiners for near-eye displays.
©2017 American Chemical Society

Electron energy loss spectroscopy is a method of choice for the characterization of both the spatial and spectral properties of localized surface plasmon resonances. The energy lost by the impinging electrons is commonly explained by the Lorentz force acting on their motion. Here, we adopt another point of view to compute the electron energy loss spectra. Coupling the energy conservation law with full-wave electromagnetic computations based on a surface integral equation method, we derive the electron energy loss spectra directly from two dissipative processes, namely, absorption and scattering. This antenna-based approach is applied to nanostructures with different sizes and materials, showing an excellent agreement with experimental observation and computations based on the evaluation of the Lorentz force. This formalism permits the easy separation of absorption losses in the nanostructures forming a coupled system and reveals the subtle interplay between absorption and scattering, which are controlled by the materials, the nanostructure size, and the energy range.
©2017 American Chemical Society

Predetermined and selective placement of nanoparticles onto large-area substrates with nanometre-scale precision is essential to harness the unique properties of nanoparticle assemblies, in particular for functional optical and electrooptical nanodevices. Unfortunately, such high spatial organization is currently beyond the reach of top-down nanofabrication techniques alone. Here, we demonstrate that topographic features comprising lithographed funnelled traps and auxiliary sidewalls on a solid substrate can deterministically direct the capillary assembly of Au nanorods to attain simultaneous control of position, orientation and interparticle distance at the nanometre level. We report up to 100% assembly yield over centimetre-scale substrates. We achieve this by optimizing the three sequential stages of capillary nanoparticle assembly: insertion of nanorods into the traps, resilience against the receding suspension front and drying of the residual solvent. Finally, using electron energy-loss spectroscopy we characterize the spectral response and near-field properties of spatially programmable Au nanorod dimers, highlighting the opportunities for precise tunability of the plasmonic modes in larger assemblies.
©2017 Macmillan Publishers Limited

This study sheds light on the short-term dynamics of pro-oxidant processes related to the exposure of C. reinhardtii microalgae to nano-TiO2 using (a) conventional fluorescence probes for cellular pro-oxidant process and (b) a recently developed cytochrome c biosensor for the continuous quantification of extracellular H2O2. The main aims are to investigate nano-TiO2 toxicity and the modifying factors thereof based on the paradigm of oxidative stress and to explore the utility of extracellular H2O2 as a potential biomarker of the observed cellular responses. This is the first study to provide continuous quantitative data on abiotic and biotic nano-TiO2-driven H2O2 generation to systematically investigate the link between extracellular and cellular pro-oxidant responses. Acute exposures of 1 h were performed in two different exposure media (MOPS and lake water), with nominal particle concentrations from 10 mg/L to 200 mg/L, with and without UV pre-illumination. Abiotic and biotic extracellular H2O2 were continuously measured with the biosensor and complemented with endpoints for abiotic ROS (H2DCFDA), oxidative stress (CellROX® Green) and damage (propidium iodide) measured by flow cytometry at the beginning and end of exposure. Results showed that nano-TiO2 suspensions generated ROS under UV light (abiotic origin) and promoted ROS accumulation in C. reinhardtii (biotic origin). However, extracellular and intracellular pro-oxidant processes differed. Hence, extracellular H2O2 cannot per se serve as a predictor of cellular oxidative stress or damage. The main predictors best describing the cellular responses included “exposure medium”, “exposure time”, “UV treatment” as well as “exposure concentration”.
©2016 The Royal Society of Chemistry

Two parallel optical surfaces often exhibit colorful fringes along the lines of equal thickness because of the interference of light. This simple phenomenon allows one to observe subwavelength corrugations on a reflective surface by simply placing on it a flat reference dielectric surface, a so-called optical flat, and inspecting the resultant interference pattern. In this work, we extend this principle from dielectric surfaces to two-dimensional plasmonic nanostructures. Optical couplings between an Au nanodisk array and an Au thin film were measured quantitatively using two different techniques, namely, the classical Newton�s rings method and a closed-loop nano-positioning system. Extremely high spectral sensitivity to the inter-surface distance was observed in the near-field coupling regime, where a 1-nm change in distance could alter the resonance wavelength by almost 10 nm, 440 times greater than the variation in the case without near-field coupling. With the help of a numerical fitting technique, the resonance wavelength could be determined with a precision of 0.03 nm, corresponding to a distance precision as high as 0.003 nm. Utilizing this effect, we demonstrated that a plasmonic nanodisk array can be utilized as a plasmonic optical flat, with which nanometer-deep grooves can be directly visualized using a low-cost microscope.
©

During the last decade, important attention has been devoted to the observation of nonlinear optical processes in plasmonic nanosystems, giving rise to a new field of research called nonlinear plasmonics. The cornerstone of nonlinear plasmonics is the use of the large field enhancement associated with the excitation of localized surface plasmon resonances to reach high nonlinear conversion yields. Among all the nonlinear optical processes, second harmonic generation (SHG), the process whereby two photons at the fundamental frequency are converted into one photon at the second harmonic frequency, is undoubtedly the most studied one due to the relative simplicity of its experimental observation. However, the physical origin of SHG from plasmonic nanostructures hides a lot of subtleties, which are mainly related to its particular behavior upon inversion symmetry. In order to catch all the peculiarities of SHG, it is mandatory to develop dedicated numerical methods able to accurately describe all the underlying physical processes and the influence of the initial assumptions needs to be well-characterized. In this presentation, we discuss and compare different methods (namely full-wave computations based on the surface integral equations method, mode analysis, the Miller’s rule, and the effective nonlinear susceptibility method) proposed for the evaluation of the SHG from plasmonic nanoparticles emphasizing their limitations and advantages. In particular, the design of double resonant antennas for efficient nonlinear conversion at the nanoscale is addressed in detail.
©2016 SPIE

Electron energy-loss spectroscopy (EELS) has become an experimental method of choice for the investigation of localized surface plasmon resonances, allowing the simultaneous mapping of the associated field distributions and their resonant energies with a nanoscale spatial resolution. The experimental observations have been well-supported by numerical models based on the computation of the Lorentz force acting on the impinging electrons by the scattered field. However, in this framework, the influence of the intrinsic properties of the plasmonic nanostructures studied with the electron energy-loss (EEL) measurements is somehow hidden in the global response. To overcome this limitation, we propose to go beyond this standard, and well-established, electron perspective and instead to interpret the EELS data using directly the intrinsic properties of the nanostructures, without regard to the force acting on the electron. The proposed method is particularly well-suited for the description of coupled plasmonic systems, because the role played by each individual nanoparticle in the observed EEL spectrum can be clearly disentangled, enabling a more subtle understanding of the underlying physical processes. As examples, we consider different plasmonic geometries in order to emphasize the benefits of this new conceptual approach for interpreting experimental EELS data. In particular, we use it to describe results from samples made by traditional thin film patterning and by arranging colloidal nanostructures.
©2016 SPIE

Efficient optical energy transfer is key to many technologies, ranging from biosensing to photovoltaics. Here, for the first time we show that by introducing a random medium with appropriate filling factor, absorption in a specific volume can be maximized. Using both numerical simulations and an analytical diffusion model, we identify design rules to maximize absorption in the system with different geometrical and scattering properties. By combining a random medium with an open photonic cavity, we numerically demonstrate a 23-fold enhancement of the absorbed energy. We also show how absorption as high as 99% can be reached in a device as thin as 500 μm for normal incidence illumination. Finally, our data indicate that introducing a non-absorbing random medium into a light trapping system for thin solar cells can enhance absorption of energy by a factor of 2.2. This absorption enhancement, caused by the random medium, is broadband and wide-angle and can help design efficient solar cells, light trapping devices, biosensors and random lasers.
©© 2016 Optical Society of America

Polarisation charge formed on nanostructure surfaces upon optical excitation provides a useful tool to understand the underlying physics of plasmonic systems. Plasmonic simulations in the frequency domain typically calculate the polarisation charge as a complex quantity. In this paper, we provide a pedagogical treatment of the complex nature of the polarisation charge and its relevance in plasmonics, and discuss how naively extracting the real part of the complex quantities to obtain physical information can lead to pitfalls. We analyse the charge distributions on various plasmonic systems and explain how to understand and visualise them clearly using techniques such as phase-correction and polarisation ellipse representation, to extract the underlying physical information.
©2016 IOP Publishing

We numerically investigate the second harmonic generation from different plasmonic systems and evidence the key role played in their nonlinear response by the phase at the fundamental wavelength. In the case of a single plasmonic nanorod, the interference between the second harmonic dipolar and quadrupolar emission modes depends on their relative phase, which is deeply related to the excitation wavelength. The knowledge obtained in this simple case is then used to describe and understand the nonlinear response from a more complex structure, namely a gold nanodolmen. The complex phase evolution associated with a Fano resonance arising at the fundamental wavelength enables dramatically modifying the second harmonic emission patterns from plasmonic metamolecules within minute wavelength shifts. These results emphasize the importance of the phase in the nonlinear optical processes arising in plasmonic nanostructures, in addition to the increase in conversion yield associated with the excitation of localized surface plasmon resonances.
©© 2016 Optical Society of America

In this article we compare the two-photon photoluminescence and second harmonic generation from single connected gold nanodimers. Analyzing the particle size-dependent nonlinear optical spectra and performing excitation polarization resolved measurements using an experimental setup combining a femtosecond laser source with a parabolic mirror, we show that second harmonic generation and two-photon photoluminescence have different behaviors despite the same expected fundamental intensity-dependence. For further understanding of the observed phenomena, the plasmon resonances of single nanodimers are investigated using dark-field optical microscopy, and calculations are performed with Green's tensor method. Furthermore, the underlying mechanisms explaining the differences between these two optical processes are investigated using a surface integral equation method for the nonlinear computations. This study reveals that the different trends in the polarization-dependences of two-photon photoluminescence and second harmonic generation with the increasing diameters of the connected discs are due to their distinct physical nature, resulting in specific rules for plasmon enhancement and different coherence properties. Furthermore, this article clearly points out that special care has to be taken when two-photon photoluminescence and second harmonic generation are used to evaluate the amplitudes of electromagnetic hot-spots generated in plasmonic nanostructures.
©© 2016 American Chemical Society

We computationally explore how the orientation of dipolar emitters placed near plasmonic nanostructures affects their radiative enhancement and spontaneous emission rate. We demonstrate that the expressions for these quantities show a subtle dependence on the molecular orientation, and this information is lost when typical calculations assume a random orientation and perform an average over all directions. This orientation dependence is strongly affected by the location of the emitter, the emission wavelength, and the symmetry of the system. While the plasmonic nanostructure can significantly modify the far-field from a molecule in its vicinity, this modification is heavily dependent on both the wavelength and the orientation of the emitter. We show that if a fluorescent molecule can be constrained to emit in a specific direction, we are able to obtain far superior control over its spontaneous emission and decay rate than otherwise and discuss implications for single molecule experiments.
©2016 American Chemical Society

The optimization of nonlinear optical processes in plasmonic structures is important for the design of efficient nonlinear nanosources of light. Considering the simple case of spherical nanoparticles, we clearly identify the most efficient channel for second-harmonic generation, thanks to physical insights provided by the generalized Mie theory. This channel corresponds to the excitation of electric dipolar modes at the fundamental wavelength and a quadrupolar second-harmonic emission. Interestingly, it is demonstrated that the second-harmonic generation intensity is directly related to the square of the absorbed power, which reproduces both the electric field enhancement and the specific size dependence of second-harmonic generation in the small-particle limit. Additionally, the absorbed power can be optimized by controlling the nanoparticle size. These results demonstrate that the optimization of the fundamental electric field is not sufficient for reaching the highest nonlinear conversion in plasmonic systems. The approach reported in this article proposes a new paradigm for the design of nonlinear plasmonic nanostructures, establishing new rules for the conception of efficient nonlinear plasmonic metamolecules on the basis of their linear response.
©2016 American Chemical Society

Using a surface integral equation approach based on the tangential Poggio–Miller–Chang–Harrington–Wu–Tsai formulation, we present a full wave analysis of the resonant modes of 3D plasmonic nanostructures. This method, combined with the evaluation of second-harmonic generation, is then used to obtain a better understanding of their nonlinear response. The second-harmonic generation associated with the fundamental dipolar modes of three distinct nanostructures (gold nanosphere, nanorod, and coupled nanoparticles) is computed in the same formalism and compared with the other computed modes, revealing the physical nature of the second-harmonic modes. The proposed approach provides a direct relationship between the fundamental and second-harmonic modes in complex plasmonic systems and paves the way for an optimal design of double resonant nanostructures with efficient nonlinear conversion. In particular, we show that the efficiency of second-harmonic generation can be dramatically increased when the modes with the appropriate symmetry are matched with the second-harmonic frequency.
©2016 Optical Society of America

Reactive oxygen species (ROS) play an important role in the life of every cell, including cellular defense and signaling mechanisms. Continuous and quantitative ROS sensing can provide valuable information about the cell state, but it remains a challenge to measure. Here, we introduce a multi-layered microfluidic chip with an integrated optical sensor for the continuous sensitive detection of extracellular hydrogen peroxide (H2O2), one of the most stable ROS. This platform includes hydraulically controlled microvalves and microsieves, which enable the precise control of toxicants and complex exposure sequences. In particular, we use this platform to study the dynamics of toxicity-induced ROS generation in the green microalga Chlamydomonas reinhardtii during short-term exposures, recovery periods, and subsequent re-exposures. Two cadmium-based toxicants with distinct internalization mechanisms are used as stress inducers: CdSe/ZnS quantum dots (Qdots) and ionic cadmium (Cd2þ). Our results show the quantitative dynamics of ROS generation by the model microalga, the recovery of cell homeostasis after stress events and the cumulative nature of two consecutive exposures. The dissolution of quantum dots and its possible influence on toxicity and H2O2 depletion is discussed. The obtained insights are relevant from ecotoxicological and physiological perspectives.
©2016 Taylor & Francis

A novel recipe for high yield and high quality nanofabrication of silver and aluminum nanostructures with features down to 10 nm is demonstrated. The degradation of silver oxide to activate the surface forms the basis of the idea. X-ray photoelectron spectroscopy and ellipsometry are used for chemical and optical characterization of the deposited thin films.
©2016 Wiley

In this article, the second-harmonic generation (SHG) from gold split-ring resonators is investigated using different theoretical methods, namely, Miller’s rule, the nonlinear effective susceptibility method, and full-wave computation based on a surface integral equation method. The results confirm that Miller’s rule is, in general, not well suited for the description of SHG from plasmonic metasurfaces. On the other hand, the comparison of the nonlinear effective susceptibility method with full-wave computations shows that this method permits us to evaluate second-harmonic (SH) emission patterns from noncentrosymmetric nanoparticles with good accuracy. However, the nonlinear effective susceptibility method fails to reproduce the multipolar nature of the SH emission from centrosymmetric nanoparticles. This shortcoming is attributed to the intrinsic nature of the nonlinear effective susceptibility method, which neglects the exact positions of the nonlinear sources. The numerical implementations of these two methods are also discussed in detail, revealing that the main limitation of the nonlinear effective susceptibility method, aside from the inaccuracy observed in specific cases, is its higher numerical requirements when several emitting directions need to be considered. This limitation stands for most of the numerical methods used for solving Maxwell’s equations at the nanoscale. This work provides clear insight into the limitations and advantages of the different methods available for evaluation of SHG from plasmonic metasurfaces.
©Optical Society of America

Nanoscale photonic systems involve a broad variety of light–matter interaction regimes beyond the diffraction limit and have opened the path for a variety of application opportunities in sensing, solid-state lighting, light harvesting, and optical signal processing. The need for numerical modeling is central for the understanding, control, and design of plasmonic and photonic nanostructures. Recently, the increasing sophistication of nanophotonic systems and processes, ranging from simple plasmonic nanostructures to multiscale and complex photonic devices, has been calling for highly efficient numerical simulation tools. This article reviews the state of the art in numerical methods for nanophotonics and describes which method is the best suited for specific problems. The widespread approaches derived from classical electrodynamics such as finite differences in time domain, finite elements, surface integral, volume integral, and hybrid methods are reviewed and illustrated by application examples. Their potential for efficient simulation of nanophotonic systems, such as those involving light propagation, localization, scattering, or multiphysical systems is assessed. The numerical modeling of complex systems including nonlinearity, nonlocal and quantum effects as well as new materials such as graphene is discussed in the perspective of actual and future challenges for computational nanophotonics.
©2015 Wiley-VCH

Plasmonics has emerged as an important research field in nanoscience and nanotechnology. Recently, significant attention has been devoted to the observation and the understanding of nonlinear optical processes in plasmonic nanostructures, giving rise to the new research field called nonlinear plasmonics. This review provides a comprehensive insight into the physical mechanisms of one of these nonlinear optical processes, namely, second harmonic generation (SHG), with an emphasis on the main differences with the linear response of plasmonic nanostructures. The main applications, ranging from the nonlinear optical characterization of nanostructure shapes to the optimization of laser beams at the nanoscale, are summarized and discussed. Future directions and developments, made possible by the unique combination of SHG surface sensitivity and field enhancements associated with surface plasmon resonances, are also addressed.
©2015 American Chemical Society

An up-scalable approach for creating Fano-resonant nanostructures on large surfaces at visible wavelengths is demonstrated. The use of processes suitable for high throughput fabrication and the choice of aluminum as a cost-efficient plasmonic material ensure that the presented insights are valuable even in consideration of typical industrial constraints. In particular, wafer-scale fabrication and the process compatibility with roll-to-roll embossing are demonstrated. It is shown that through adjustment of readily accessible evaporation parameters, the shape and position of the optical resonance can be tuned within a spectral band of more than 70 nm. The experimental data are complemented with rigorous coupled wave analysis and surface integral equation simulations. Calculated electric fields as well as surface charges shed light onto the physics behind the present resonances. In particular, a surface plasmon polariton is found to couple to a localized plasmonic mode with a hexapolar charge distribution, leading to a Fanolike resonance. Further understanding of the interactions at hand is gained by considering both aluminum and gold nanostructures.
©2015 Royal Society of Chemistry

A new concept for generating visual effects with metallized gratings is introduced. Shadow evaporation of aluminum onto dielectric gratings is shown to produce strongly asymmetric color effects. Zero-order effects are combined with ±first-order transmission in the present structures to generate not only polarization but also orientation-dependent colors. We show that a wide palette of colors can be obtained by simply scaling the parameters of the dielectric base grating. The present approach therefore enables the fabrication of entire asymmetric images in a two-step process. Additionally, a floating screen film is created by placing the grating at an increased distance to the light source. The present structures could find applications as security or decorative elements and are well suited for mass production using high-throughput techniques such as roll-to-roll fabrication.
©2015 Wiley

We present a computational study of the internal optical forces arising in plasmonic gap antennas, dolmen structures and split rings. We find that very strong internal forces perpendicular to the propagation direction appear in these systems. These internal forces show a rich behaviour with varying wavelength, incident polarisation and geometrical parameters, which we explain in terms of the polarisation charges induced on the structures. Various interesting and anomalous features arise such as lateral force reversal, optical pulling force, and circular polarisation-induced forces and torques along directions symmetry-forbidden for orthogonal linear polarisations. Understanding these effects and mastering internal forces in plasmonic nanostructures will be instrumental in implementing new functionalities in these nanophotonic systems.
©© 2015 Optical Society of America

Determination of homocysteine (HcySH) is highly beneficial in human physiology and pathophysiology for diagnosis and prognosis of cardiovascular diseases (CVD). Unfortunately, the practicability of the existing methodologies for the determination of HcySH is limited in terms of sample requirements, preparation time and instrumentation cost. To overcome these limitations, we have developed a new miniaturized electrochemical assay for HcySH in which cytochrome c (cyt c) immobilized on gold nanoparticle (GNP) modified screen printed carbon electrode (SPE) is employed as a biosensing element. The electrochemical characterization of the biosensor (cyt c-GNP-SPE) shows quasi-reversible redox peaks at the potentials 0 and -0.2 V, confirming the cyt c binding. The methodology of quantification is based on the electrochemical oxidation of HcySH by the Fe3+/ Fe2+ crevice of cyt c, observed at a potential of + 0.56 V. Using the amperometric technique, the detection limit of HcySH is found to be 0.3 +/- 0.025 mu M in the linear range between 0.4 mu M and 700 mu M, with a sensitivity of 3.8 +/- 0.12 nA mu M-1 cm(-2). The practical application of the present assay is validated through the measurement of HcySH in blood plasma samples and the selectivity is ensured by eliminating the impact of the common interfering biological substrates using a Nafion membrane. This biosensor shows striking analytical properties of good repeatability, reproducibility (2.85% SD) and high stability (83% of its initial current response after 4 weeks). This work paves the way for cheap, efficient and reliable point-of-care biosensors for screening one of the major causes of deaths both in the developed and developing countries.
©ROYAL SOC CHEMISTRY

Using full-wafer processing, we demonstrate a sophisticated nanotechnology for the realization of an ultrahigh sensitive cavity-coupled plasmonic device that combines the advantages of Fabry-Perot microcavities with those of metallic nanostructures. Coupling the plasmonic nanostructures to a Fabry-Perot microcavity creates compound modes, which have the characteristics of both Fabry-Perot and localized surface plasmon resonance (LSPR) modes, boosting the sensitivity and figure-of-merit of the structure. The significant trait of the proposed device is that the sample to be measured is located in the substrate region and is probed by the compound modes. It is demonstrated that the sensitivity of the compound modes is much higher than that of LSPR of plasmonic nanostructures or the pure Fabry-Perot modes of the optical microcavity. The response of the device is also investigated numerically and the agreement between measurements and calculations is excellent. The key features of the device introduced in this work are applicable for the realization of ultrahigh sensitive plasmonic devices for biosensing, optoelectronics, and related technologies
©ACS

The plasmon enhancement of molecular hyper-Raman scattering, the nonlinear counterpart of Raman scattering, which involves the absorption of two fundamental photons, is investigated with emphasis on low energy molecular vibrations. The two-photon excitation of the molecule is treated using its hyperpolarizability β, and the emission of the hyper-Raman photons is computed using a dipole emitter located at the molecule position. The electromagnetic response of the plasmonic systems is evaluated using a surface integral equation method, which makes possible considering both planewave and dipole excitations in a single formalism. Taking into account different geometries (including multiresonant antennas and silver heptamers supporting Fano resonances), the experimental parameters influencing the enhancement of the molecular hyper-Raman scattering are discussed in detail. In particular, it is shown that a good excitation at the fundamental stage is not sufficient for reaching a good enhancement factor and that an optimization of the electromagnetic response of the plasmonic substrate is also important at the emission wavelength. The competition between the molecular hyper-Raman scattering signal and the background signal, that is, the second harmonic generation, is discussed. The latter can be reduced in specific structures by taking advantages of the key role played by the symmetry of the structure for hyper-Raman scattering and second harmonic generation. This way, we propose a nanostructure where the second harmonic generation can be reduced in the detection direction, enabling the hyper-Raman scattering signal from single donor–acceptor “push–pull” chromophores to be experimentally recorded with a low noise level using surface-enhanced hyper-Raman scattering. It is particularly remarkable that the hyper-Raman signal from one molecule can be stronger than the second harmonic generation from a complete plasmonic nanostructure, despite the considerable volume difference between both nano-objects. This fundamental observation stems from the different selection rules for both nonlinear optical processes.
©2015 American Physical Society

The continuous measurement of uptake or release of biomarkers provides invaluable information for understanding and monitoring the metabolism of cells. In this work, a multiscattering-enhanced optical biosensor for the multiplexed, non-invasive, and continuous detection of hydrogen peroxide (H2O2), lactate and glucose is presented. The sensing scheme is based on optical monitoring of the oxidation state of the metalloprotein cytochrome c (cyt c). The analyte of interest is enzymatically converted into H2O2 leading to an oxidation of the cyt c. Contact microspotting is used to prepare nanoliter-sized sensing spots containing either pure cyt c, a mixture of cyt c with glucose oxidase (GOx) to detect glucose, or a mixture of cyt c with lactate oxidase (LOx) to detect lactate. The sensing spots are embedded in a multiscattering porous medium that enhances the optical signal. We achieve limits of detection down to 240 nM and 110 nM for lactate and glucose, respectively. A microfluidic embodiment enables multiplexed and crosstalk-free experiments on living organisms. As an example, we study the uptake of exogenously supplied glucose by the green algae Chlamydomonas reinhardtii and simultaneously monitor the stress-related generation of H2O2. This multifunctional detection scheme provides a powerful tool to study biochemical processes at cellular level.
©2015 Optical Society of America

In this article, we share our vision for a future nanofactory, where plasmonic trapping is used to control the different manufacturing steps associated with the transformation of initial nanostructures to produce complex compounds. All the different functions existing in a traditional factory can be translated at the nanoscale using the optical forces produced by plasmonic nanostructures. A detailed knowledge of optical forces in plasmonic nanostructures is however essential to design such a nanofactory. To this end, we review the numerical techniques for computing optical forces on nanostructures immersed in a strong optical field and show under which conditions approximate solutions, like the dipole approximation, can be used in a satisfactory manner. Internal optical forces on realistic plasmonic antennas are investigated and the reconfiguration of a Fano-resonant plasmonic system using such internal forces is also studied in detail.
©The Royal Society of Chemistry 2015

We study the convergence of the integrals required to be evaluated in the surface integral equation (SIE) formulation (or method of moments) for simulating the optical response of plasmonic nanostructures. We analyze how the numerical quadratures used to compute the integrals affect the accuracy of the SIE matrix elements and, in turn, that of the relevant physical quantities calculated using the method. Based on these studies, we propose an optimized algorithm for evaluation of the integrals, which improves the accuracy of the results without significantly increasing the calculation overhead
©2015 Optical Society of America

An original scheme for sensitive absorption measurements, particularly well-suited for low analyte concentrations, is presented. The technique is based on multiscattering-enhanced absorption spectroscopy (MEAS) and benefits from the advantages of conventional absorption spectroscopy: simplicity, rapidity, and low costs. The technique relies on extending the optical path through the sensing volume by suspending dielectric beads in the solution containing the analytes of interest, resulting in multiple scattering of light, which increases the optical path length through the sample. This way, a higher sensitivity and lower limit of detection, compared to those of conventional absorption spectroscopy, can be achieved. The approach is versatile and can be used for a broad variety of analytes. Here, it is applied to the detection of phenol red, 10 nm gold nanoparticles, and envy green fluorescence dye; the limit of detection is decreased by a factor of 7.2 for phenol red and a factor of 3.3 for nanoparticles and dye. The versatility of this approach is illustrated by its application in increasing the sensitivity of colorimetric detection with gold nanoparticle probes and a commercially available hydrogen peroxide bioassay. The influence of different parameters describing the scattering medium is investigated in detail experimentally and numerically, with very good agreement between the two. Those parameters can be effectively used to tailor the enhancement for specific applications and analytes.
©2014 American Chemical Society

Assemblies of plasmonic nanoparticles possess exotic properties that are used in numerous applications. Their efficiency for specific applications strongly depends on the modes supported by the structure. In this paper, we extend the Green’s tensor formalism to compute the eigenmodes of an assembly of plasmonic nanoparticles. Using the developed technique, we investigate the specific cases of a nanoparticle monomer, dimer, and trimer. The influence of various geometrical parameters and of symmetry breaking on the eigenmodes of the assemblies is studied in detail, as well as the illumination conditions required to excite specific eigenmodes
©2015 Optical Society of America

The influence of a reflecting surface on the optical bistability in a nanoantenna array is investigated theoretically. The optical response of the array is modeled using a surface integral equation method developed for periodic structures, and the description of the Kerr effect is based on an analytical model. Different behaviors are observed when the distance between the nanoantenna array and the silver layer is changed. Indeed, a modification of the nanoantennas radiative properties permit to control important parameters of the nonlinear response such as the intensity threshold and the area of the hysteresis cycle. The results presented in this article demonstrate that a reflecting surface is a convenient and flexible tool for controlling the operating of nonlinear optical systems based on the Kerr effect.
©Springer Science+Business Media New York 2014

Reactive oxygen species (ROS) generated by aerobic organisms are essential for physiological processes such as cell signaling, apoptosis, immune defense and oxidative stress mechanisms. Unbalanced oxidant/antioxidant budgets are involved in many diseases and, therefore, the sensitive measurement of ROS is of great interest. Here, we present a new device for the real-time monitoring of oxidative stress by measuring one of the most stable ROS, namely hydrogen peroxide (H2O2). This portable oxidative stress sensor contains the heme protein cytochrome c (cyt c) as sensing element whose spectral response enables the detection of H2O2 down to a detection limit of 40 nM. This low detection limit is achieved by introducing cyt c in a random medium, enabling multiscattering that enhances the optical trajectory through the cyt c spot. A contact microspotting technique is used to produce reproducible and reusable cyt c spots which are stable for several days. Experiments in static and microfluidic regimes, as well as numerical simulations demonstrate the suitability of the cyt c/H2O2 reaction system for the real-time sensing of the kinetics of biological processes without H2O2 depletion in the measurement chamber. As an example, we detect the release of H2O2 from the green alga Chlamydomonas reinhardtii exposed to either 180 nM functionalized CdSe/ZnS core shell quantum dots, or to 10 mg/l TiO2 nanoparticles. The continuous measurement of extracellular H2O2 by this optical sensor with high sensitivity is a promising new means for real-time cytotoxicity tests, the investigation of oxidative stress and other physiological cell processes.
©2015 Elsevier B.V.

We drive an explict expression for the resonance frequency shift for a subwavelength plasmonic nanocavity upon the adsorption or trapping of a single nanoparticle using rigorous perturbation theory. It reveals a simple linear dependence of the resonance frequency shift on the product of the local field intensity of a resonance mode, the material dispersion factor d omega(1)/d epsilon of the nanocavity, and the polarizability of the nanoparticle. To verify this linear relation, we numerically simulate the nanoparticle-induced resonance shifts for subwavelength ellipsoids, rods, rod pairs, and split rings with different sizes and materials, and a very good agreement is found between the theory and the numerically results. Moreover, we discuss this approach from the energy perspective and find that the linear relation can be understood in the context of optical trapping. This work not only reveals the underlining physics of near-field couplings in plasmonic nanocavities but also provides theoretical guidelines for the design of ultrasensitive nanosensors.
©2014 American Chemical Society

The coupling between metallic nanostructures is a common and easy way to control the optical properties of plasmonic systems. Even though the coupling between plasmonic oscillators has been widely studied in the linear regime, its influence on the nonlinear optical response of metallic nanostructures has been sparsely considered. Using a surface integral equation method, we investigate the second order nonlinear optical response of plasmonic metamolecules supporting Fano resonances revealing that the typical lineshape of Fano resonances is also clearly observable in the nonlinear regime. The physical mechanisms leading to nonlinear Fano resonances are revealed by the coupled oscillator model and the symmetry subgroup decomposition. It is found that the origin of the nonlinear scattered wave, i. e. the active plasmonic oscillator, can be selectively chosen. Furthermore, interferences between nonlinear emissions are clearly observed in specific configurations. The results presented in this article pave the way for the design of efficient nonlinear plasmonic metamolecules with controlled nonlinear radiation.
©2014 Optical Society of America

An innovative class of coupling gratings for efficiently redirecting light beams is described. They are based on symmetric binary gratings equipped with an asymmetric high refractive index coating. This kind of structure exhibits a grating blaze effect and produces high diffraction efficiencies. Coupling gratings based on the described principle have been fabricated by UV casting and subsequent oblique ZnS evaporation. The gratings were characterized by measuring their diffraction efficiencies. For unpolarized light, efficiencies of around 70% have been measured at 510 nm wavelength. Simulations revealed that diffraction efficiencies of up to 90% can be obtained for polarized light.
©2014 Optical Society of America

We investigate experimentally and theoretically the role of periodicity on the optical response of dolmen plasmonic arrays that exhibit a Fano line shape. Contrary to previous works on single nanostructures, this study deals with the in-plane near-field coupling between adjacent unit cells. By making an analogy to the electronic properties of atoms in the tight-binding model, specific behaviors of photonic states are investigated numerically as a function of the structural asymmetry for different coupling directions. These predictions are verified experimentally with dark-field measurements on nanostructure arrays which exhibit high tunability and fine control of their spectral features as a function of the lattice constants. These effects, originated from symmetry-breaking and selective excitation of the subradiant mode, provide additional degree of freedom for tuning the spectral response and can be used for the sensitive detection of local perturbations. This study provides a general understanding of the near-field interactions in Fano resonant lattices that can be used for the design of plasmonic nanostructures and planar metamaterials.
©2014 American Chemical Society

Sensing using surface plasmon resonances is one of the most promising practical applications of plasmonic nanostructures and Fano resonances allow achieving a lower detection limit thanks to their narrow spectral features. However, a narrow spectral width of the subradiant mode in a plasmonic system, as observed in the weak coupling regime, is in general associated with a low modulation of the complete spectral response. In this article, we show that this limitation can be overcome by a nonlinear approach based on second harmonic generation and its dependence on symmetry at the nanoscale. The Fano resonant systems considered in this work are gold nanodolmens. Their linear and nonlinear responses are evaluated using a surface integral equation method. The numerical results demonstrate that a variation of the refractive index of the surrounding medium modifies the coupling between the dark and bright modes, resulting in a modification of the electromagnetic wave scattered at the second harmonic wavelength, especially the symmetry of the nonlinear emission. Reciprocally, we show that evaluating the asymmetry of the nonlinear emission provides a direct measurement of the gold nanodolmens dielectric environment. Interestingly, the influence of the refractive index of the surrounding medium on the nonlinear asymmetry parameter is approximately 10 times stronger than on the spectral position of the surface plasmon resonance: hence, smaller refractive index changes can be detected with this new approach. Practical details for an experimental realization of this sensing scheme are discussed and the resolution is estimated to be as low as Δn = 1.5 × 10−3, respectively 1.5 × 10−5, for an acquisition time of 60 s for an isolated gold nanodolmen, respectively an array of 10 × 10 nanodolmens.
© Royal Society of Chemistry 2015

Single nanoantenna spectroscopy was carried out on realistic dipole nanoantennas with various arm lengths and gap sizes fabricated by electron-beam lithography. A significant difference in resonance wavelength between realistic and ideal nanoantennas was found by comparing their spectral response. Consequently, the spectral tunability (96 nm) of the structures was significantly lower than that of simulated ideal nanoantennas. These observations, attributed to the nanofabrication process, are related to imperfections in the geometry, added metal adhesion layer, and shape modifications, which are analyzed in this work. Our results provide important information for the design of dipole nanoantennas clarifying the role of the structural modifications on the resonance spectra, as supported by calculations.
© (C) 2014 AIP Publishing LLC

We develop a novel formalism to calculate the optical forces and torques on complex and realistic nanostructures by combining the surface integral equation (SIE) technique with Maxwell’s stress tensor. The optical force is calculated directly on the scatterer surface from the currents obtained from the SIE, which does not require an additional surface to evaluate Maxwell’s stress tensor; this is especially useful for intricate geometries such as plasmonic antennas. SIE enables direct evaluation of forces from the surface currents very efficiently and accurately for complex systems. As a proof of concept, we establish the accuracy of the model by comparing the results with the calculations from the Mie theory. The flexibility of the method is demonstrated by simulating a realistic plasmonic system with intricate geometry.
©2014 Optical Society of America

The surface second-harmonic generation from interacting spherical plasmonic nanoparticles building different clusters (symmetric and asymmetric dimers, trimers) is theoretically investigated. The plasmonic eigenmodes of the nanoparticle clusters are first determined using an ab initio approach based on the Green’s functions method. This method provides the properties, such as the resonant wavelengths, of the modes sustained by a given cluster. The fundamental and second-harmonic responses of the corresponding clusters are then calculated using a surface integral method. The symmetry of both the linear and nonlinear responses is investigated, as well as their relationship. It is shown that the second-harmonic generation can be significantly enhanced when the fundamental field is such that its second harmonic matches modes with suitable symmetry. The role played by the nanogaps in second-harmonic generation is also underlined. The results presented in this article demonstrate that the properties of the second-harmonic generation from coupled metallic nanoparticles cannot be fully predicted from their linear response only, while, on the other hand, a detailed knowledge of the underlying modal structure can be used to optimize the generation of the second harmonic.
©2014 American Physical Society

The evaluation of distances as small as few nanometers using optical waves is a very challenging task that can pave the way for the development of new applications in biotechnology and nanotechnology. In this article, we propose a new measurement method based on the control of the nonlinear optical response of plasmonic nanostructures by means of Fano resonances. It is shown that Fano resonances resulting from the coupling between a bright mode and a dark mode at the fundamental wavelength enable unprecedented and direct manipulation of the nonlinear electromagnetic sources at the nanoscale. In the case of second harmonic generation from gold nanodolmens, the different nonlinear sources distributions induced by the different coupling regimes are clearly revealed in the far-field distribution. Hence, the configuration of the nanostructure can be accurately determined in 3-dimensions by recording the wave scattered at the second harmonic wavelength. Indeed, the conformation of the different elements building the system is encoded in the nonlinear far-field distribution, making second harmonic generation a promising tool for reading 3-dimension plasmonic nanorulers. Furthemore, it is shown that 3-dimension plasmonic nanorulers can be implemented with simpler geometries than in the linear regime while providing complete information on the structure conformation, including the top nanobar position and orientation.
©2014 American Chemical Society

We establish a general method to bridge the gap between full-field electromagnetic calculations and equivalent lumped circuit elements to describe the optical response of plasmonic nanostructures. The exact value of each lumped element is extracted from one single full-field calculation using the Poynting vector and considerations on the energy flow in the system. The equivalent circuit obtained this way describes the complete response of the system at any frequency and can be used to optimize it for specific applications or perform parametric studies. This powerful approach can accurately reproduce the behavior of complex plasmonic nanostructures, such as Fano resonances, retardation effects, and polarization coupling. Furthermore, the influence of coupling parameters within the different modes supported by a given plasmonic structure can be investigated, thus providing new physical insights into its functioning mechanisms.
©2014 American Chemical Society

Double-layer plasmonic nanostructures are fabricated by depositing metal at normal incidence onto various resist masks, forming an antenna layer on top of the resist post and a hole layer on the substrate. Antenna plasmon resonances are found to couple to the hole layer, inducing image charges which enhance the near-field for small layer spacings. For continued evaporation above the resist height, a sub-10 nm gap channel develops due to a self-aligned process and a minimal undercut of the resist sidewall. For such double layers with nanogap channels, the average surface-enhanced Raman scattering intensity is improved by a factor in excess of 60 in comparison to a single-layer antenna with the same dimensions. The proposed design principle is compatible with low-cost fabrication, straightforward to Implement, and applicable over large areas. Moreover, it can be applied for any particular antenna shape to improve the signals In surface-enhanced spectroscopy applications.
©American Chemical Society

A genericopticalbiosensingstrategywasdevelopedthatreliesontheabsorbanceenhancementphenom- enon occurringinamultiplescatteringmatrix.Experimentally,insertsmadeofglass fiber membranewere placedintomicroplatewellsinordertosignificantlylengthenthetrajectoryoftheincidentlightthroughthe sampleandthereforeincreasethecorrespondingabsorbance.Enhancementfactorwascalculatedby comparingtheabsorbancevaluesmeasuredforagivenamountofdyewithandwithouttheabsorbance- enhancinginsertsinthewells.Moreover,thedilutionofdyeinsolutionswithdifferentrefractiveindices(RI) clearlyrevealedthattheenhancementfactorincreasedwiththe ÄRI betweenthemembraneandthe surroundingmedium,reachingamaximumvalue(EF425)whenthemembranesweredried.Onthisbasis, two H2O2-biosensingsystemsweredevelopedbasedonthebiofunctionalizationoftheglass fiberinserts either withcytochrome c or horseradishperoxidase(HRP)andthe analyticalperformancesweresystem- aticallycomparedwiththecorrespondingbioassayinsolution.Theefficiencyoftheabsorbance-enhancement approachwasparticularlyclearinthecaseofthecytochrome c-basedbiosensorwithasensitivitygainof40 folds andwiderdynamicrange.Therefore,thedevelopedstrategyrepresentsapromisingwaytoconvert standardcolorimetricbioassaysintoopticalbiosensorswithimprovedsensitivity.
©2014 Elsevier B.V.

Compound plasmonic resonances arise due to the interaction between discrete and continuous metallic nanostructures. Such combined nanostructures provide a versatility and tunability beyond that of most other metallic nanostructures. In order to observe such resonances and their tunability, multiple nanostructure arrays composed of periodic metallic gratings of varying width and an underlying metallic film should be studied. Large-area compound plasmonic structures composed of various Au grating arrays with sub-100 nm features spaced nanometers above an Au film were fabricated using extreme ultraviolet interference lithography. Reflection spectra, via both numerical simulations and experimental measurements over a wide range of incidence angles and excitation wavelengths, show the existence of not only the usual propagating and localized plasmon resonances, but also compound plasmonic resonances. These resonances exhibit not only propagative features, but also a spectral evolution with varying grating width. Additionally, a reduction of the width of the grating elements results in coupling with the localized dipolar resonance of the grating elements and thus plasmon hybridization. This newly acquired perspective on the various interactions present in such a plasmonic system will aid in an increased understanding of the mechanisms at play when designing plasmonic structures composed of both discrete and continuous elements.
©2014 SPIE

Reactive oxygen species play a key role in cell signalling and oxidative stress mechanisms, therefore, sensing their production by living organisms is of fundamental interest. Here we describe a novel biosensing method for extracellular detection of endogenous hydrogen peroxide (H2O2). The method is based on the enhancement of the optical absorption spectrum of the hemoprotein cytochrome c when loaded into a highly scattering random medium. Such a configuration enables, in contrast to existing techniques, non-invasive and dynamic detection of the oxidation of cyt c in the presence of H2O2 with unprecedented sensitivity. Dynamic information on the modification of the cell oxidative status of Chlamydomonas reinhardtii, an aquatic green algae, was obtained under oxidative stress conditions induced by the presence of trace concentrations of Cd(II). Furthermore, the dynamics of H2O2 production was investigated under different lighting conditions confirming the impact of Cd(II) on the photosynthetic activity of those phytoplanktonic cells.
©

The second harmonic generation from gold nanoparticles trapped into realistic and idealized gold nanoantennas is numerically investigated using a surface integral equations technique. It is observed that the presence of a nanoparticle in the nanoantenna gap dramatically modifies the second harmonic intensity scattered into the far-field. These results clearly demonstrate that second harmonic generation is a promising alternative to the conventional linear optical methods for the detection of trapping events at the nanoscale.
©2013 OSA

We study the coupling interactions between a progressively elongated silver nanoparticle and a silver film on a glass substrate. Specifically, we investigate how the coupling between localized surface plasmons (LSPs) and propagating surface plasmon polaritons (SPPs) is influenced by nanoparticle length. Although the multiple resonances supported by the nanoparticle are effectively standing wave surface plasmons, their interaction with the SPP continuum of the underlying Ag film indicates that their spectral response is still localized in nature. It is found that these LSP–SPP interactions are not limited to small particles, but that they are present as well for extremely long particles, with a transition to the SPP coupling interactions of a bilayer metallic film system beginning at a particle length of approximately 5 ìm.
©2013 Optical Society of America

A surface integral formulation for the second-harmonic generation (SHG) from periodic metallic–dielectric nanostructures is described. This method requires the discretization of the scatterers’ surface in the unit cell only. All the physical quantities involved in this problem are derived in the unit cell by applying specific periodic boundary conditions both at the fundamental and the second-harmonic (SH) frequencies. Both the fundamental and the SH electric fields are computed using the method of moments and periodic Green’s function evaluated with the Ewald’s method. The accuracy of the method is carefully assessed using two specific cases, namely the surface plasmon enhancement of SHG from a gold film and the SHG from L-shaped nanoparticle arrays. These two examples emphasize the accuracy and versatility of the proposed method, which can be applied to a broad range of periodic metallic structures, including plasmonic arrays on arbitrary substrates and metamaterials.
©OSA 2013

ABSTRACT: Pairs of metal nanoparticles with a sub-10 nm gap are an efficient way to achieve extreme near-field enhancement for sensing applications. We demonstrate an attractive alternative based on Fabry−Perot type nanogap resonators, where the resonance is defined by the gap width and vertical elongation instead of the particle geometry. We discuss the crucial design parameters for such gap plasmons to produce maximum near-field enhancement for surface-enhanced Raman scattering and show compatibility of the pattern processing with low-cost and low-resolution lithography. We find a minimum critical metal thickness of 80 nm and observe that the mode coupling from the far field increases by tapering the gap opening. We also show the saturation of the Raman signal for nanogap periodicities below 1 μm, demonstrating efficient funneling of light into such nanogap arrays. KEYWORDS: Nanogap, gap plasmon, surface-enhanced Raman scattering, near-field enhancement, light funneling, Fabry−Perot
©2013 American Chemical Society

We study the effect of realistically rounding nanorod antennae and gap antennae on their far field and near field properties. The simulations show that both scattering behaviour and polarisation charge distribution depend significantly on rounding. Rounding is also seen to have a major effect on coupling between nanostructures. The results suggest that it is important to incorporate the effect of rounding to be able to design plasmonic nanostructures with desired properties.
©2013 Optical Society of America

The far-field polarization of the optical response of a plasmonic antenna can be tuned by subtly engineering of its geometry. In this paper, we develop design rules for nano antennas which enable the generation of circular polarized light via the excitation of circular plasmonic modes in the structure. Two initially orthogonal plasmonic modes are coupled in such a way that a rotational current is excited in the structure. Modifying this coupling strength from a weak to a strong regime controls the helicity of the scattered field. Finally, we introduce an original sensing approach that relies on the rotation of the incident polarization and demonstrates a sensitivity of 0.23 deg·nm−1 or 33 deg·RIU−1, related to changes of mechanical dimensions and the refractive index, respectively.
©2013 American Chemical Society

ABSTRACT Plasmonic modes with long radiative lifetimes, subradiant modes, combine strong confinement of the electromagnetic energy at the nanoscale with a steep spectral dispersion, which makes them promising for biochemical sensors or immunoassays. Subradiant modes have three decay channels: Ohmic losses, their extrinsic coupling to radiation, and possibly their intrinsic dipole moment. In this work, the performance of subradiant modes for refractive index sensing is studied with a general analytical and numerical approach. We introduce a model for the impact that has different decay channels of subradiant modes on the spectral resolution and contrast. It is shown analytically and verified numerically that there exists an optimal value of the mode coupling for which the spectral dispersion of the resonance line shape is maximal. The intrinsic width of subradiant modes determines the value of the dispersion maximum and depends on the penetration of the electric field in the metallic nanostructure. A figure of merit, given by the ratio of the sensitivity to the intrinsic width, which are both intrinsic properties of subradiant modes, is introduced. This figure of merit can be directly calculated from the line shape in the far-field optical spectrum and accounts for the fact that both the spectral resolution and contrast determine the limit of detection. An expression for the intrinsic width of a plasmonic mode is derived and calculated from the line shape parameters and using perturbation theory. The method of analysis introduced in this work is illustrated for dolmen and heptamer nanostructures. Fano-resonant systems have the potential to act as very efficient refractive index sensing platforms compared to Lorentz-resonant systems, due to control of their radiative losses. This study paves the way toward sensitive nanoscale biochemical sensors and immunoassays with a low limit of detection and, in general, any nano-optical device where Ohmic losses limit the performance.
©2013 American Chemical Society

The universal scaling of plasmon coupling in metallic nanostructures is now a well-established feature. However, if the interaction between dipolar plasmon modes has been intensively studied, this is not the case of the coupling between higher order ones. Using Mie theory extended to second harmonic generation, we investigate the coupling between quadrupolar plasmon modes in metallic nanoshells. Like in the case of dipolar plasmon modes, a universal scaling behavior is observed in agreement with the plasmon hybridization model.
©2013 American Physical Society

Fano resonances in hybridized systems formed from the interaction of bright modes only are reported. Despite precedent works, we demonstrate theoretically and experimentally that Fano resonances can be obtained by destructive interference between two bright dipolar modes out of phase. A simple oscillator model is provided to predict and fit the far-field scattering. The predictions are verified with numerical calculations using a surface integral equation method for a wide range of geometrical parameters. The validity of the model is then further demonstrated with experimental dark-field scattering measurements on actual nanostructures in the visible range. A remarkable set of properties like crossings, avoided crossings, inversion of subradiant and superradiant modes and a plasmonic equivalent of a bound state in the continuum are presented. The nanostructure, that takes advantage of the combination of Fano resonance and nanogap effects, also shows high tunability and strong near-field enhancement. Our study provides a general understanding of Fano resonances as well as a simple tool for engineering their spectral features.
©2013 American Chemical Society

Significant augmentation of second harmonic generation using Fano resonances in plasmonic heptamers made of silver is theoretically and experimentally demonstrated. The geometry is engineered to simultaneously produce a Fano resonance at the fundamental wavelength, resulting in a strong localization of the fundamental field close to the system, and a higher order scattering peak at the second harmonic wavelength. These results illustrate the versatility of Fano resonant structures to engineer specific optical responses both in the linear and nonlinear regimes thus paving the way for future investigations on the role of dark modes in nonlinear and quantum optics.
©2013 American Chemical Society

The construction, alignment, and performance of a setup for broadband wide-angle dispersion measurements, with emphasis on surface plasmon resonance (SPR) measurements, are presented in comprehensive detail. In contrast with most SPR instruments working with a monochromatic source, this setup takes advantage of a broadband/white light source and has full capability for automated angle vs. wavelength dispersion measurements for any arbitrary nanostructure array. A cylindrical prism is used rather than a triangular one in order to mitigate refraction induced effects and allow for such measurements. Although seemingly simple, this instrument requires use of many non-trivial methods in order to achieve proper alignment over all angles of incidence. Here we describe the alignment procedure for such a setup, the pitfalls introduced from the finite beam width incident onto the cylindrical prism, and deviations in the reflected/transmitted beam resulting from the finite thickness of the sample substrate. We address every one of these issues and provide experimental evidences on the success of this instrument and the alignment procedure used.
©2013 American Institute of Physics

Second harmonic generation from plasmonic nanoantennas is investigated numerically using a surface integral formulation for the calculation of both the fundamental and the second harmonic electric field. The comparison between a realistic and an idealized gold nanoantenna shows that second harmonic generation is extremely sensitive to asymmetry in the nanostructure shape even in cases where the linear response is barely modified. Interestingly, minute geometry asymmetry and surface roughness are clearly revealed by far-field analysis, demonstrating that second harmonic generation is a promising tool for the sensitive optical characterization of plasmonic nanostructures. Furthermore, defects located where the linear field is strong (e.g., in the antenna gap) do not necessarily have the strongest impact on the second harmonic signal.
©2013 American Chemical Society

Adhesion layers, required to stabilize metallic nanostructures, dramatically deteriorate the performances of plasmonic sensors, by severely damping the plasmon modes. In this article, we show that these detrimental effects critically depend on the overlap of the electromagnetic near-field of the resonant plasmon mode with the adhesion layer and can be minimized by careful engineering of the latter. We study the dependence of the geometrical parameters such as layer thickness and shape on the near-field of localized plasmon resonances for traditional adhesion layers such as Cr, Ti, and TiO2. Our experiments and simulations reveal a strong dependence of the damping on the layer thickness, in agreement with the exponential decay of the plasmon near-field. We developed a method to minimize the damping by selective deposition of thin adhesion layers (<1 nm) in a manner that prevents the layer to overlap with the hotspots of the plasmonic structure. Such a designed structure enables the use of standard Cr and Ti adhesion materials to fabricate robust plasmonic sensors without deteriorating their sensitivity.
©2013 American Chemical Society

In this work a portable analytical biosensor for real-time extracellular monitoring of released hydrogen peroxide (H2O2)is presented. The biosensor is based on the optical detection of the cytochrome c (cyt c) oxidation state. The setup consists of an integrated microscope combined with a compact spectrometer. The light being absorbed by cyt c is enhanced via multiscattering produced by random aggregates of polystyrene beads in a cross-linked cyt c matrix. Using ink-jet printing technique, the sensing elements, namely cyt c loaded polystyrene aggregates, are fabricated with high reliability in terms of repeatability of size and sensitivity. Additionally, the sensing elements are enclosed in a microfluidic channel assuring a fast and efficient analytes delivery. As an example, the effect of trace concentrations of functionalized cadmium selenide/zinc sulfide (CdSe/ZnS) core shell quantum dots on the green algae Chlamydomonas reinhardtii is investigated, showing extracellular H2O2 release with different production rates over a period of 1 hour. In conclusion, the presented portable biosensor enables the highly sensitive and non-invasive real-time monitoring of the cell metabolism of C. reinhardtii.
©2013 SPIE

Surface-enhanced Raman scattering (SERS) has become increasingly popular in the scientific and industrial communities because of its analytical capabilities and potential to study fundamentals in plasmonics. Although under certain conditions extremely high sensitivity is possible, the practical use of SERS is frequently limited by instability and poor reproducibility of the enhancement factor. For analytical applications or for comparative measurements to enable the distinction between electromagnetic and chemical enhancement, the development of standardized and recyclable SERS substrates, having uniform and persistent performance, is proposed. To this end, we have fabricated periodic nanoslit arrays using extreme ultraviolet lithography that provide average large (2*106) and homogeneous SERS enhancement factors with a spot-to-spot variability of less than 3%. In addition, they are reusable without any degradation or loss of enhancement. The fabrication of such arrays consists of two steps only, lithographic patterning followed by metal evaporation. Both processes may be performed over areas of several square mm on any planar substrate. The sensor capabilities were demonstrated by substrates with monomolecular films of several different thiols. The concept of reusable SERS substrates may open a powerful platform within an analytical tool and in particular for systematic SERS studies for the investigation of fundamental parameters such as chemical enhancement, surface selection rules, and molecular alignment.
©2012 John Wiley & Sons, Ltd.

High-resolution multilayer plasmonic nanostructures are made in a single fabrication step by nanocutting a gold and parylene C bilayer. A silicon stamp is used to cut the gold film and vertically displace the nanocut shapes into the polymer. Periodic Au nanopatterns in single and double-layer configurations are fabricated down to the sub-100 nm range, supporting plasmon resonances in the visible range.
©2013 WILEY-VCH Verlag GmbH & Co.

A novel third generation biosensor was developed based on one-shot adsorption of chemically-modified cytochrome c (cyt c) onto bare gold electrodes. In contrast to the classic approach which consists of attaching cyt c onto an active self-assembled monolayer (SAM) priory chemisorbed on gold, here short-chain thiol derivatives (mercaptopropionicacid,MPA) were chemically introduced on cyt c protein shell via its lysine residues enabling the very fast formation (< 5 min) of an electroactive biological self-assembled monolayer (SAM) exhibiting a quasi-reversible electrochemical behavior and a fast direct electrontransfer (ET). The heterogeneous ET rate constant was estimated to be ks=1600 s^(-1), confirming that short anchors facilitate the ET via an efficient orientation of the heme pocket. In comparison, no ET was observed in the case of native and long-anchor (mercaptoundecanoic acid, MUA) modified cyt c directly adsorbed on gold. However, in both cases the ET was efficiently restored upon in-bulk generation of gold nanoparticles which acted as electron shuttles. This observation emphasizes that the lack of electroactivity might be caused by either an inappropriate orientation of the protein (native cyt c) or a critical distance (MUA-cyt c). Finally, the sensitivity of the bare gold electrode directly modified with MPA-cyt c to hydrogenperoxide (H2O2) was evaluated by amperometry and the so-made amperometric biosensor was able to perform real-time and non-invasive detection of endogeneous H2O2 released by unicellular aquatic microorganisms, Chlamydomonas reinhardtii, upon cadmium exposure.
©2013 Elsevier B.V.

Plasmonic modes with long radiative lifetimes combine strong nanoscale light confinement with a narrow spectral line width carrying the signature of Fano resonances, making them very promising for nanophotonic applications such as sensing, lasing, and switching. Their coupling to incident radiation, also known as radiance, determines their optical properties and optimal use in applications. In this work, we theoretically and experimentally demonstrate that the radiance of a plasmonic mode can be classified into three different regimes. In the weak coupling regime, the line shape exhibits remarkable sensitivity to the dielectric environment. We show that geometrical displacements and deformations at the Angström scale can be detected optically by measuring the radiance. In the intermediate regime, the electromagnetic energy stored in the mode is maximal, with large electric field enhancements that can be exploited in surface enhanced spectroscopy applications. In the strong coupling regime, the interaction can result in hybridized modes with tunable energies.
©2013 American Chemical Society

We study a compound plasmonic system composed of a periodic Au grating array placed close to a thin Au film. The study is not limited to normal incidence and dispersion diagrams are computed for a broad variety of parameters. In addition to identifying localized and propagating modes and the coupling/hybridization interactions between them, we go further and identify modes of compound nature, i.e. those exhibiting both localized and propagating characteristics, and discuss which plasmon modes can exhibit such a behavior in the system at hand and how structural parameters play a central part in the spectral response of such modes.
©2012 Optical Society of America

Using a generalization of Fano formula that includes the intrinsic losses associated with metals, we retrieve the underlying modal structure that produces Fano resonances from the interference between a bright and a dark mode. This knowledge is used to determine the most efficient plasmonic system for a specific application by tuning its mode of operation from weak coupling to best energy storage or strong coupling. This approach is illustrated with examples from sensing.
©2012 American Institute of Physics

We show bending of light on the same side of the normal in a free-standing corrugated metal film under bidirectional illumination. Coherent perfect absorption (CPA) is exploited to suppress the specular zeroth order leading to effective back-bending of light into the “−1” order, while the “
1” order is resonant with the surface mode. The effect is shown to be phase sensitive, yielding CPA and superscattering in the same geometry.
©2012 Optical Society of America

The enhancement of fluorescence and Raman scattering by plasmonic nanostructures is studied theoretically with special focus on the effects of the observed molecule’s properties and the realistic geometry of the plasmonic nanostructure. Numerical experiments show that the enhancement factor may vary by many orders of magnitude depending on a fluorophore’s transition rates or intrinsic quantum yield. For different molecules, boosting fluorescence enhancement means optimizing different factors, leading to a different ideal geometric and spectral configuration. This framework, coupled with powerful new simulation tools, will facilitate the design and characterization of fluorescence-enhancing plasmonic nanostructures as well as yield experimental access to the intrinsic properties of the molecules under study.
©2012 American Chemical Society

We introduce photothermal optical lock-in Optical Coherence Microscopy (poli-OCM), a volumetric imaging technique, which combines the depth sectioning of OCM with the high sensitivity of photothermal microscopy while maintaining the fast acquisition speed inherent to OCM. We report on the detection of single 40 nm gold particles with a 0.5 ìm lateral and 2 ìm axial resolution over a 50 ìm depth of field and the three-dimensional localization of gold colloids within living cells. In combination with intrinsic sample contrast measured with dark-field OCM, poli-OCM offers a versatile platform for functional cell imaging.
©2012 Optical Society of America

We present a novel plasmonic antenna geometry – the double resonant antenna (DRA) – that is optimized for second-harmonic generation (SHG). This antenna is based on two gaps coupled to each other so that a resonance at the fundamental and at the doubled frequency is obtained. Furthermore, the proximity of the localized hot spots allows for a coupling and spatial overlap between the two field enhancements at both frequencies. Using such a structure, both the generation of the second-harmonic and its re-emission into the far-field are significantly increased when compared with a standard plasmonic dipole antenna. Such DRA are fabricated in aluminium using electron beam lithography and their linear and nonlinear responses are studied experimentally and theoretically.
©2012 Optical Society of America

The mechanism of using the anisotropic Purcell factor to control the spontaneous emission linewidths in a four-level atom is theoretically demonstrated; if the polarization angle bisector of the two dipole moments lies along the axis of large/small Purcell factor, destructive/constructive interference narrows/widens the fluorescence center spectral lines. Large anisotropy of the Purcell factor, confined in the subwavelength optical mode volume, leads to rapid spectral line narrowing of atom approaching a metallic nanowire, nanoscale line width pulsing following periodically varying decay rates near a periodic metallic nanostructure, and dramatic modification on the spontaneous emission spectrum near a custom-designed resonant plasmon nanostructure. The combined system opens a good perspective for applications in ultracompact active quantum devices.
©2012 American Chemical Society

Second-harmonic generation (SHG) from centrosymmetric nanostructures originating from the breaking of inversion symmetry at their surfaces is a well-known phenomenon and is extensively used as a surface probe in nonlinear optical microscopy. In recent years, SHG and its subsequent enhancement using plasmonics has been observed from nanostructures such as sharp metallic tips, nanoantennae and nanodimers. However, the process is still inefficient, its mechanism not well understood, and an improvement is required. In order to achieve a higher conversion efficiency, we investigate experimentally a way to minimize the radiative losses at the fundamental frequency. In the present investigation, we use silver heptamer nanostructures and tune the subradiant mode of the Fano resonance to the fundamental of the pump source, while tuning a higher order multipolar term to the second harmonic and in the process we obtain a significant enhancement of the second harmonic signal. A detailed explanation and analysis of this is provided by considering the contribution and effect of varying different parameters, such as gap size and radius, as well as the overall symmetry of the structure. In fact, recently gold heptamers have been studied and have indeed shown strong hybridization of their constituent resonant primitive plasmonic modes, leading to new hybridized superradiant ‘bright’ and subradiant ‘dark’ modes. The ease of fabrication and possible tunability achievable, make these structures very versatile tools for studying surface SHG in nanostructures.
©2012 SPIE

Recently stacked metamaterial structures coupled to a conductive plane have been investigated and have been shown to exhibit the same properties as stacked structures with double the layers, due to dipole mirror coupling. Here we study a system of stacked subwavelength metallic grating layers coupled to a metal film and show that this system not only supports the localized modes of a doubly layered structure, but also, for non-normal incidence, supports modes that exhibit a clear propagation and in one case a simultaneous localization of the electromagnetic field in the region between the metal film and the first grating layer. Furthermore we show that this hybridized propagating mode, excited for any N number of periodic layers, is further influenced as it couples with the highest energy localized mode of the periodic layered stack. Additionally it is found that the localized modes of the structure can be spectrally positioned in a directly adjacent manner, resulting in wideband absorption that can effectively be tuned by varying the grating film spacing.
©2012 SPIE

We theoretically investigate the dependence of the different parameters of an optical biosensor for the detection of Hydrogen peroxide (H2O2) based on absorption enhancement of Cytochrome c molecules near gold nanoparticles. H2O2 is a major reactive oxygen species which is involved in signaling pathways and oxidative stress in cells. We use the Green’s function approach as well as confirm the corresponding simulation results using the surface integral formulation. Further we show that this technique can be applied for detection of other small molecules, like oxygen and carbon monoxide.
©© 2012 SPIE

Due to their extreme sensitivity to refractive index changes, surface plasmon resonance (SPR) sensors have long been established as extremely valuable tools for biosensing. In the past few years researchers have begun investigating various other metallic nanostructures as candidates for localized SPR (LSPR) sensing. Although LSPR is not nearly as sensitive to bulk refractive index changes as standard SPR, is has the advantage of being extremely sensitive to local refractive index changes, thereby providing detection on the level of a single molecule. In practice such sensitivity criterion is of paramount importance since the analyte layer under investigation is often only a few nanometers thick and deposited directly on the surface of the metal. Most desirable, however, is a sensor that retains the total integrated sensitivity of a traditional SPR sensor and at the same time localizes this sensitivity right at the sensor surface. For this reason, we have investigated a hybrid structure composed of a 2D Au nanoparticle array coupled to a Au film. We show that this structure, when excited in the Kretschmann configuration, retains to a surprising degree the total integrated sensitivity of an ideal SPR sensor and is able to concentrate that sensitivity within a few nanometers of the sensor surface, thereby yielding a hybrid sensor with the advantages of both LSPR and SPR sensing, i.e. both a high local sensitivity and a high figure of merit (FOM).
©2012 SPIE

We investigate the mediation of symmetry-forbidden atomic transitions using plasmonic nanostructures. We show that the excitation of the electric dipole-forbidden, quadrupole-allowed 6(2)S(1/2) - 5(2)D(5/2) transition in cesium may be enhanced by more than 6 orders of magnitude in the intense, inhomogeneous near field of a plasmonic nanoantenna. Using optical reciprocity, the enhancement can be understood to apply to spontaneous emission as well, allowing the fast and efficient optical detection of excited atoms.
©2012 American Physical Society

We exploit the versatility provided by metal–dielectric composites to demonstrate controllable coherent perfect absorption (CPA) or anti-lasing in a slab of heterogeneous medium. The slab is illuminated by coherent light from both sides, at the same angle of incidence and the conditions required for CPA are investigated as a function of the different system parameters. Our calculations clearly elucidate the role of absorption as a necessary prerequisite for CPA. We further demonstrate the controllability of the CPA frequency to the extent of having the same at two distinct frequencies even in presence of dispersion, rendering the realization of anti-lasers more flexible.
©2012 Optical Society of America

A central goal in bioanalytics is to determine the concentration of and interactions between biomolecules. Nanotechnology allows performing such analyses in a highly parallel, low-cost, and miniaturized fashion. Here we report on label-free volume, concentration, and mobility analysis of single protein molecules and nanoparticles during their diffusion through a subattoliter detection volume, confined by a 100 nm aperture in a thin gold film. A high concentration of small fluorescent molecules renders the aqueous solution in the aperture brightly fluorescent. Nonfluorescent analytes diffusing into the aperture displace the fluorescent molecules in the solution, leading to a decrease of the detected fluorescence signal, while analytes diffusing out of the aperture return the fluorescence level. The resulting fluorescence fluctuations provide direct information on the volume, concentration, and mobility of the nonfluorescent analytes through fluctuation analysis in both time and amplitude.
©2012 American Chemical Society

We report a high-throughput method for the fabrication of metallic nanogap arrays with high-accuracy over large areas. This method, based on shadow evaporation and interference lithography, achieves sub-10 nm gap sizes with a high accuracy of +/- 1.5 nm. Controlled fabrication is demonstrated over mm(2) areas and for periods of 250 nm. Experiments complemented with numerical simulations indicate that the formation of nanogaps is a robust, self-limiting process that can be applied to wafer-scale substrates. Surface-enhanced Raman scattering (SERS) experiments illustrate the potential for plasmonic sensing with an exceptionally low standard-deviation of the SERS signal below 3% and average enhancement factors exceeding 1 x 10(6).
©2011 American Institute of Physics

The optical properties of plasmonic nanostructures supporting Fano resonances are investigated with an electromagnetic theory. Contrary to the original work of Fano, this theory includes losses in the materials composing the system. As a result, a more general formula is obtained for the response of the system and general conclusions for the determination of the resonance parameters are drawn. These predictions are verified with surface integral numerical calculations in a broad variety of plasmonic nanostructures including dolmens, oligomers, and gratings. This work presents a robust and consistent analysis of plasmonic Fano resonances and enables the control of their line shape based on Maxwell's equations. The insights into the physical understanding of Fano resonances gained this way will be of great interest for the design of plasmonic systems with specific spectral responses for applications such as sensing and optical metamaterials.
©2011 American Chemical Society

The relation between the near–field and far–field properties of plasmonic nanostructures that exhibit Fano resonances is investigated in detail. We show that specific features visible in the asymmetric lineshape far–field response of such structures originate from particular polarization distributions in their near–field. In particular we extract the central frequency and width of plasmonic Fano resonances and show that they cannot be directly found from far–field spectra. We also address the effect of the modes coupling onto the frequency, width, asymmetry and modulation depth of the Fano resonance. The methodology described in this article should be useful to analyze and design a broad variety of Fano plasmonic systems with tailored near–field and far–field spectral properties.
©2011 Optical Society of America

We use numerical simulations based on the surface integral technique to study the detection limit of plasmonic trapping with realistic dipole antennas. The induced plasmon resonance shift due to the coupling between an antenna and a nanoparticle is studied for different antennas geometries, different positions, sizes, and materials for the trapped nanoparticle. The shift of the antenna resonance is found to be linear with the near-field intensity enhancement caused by the antenna and further dependents on the volume and refractive index of the trapped nanoparticle. Detection limit of 5 nm for plasmonic particles and 6.5 nm for high index dielectrics is reported.
©2011 American Institute of Physics

We report on the derivation of analytical formulas for the lineshape of Fano resonances in plasmonic nanostructures as a function of their electromagnetic response. Contrary to the original work of Fano, the formalism proposed here includes losses in the materials composing the system. As a result, a more general formula is obtained for the response of the system and general conclusions for the determination of the resonance parameters are drawn, in particular on its width and asymmetry. The insights into the physical understanding of Fano resonances gained this way will be of great interest for the design of plasmonic sensing platforms and metamaterials.
©2011 American Institute of Physics

Using a finite-element, full-wave modeling approach, we present a flexible method of analyzing and simulating dielectric and plasmonic waveguide structures as well as their mode coupling. This method is applied to an integrated plasmonic circuit where a straight dielectric waveguide couples through a straight hybrid long-range plasmon waveguide to a uniformly bent hybrid one. The hybrid waveguide comprises a thin metal core embedded in a two–dimensional dielectric waveguide. The performance of such plasmonic circuits in terms of insertion losses is discussed.
©2011 Optical Society of America

We investigate theoretically the strong coupling between surface plasmon resonances (SPRs) and absorption bands of hemoglobin. When the surface plasmon resonance spectrally overlaps the absorption bands of hemoglobin, the system is strongly coupled and its dispersion diagram exhibits an anti-crossing. Working in the conditions of strong coupling enhances the sensitivity of a SPR sensor up to a factor of 10. A model for the permittivity of hemoglobin, both in oxygenated and deoxygenated states, is presented and the study is carried out for both angle and wavelength modulated SPR sensors. Finally, a differential measurement is shown to increase the sensitivity further.
©2011 American Institute of Physics

Plasmonic dipole antennas are powerful optical devices for many applications since they combine a high field enhancement with outstanding tunability of their resonance frequency. The field enhancement, which is mainly localized inside the nanogap between both arms, is strong enough to generate attractive forces for trapping extremely small objects flowing nearby. Furthermore it dramatically enhances their Raman scattering cross-section generating SERS emission. In this publication, we demonstrate how plasmonic antennas provide unique means for bringing analyte directly into hotspots by merely controlling the optical force generated by the plasmon resonance. This technique is very suitable for immobilizing objects smaller that the diffraction limit and requires a very little power density. In this work, 20nm gold nanoparticles functionalized with Rhodamine 6G are trapped in the gap of nanoantennas fabricated with e-beam lithography on glass substrate. The entire system is integrated into a microfluidic chip with valves and pumps for driving the analyte. The field enhancement is generated by a near-IR laser (λ=808nm) that provides the trapping energy. It is focused on the sample through a total internal reflection (TIRF) objective in dark field configuration with a white light source. The scattered light is collected through the same objective and the spectrum of one single antenna spectrum is recorded and analyzed every second. A trapping event is characterized by a sudden red-shift of the antenna resonance. This way, it is possible to detect the trapping of extremely small objects. The SERS signal produced by a trapped analyte can then be studied by switching from the white light source to a second laser for Raman spectroscopy, while keeping the trapping laser on. The trapping and detection limit of this approach will be discussed in detail.
©2011 SPIE

In this work, we pave the route towards the engineering of strong and spectrally sharp Fano resonances in plasmonic nanostructures and derive analytical formulas for their line shape as a function of their electromagnetic response. Contrary to the original work of Fano, the formalism proposed here includes losses in the materials composing the system. As a result, a more general formula is obtained for the response of the system and general conclusions for the determination of the resonance parameters are drawn, in particular on its width and asymmetry. Using a surface integral simulation technique for electromagnetic scattering on three-dimensional individual and periodic nanostructures, we numerically validate our model for structures that are currently under extensive investigation in the plasmonic and metamaterial communities. The insights into the physical comprehension of Fano resonances gained this way will be of great interest for the design of plasmonic sensing platforms and metamaterials.
©2011 SPIE

An ab initio theory for Fano resonances in plasmonic nanostructures and metamaterials is de- veloped using Feshbach formalism. It reveals the role played by the electromagnetic modes and material losses in the system, and enables the engineering of Fano resonances in arbitrary geome- tries. A general formula for the asymmetric resonance in a non-conservative system is derived. The influence of the electromagnetic interactions on the resonance line shape is discussed and it is shown that intrinsic losses drive the resonance contrast, while its width is mostly determined by the coupling strength between the non-radiative mode and the continuum. The analytical model is in perfect agreement with numerical simulations.
©2011 American Physical Society

The transition from localized to delocalized plasmons (i.e. the transition from a situation where the decay length of a travelling surface plasma wave is greater than its propagation distance to a situation where it is smaller) and hence the onset of plasmon delocalization is studied in a single 2D silver nanoparticle of increasing length. A fourier analysis in the near-field of the nanoparticle is used as the main tool for analysis. This method, along with far-field scattering spectra simulations and the near-field profile directly above and along the length of the nanoparticle are used to investigate and clearly show the transition from localized to delocalized modes. In particular, it is found that for a finite sized rectangular nanoparticle, both the emerging odd and even delocalized modes are nothing but a superposition of many standing wave plasmon modes. As a consequence, even very short metal films can support delocalized plasmons that bounce back and forth along the film.
©2011 Optical Society of America

The electric field enhancement associated with detailed structure within novel optical antenna nanostructures is modeled using the surface integral equation technique in the context of surface-enhanced Raman scattering (SERS). The antennae comprise random arrays of vertically aligned, multiwalled carbon nanotubes dressed with highly granular Ag. Different types of “hot-spot” underpinning the SERS are identified, but contrasting characteristics are revealed. Those at the outer edges of the Ag grains are antenna driven with field enhancement amplified in antenna antinodes while intergrain hotspots are largely independent of antenna activity. Hot-spots between the tops of antennae leaning towards each other also appear to benefit from antenna amplification.
©2011 American Chemical Society

Remote sensing finds more and more applications, from industrial control, to face recognition, not forgetting terrain surveying. This trend is well exemplified by fringe projection techniques, which enjoyed a considerable development in the recent years. In addition of high requirement in terms of measurement accuracy and spatial resolution, the end-users of full-field techniques show a growing interest for dynamic regimes. We report here what we believe to be the use for the first time of a CMOS 3-layer color sensor (Foveon X3) as the key element of a RGB fringe projection system, together with the processing specifically elaborated for this sensor. The 3-layer architecture allows the simultaneous recording of three phase-shifted fringe patterns and features the precious asset of an unambiguous relationship between the physical sensor pixel and the picture pixel and this for each color layer, on the contrary of common color sensor arrays (Bayer mosaic and tri-CCD). Due to the overlapping of the spectral responses of the layers, color transformation is mandatory to achieve the separation of the three phase-shifted RGB projected fringe patterns. In addition, we propose the use of the Empirical Mode Decomposition to equalize the non-uniform responses of the three layers. Although the conversion of the phase into a height is of primary importance in an actual measurement, it is not treated here, the literature being profuse on the central projection model.
©2011 Optical Society of America

The enhancement of excitation and reemission of molecules in close proximity to plasmonic nanostructures is studied with special focus on the comparison between idealized and realistically shaped nanostructures. Numerical experiments show that for certain applications choosing a realistic geometry closely resembling the actual nanostructure is imperative, an idealized simulation geometry yielding significantly different results. Finally, a link between excitation and reemission processes is formed via the theory of optical reciprocity, allowing a transparent view of the electromagnetic processes involved in plasmon-enhanced fluorescence and Raman-scattering.
©2011 American Chemical Society

In many respects, speckle interferometry (SI) techniques are being considered as mature tools in the experimental mechanics circles. These techniques have enlarged considerably the field of optical metrology, featuring nanometric sensitivities in whole-field measurements of profile, shape and deformation of mechanical rough surfaces. Nonetheless, when we consider classical fringe processing techniques, e.g. phase-shifting methods, the deformation range is intrinsically limited to the correlation volume of the speckle field. In addition, the phase evaluation from such patterns is still computationally intensive, especially in the characterisation of dynamic regimes, for which there is a growing interest in a wide range of research and engineering activities. A promising approach lies in the pixel history analysis. We propose in this paper to implement the empirical mode decomposition (EMD) algorithm in a fast way, to put the pixel signal in an appropriate shape for accurate phase computation with the Hilbert transform.
©2008 Blackwell Publishing Ltd

A surface integral formulation for light scattering on periodic structures is presented. Electric and magnetic field equations are derived on the scatterers’ surfaces in the unit cell with periodic boundary conditions. The solution is calculated with the method of moments and relies on the evaluation of the periodic Green’s function performed with Ewald’s method. The accuracy of this approach is assessed in detail. With this versatile boundary element formulation, a very large variety of geometries can be simulated, including doubly periodic structures on substrates and in multilayered media. The surface discretization shows a high flexibility, allowing the investigation of irregular shapes including fabrication accuracy. Deep insights into the extreme near-field of the scatterers as well as in the corresponding far-field are revealed. This method will find numerous applications for the design of realistic photonic nanostructures, in which light propagation is tailored to produce novel optical effects.
©2010 Optical Society of America

With the development of nanotechnology, many new optical phenomena in nanoscale have been demonstrated. Through the coupling of optical waves and collective oscillations of free electrons in metallic nanostructures, surface plasmon polaritons can be excited accompanying a strong near field enhancement that decays in a subwavelength scale, which have potential applications in the surface-enhanced Raman scattering, biosensor, optical communication, solar cells, and nonlinear optical frequency mixing. In the present article, we review the Green’s matrix method for solving the surface plasmon resonances and near field in arbitrarily shaped nanostructures and in binary metallic nanostructures. Using this method, we design the plasmonic nanostructures whose resonances are tunable from the visible to near-infrared, study the interplay of plasmon resonances, and propose a new way to control plasmonic resonances in binary metallic nanostructures.
©2010 Science China Press and Springer-Verlag Berlin Heidelberg

Raman spectroscopy (SERS), localized surface plasmon resonance (LSPR) –based sensing and optical trapping. An analytical model is implemented to link the local electric field enhancement with the gradient forces, as well as the resonance shift caused by the presence of the analyte which can be a molecule or a nanoparticle. We find that a higher local field enhancement induces stronger trapping forces and a larger resonance wavelength shift. Experiments were also performed using plasmonic dipole antennas. Strong SERS signals were observed from the nanogap of an antenna, trapping of Au nanoparticles as small as 10 nm was achieved with a moderate laser power, and evident resonance shifts of the antenna associated with the trapping events were also observed. These results are consistent with our theoretical result that the giant field enhancement generated by a plasmonic dipole antenna also generates strong gradient forces and a high spectral sensitivity.
©2010 SPIE

In this work, we use a multi-heterodyne scanning near-field optical microscope to investigate the polarization and propagation of Bloch surface waves in an ultrathin (~λ/10) ridge waveguide. First, we show that the structure sustains three surface modes, and demonstrate selective excitation of each. Then, by numerically processing the experimental data, we retrieve the transverse and longitudinal components of each of the modes, in good agreement with the calculated fields. Finally, we provide an experimental estimation of the effective indices and the dispersion relations of the modes.
©2010 American Optical Society

We describe a general theoretical framework based on the Bergman spectral representation to study how a nanostructure interacts with an external electromagnetic field. The selection rules for localized surface plasmon resonances (LSPRs) are obtained by implementing the group theory upon the electric vector field. The influence of symmetry breaking on the splitting of degenerated modes and the switching of dark modes by specific illuminations are discussed. These results emphasize the fact that the selection rules for a vector field are different from the case of a scalar field and essentially induced by the geometry of the structure. Finally, this work not only points out that measurements of LSPRs may result in very different results with different external fields, but also provides a strategy to selectively excite specific LSPRs of plasmonic structures.
©2010 American Institute of Physics

Investigation on the interplay of plasmonic resonances in binary nanostructures indicated that, at a fixed wavelength, with a variation in the difference permittivity ratio η =(ε2−ε0 /ε1−ε0), resonances exhibit the dielectric effect, resonance chaos, collective resonance, resonance flat, and new branch regions. This means that plasmonic resonances can be controlled by material parameters ε1 and ε2. In this work, using the Green’s matrix method of solving the surface plasmon resonances, we first study the resonance combination of symmetrical binary three-nanostrip systems. Several resonance branches extend across the above mentioned regions. Near fields within the gaps and at the ends of nanostrips are greatly enhanced due to the influence of neighboring metallic material. Then, along each resonance branch, resonances in the dielectric permittivity region are mapped into the wavelength region of gold. Through adjusting material parameters ε1 and ε2, the resonance wavelength is tuned from λR=500 to 1500 nm, while for a single nanostrip it is only at λR =630 nm. We also find that comparable permittivity parameters ε1 (or ε2) and εAu(ω) can control resonance wavelength and intensity effectively. High dielectric permittivity of the neighboring metal has also an advantage in a giant enhancement of the near field. These findings provide new insights into design of hybrid plasmonic devices as plasmonic sensors.
©2010 American Institute of Physics

The surface integral formulation is a flexible, multiscale and accurate tool to simulate light scattering on nanostructures. Its generalization to periodic arrays is introduced in this paper. The general electromagnetic scattering problem is reduced to a discretizated model using the Method of Moments on the surface of the scatterers in the unit cell. The study of the resonances of an array of bowtie antennas illustrates the main features of the method. When placed into an array, the bowtie antennas show additional resonances compared to those of an individual antenna. Using the surface integral formulation, we are able to investigate both nearfield and far-field properties of these resonances, with a high level of accuracy.
©2010 Elsevier B.V.

Calculation of electromagnetic cross sections from surface integral equation simulations, a popular approach in microwave studies and recently also in optics and plasmonics, requires only a single post-processing step, which can, however, be very sensitive to the precision of the simulation result. We investigate the accuracy and robustness of two methods for cross section calculation, displaying when and why errors may occur, in certain cases even unphysical behavior. A calculation recipe which avoids unphysical results is given, ensuring convergence of all obtained cross sections. This study will help judge the accuracy of performed simulations and can prevent misinterpretation of modeling results.
©2010 IEEE

The resonance fluorescence of a two-level single molecular system interacting with a plasmonic nanostructure is investigated. Specific regions of space are identified, where a balance exists between the near-field enhancement and the modification of the decay rate, such that the fluorescence spectrum of the molecule exhibits the Mollow triplet and the emission photons are antibunched. The utilization of such quantum phenomena at the vicinity of custom-designed plasmonic nanostructures paves the way for applications in nanoscale quantum devices and quantum information processing.
©2010 American Physical Society

We present a direct evidence of Bloch surface waves (BSWs) waveguiding on ultrathin polymeric ridges, supported by near-field measurements. It is demonstrated that near-infrared BSWs sustained by a silicon-based multilayer can be locally coupled and guided through dielectric ridges of nanometric thickness with low propagation losses. Using a conventional prism-based configuration, we demonstrate a wavelength-selective BSW coupling inside and outside the ridge. Such a result can open interesting opportunities in surface wave-mediated sensing applications, where light could be selectively coupled in specific regions defined by nanometric reliefs.
©2010 American Chemical Society

Light scattering by an array of alternating electric and magnetic nanoparticles is analyzed in detailed. Specific geometrical conditions are derived, where such an array behaves like double-negative particles, leading to a suppression of the backscattered intensity. This effect is very robust and could be used to produce double-negative metamaterials using singlenegative components.
©2010 Optical Society of America

A broad bandwidth and high gain rectangular patch antenna was specifically designed in this paper using planar-patterned metamaterial concepts. Based on an ordinary patch antenna, the antenna has isolated triangle gaps and crossed strip-line gaps etched on the metal patch and ground plane, respectively. Demonstrated to have left-handed characteristics, the patterned metal patch and finite ground plane form a coupled capacitive-inductive circuit of negative index metamaterial. It is shown to have great impact on the antenna performance enhancement in terms of the bandwidth significantly broadened from a few hundred megahertz to a few gigahertz, and also in terms of high efficiency, low loss, and low voltage standing wave ratio. Experimental data show a reasonably good agreement between the simulation and measured results. This antenna has strong radiation in the horizontal direction for some specific applications within the entire band.
©2010 American Institute of Physics

The optical trapping of Au nanoparticles with dimensions as small as 10 nm in the gap of plasmonic dipole antennas is demonstrated. Single nanoparticle trapping events are recorded in real time by monitoring the Rayleigh scattering spectra of individual plasmonic antennas. Numerical simulations are also performed to interpret the experimental results, indicating the possibility to trap nanoparticles only a few nanometers in size. This work unveils the potential associated with the integration of plasmonic trapping with localized surface plasmon resonance based sensing techniques, in order to deliver analyte to specific, highly sensitive regions (“hot spots”).
©2010 American Chemical Society

By introducing the difference permittivity ratio η = (ε2 − ε0)/(ε1 − ε0), the Green matrix method for computing surface plasmon resonances is extended to binary nanostructures. Based on the near field coupling, the interplay of plasmon resonances in two closely packed nanostrips is investigated. At a fixed wavelength, with varying η the resonances exhibit different regions: the dielectric effect region, resonance chaos region, collective resonance region, resonance flat region, and new branches region. Simultaneously, avoiding crossing and mode transfer phenomena between the resonance branches are observed. These findings will be helpful to design hybrid plasmonic subwavelength structures.
©2009 Springer-Verlag

Light scattered by a dielectric object when it is trapped in the field of a plasmonic nanostructure is studied theoretically and experimentally using both dielectric spheres and S. cerevisiae cells. A dramatic enhancement of the scattered light is observed for short separation distances between scatterer and plasmonic trap. It is shown that this effect can serve to selectively image cells after their immobilization and distinguish them from a turbid background. The high sensitivity of the scattered light to the separation distance and lateral displacement also provides additional insights in the configuration of the cell within the trap.
©2010 American Institute of Physics

A novel method is elaborated for the electromagnetic scattering from periodical arrays of scatterers embedded in a polarizable background. A dyadic periodic Green's function is introduced to calculate the scattered electric field in a lattice of dielectric or metallic objects. The method exhibits strong advantages: discretization and computation of the field are restricted to the volume of the scatterers in the unit cell, open and periodic boundary conditions for the electric field are included in the Green's tensor, and finally both near and far-fields physics are directly revealed, without any additional computational effort. Promising applications include the design of periodic structures such as frequency-selective surfaces, photonic crystals and metamaterials.
©2009 American Institute of Physics

We present a systematic numerical investigation of conical metal tips which are commonly used in tip-enhanced Raman spectroscopy (TERS). Different from previous studies, we focus on how the tip length and the illumination condition influence the local field enhancement at the tip apex, and provide a useful reference for real experiments. In particular, we show that the type of illumination has a dramatic influence on the field enhancement: a localized illumination spot – as used in experiments – producing a very different response than a plane wave illumination – as usually used in previous models. Also, the effect of the different geometrical parameters, such as the sharpness of the tip apex and the cone angle, provides guidance to optimize the tip design. Finally, we investigate the influence of the substrate and compare numerical data with results deduced from a simplified model, finding good agreement. This brings new insights into the enhancementmechanism of TERS.
©2009 John Wiley & Sons, Ltd.

Recent developments in nanofabrication and optical near-field metrology have faced complementary modeling techniques with new demands. We present a surface integral formulation that accurately describes the extreme near-field of a plasmonic nanoparticle in addition to its far-field properties. Flexible surface meshing gives precise control over even complex geometries allowing investigation of the effects of fabrication accuracy and material homogeneity on a particle’s optical response. Using this technique, the influence of a particle’s symmetry and shape on surrounding “hot spots” of extremely large field enhancement is explored, giving insight into the mechanisms of surface enhanced Raman scattering and single-molecule detection techniques.
©2009 SPIE

Light or electromagnetic wave scattered by a single sphere or a coated sphere has been considered as a classic Mie theory. There have been some further extensions which were made further based on the Mie theory. Recently, a closed form analytial model of the scattering cross section of a single nanoshell has been considered. The present paper is documented further, based on the work in 2006 by Alam and Massoud, to derive another different closed form solution to the problem of light scattered by the nanoshells using polynomials of up to order 6. Validation is made by comparing the present closed form solution to the exact Mie scattering solution and also to the other closed form solution by Alam and Massoud. The present work is found to be, however, more generalized and also more accurate for the coated spheres of either tiny/small or medium sizes than that of Alam and Massoud. Therefore, the derived formulas can be used for accurately characterizing both surface plasmon resonances of nanoparticles (of small sizes) or nano antenna near-field properties (of medium sizes comparable with half wavelength).
©2009 IEEE

Mode-selective surface-enhanced Raman spectroscopy (SERS) is demonstrated using plasmonic dipole antennas fabricated with electron beam lithography. An ∼10× change of the relative enhancement between two different Raman modes is observed when the resonance frequency of the plasmonic antennas is tuned over the Raman modes by varying the geometrical parameters of the antennas, i.e., changing their lengths or narrowing their feeding gaps. The comparison between the Rayleigh scattering spectra and the SERS spectra from the same individual plasmonic dipole antennas indicates that this mode-selective SERS phenomenon is a pure electromagnetic effect, providing a quantitative verification of the electromagnetic mechanism of SERS on a single nanoantenna level.
©2009 American Chemical Society

In this paper, we present the design, fabrication, and characterization of wire grid polarizers. These polarizers show high extinction ratios and high transmission with structure dimensions that are compatible with current complementary metal-oxide-semiconductor (CMOS) technology. To design these wire grids, we first analyze the transmission properties of single apertures. From the understanding of a single aperture, we apply a modal expansion method to model wire grids. The most promising grids are fabricated on both a glass substrate and CMOS photodiode. An extinction ratio higher than 200 is measured.
©2009 American Institute of Physics

We study the coupling of localized surface plasmons (LSP) and surface- plasmon polaritons (SPP) in a system composed of a metallic nanoparticle chain separated by a dielectric spacer from a thin metallic Film. The thickness of such a spacer determines the level of interaction between LSP and SPP modes, and controls the electromagnetic enhancement in this system. A dramatic enhancement with extremely narrow resonances can be observed for appropriate parameters. The high resonance quality factor and tunability of this system makes it a very promising candidate for bio-sensing and surface-enhanced spectroscopy applications.
©2009 Optical Society of America

Near field generated by plasmonic structures has recently been proposed to trap small objects. We report the first integration of plasmonic trapping with microfluidics for lab--on--a--chip applications. A three-layer plasmo-microfluidic chip is used to demonstrate the trapping of polystyrene spheres and yeast cells. This technique enables cell immobilization without the complex optics required for conventional optical tweezers. The benefits of such devices are optical simplicity, low power consumption and compactness; they have great potential for implementing novel functionalities for advanced manipulations and analytics in lab-on-a-chip applications.
©2009 Optical Society of America

Among the most popular approaches used for simulating plasmonic systems, the discrete dipole approximation suffers from poorly scaling volume discretization and limited near-field accuracy. We demonstrate that transformation to a surface integral formulation improves scalability and convergence and provides a flexible geometric approximation allowing e.g. to investigate the influence of fabrication accuracy. The occurring integrals can be solved quasi-analytically, permitting even rapidly changing fields to be determined arbitrarily close to a scatterer. This insight into the extreme near-field behavior is useful for modeling closely-packed particle ensembles and to study "hot spots" in plasmonic nanostructures used for plasmon-enhanced Raman scattering.
©2009 Optical Society of America

Nanostructures used in near field optics, material science, and biological applications can easily be realised using Focused Ion Beam (FIB) technique. In this work three applications developed in our laboratories are presented in order to highlight the versatility of FIB technique. Sophisticated nanostructures can be written in a single step into substrates without any need of masks, stamps or other additional means and, most notably, FIB technique enables postprocessing of prefabricated devices.
©2009 Carl Hans Verlag Munich

We investigate numerically the effect of a finite metal film thickness on the propagation characteristics of the channel plasmon polariton (CPP) and wedge plasmon polariton (WPP) modes, both in a symmetric and asymmetric environment. We observe that decreasing the metal thickness results in an improvement of the field localization near the groove tip and an increase of the losses for both types of mode. This behavior stems from the typical symmetric charge distribution of both modes across the metal film. When considering an asymmetric dielectric environment, the CPP mode is found to evolve into short range plasmon modes propagating along the groove walls, in contrast to the WPP mode which remains essentially confined at the tip apex. These results can be useful to tailor the properties of such plasmon modes, using the metal thickness as the variable parameter.
©2009 Optical Society of America

The optical properties of plasmonic dipole and bowtie antennas are investigated experimentally. The structures are fabricated using electron beam lithography allowing for controlled structure sizes down to 15\;nm. A home-built darkfield microscope is used to measure the scattering spectra of isolated antennas. The experimental results obtained from measurements on dipole and bowtie antennas are compared to numerical simulations. Furthermore extinction measurements are performed of dense antenna arrays to determine the influence of the antenna length and the antenna gap width on the antenna spectra.
©2009 American Institute of Physics

In a wider and wider range of research and engineering activities, there is a growing interest for full-field techniques, featuring nanometric sensitivities, and able to be addressed to dynamic behaviors characterization. Speckle interferometry (SI) techniques are acknowledged as good candidates to tackle this challenge. To get rid of the intrinsic correlation length limitation and simplify the unwrapping step, a straightforward approach lies in the pixel history analysis. The need of increasing performances in terms of accuracy and computation speed is permanently demanding new efficient processing techniques. We propose in this paper a fast implementation of the Empirical Mode Decomposition (EMD) to put the SI pixel signal in an appropriate shape for accurate phase computation. As one of the best way to perform it, the analytic method based on the Hilbert transform (HT) of the so-transformed signal will then be reviewed. For short evaluation, a zero-crossing technique which exploits directly the extrema finding step of the EMD will be presented. We propose moreover a technique to discard the under-modulated pixels which yield wrong phase evaluation. This work is actually an attempt to elaborate a phase extraction procedure which exploits all the reliable information in 3D, – two space and one time coordinates –, to endeavor to make the most of SI raw data.
©2009 American Optical Society

We study how retardation leads to interference effects in radiatively coupled plasmonic nanoparticles. We show that inclined illumination through a glass substrate on two plasmonic particles results in either an enhanced field or an attenuated field localized at the position of the first particle. Periodic intensity blinking of the first particle is observed as a function of the particle separation. This phenomenon is non-symmetric and almost no blinking is observed on the second particle. The effect is strongest when the illumination angle is chosen such that the optical retardation path in the substrate coincides with the particle distance. Implications of this plasmonic blinking for near-field measurements are discussed.
©2009 Optical Society of America

In many respects, speckle interferometry (SI) techniques are being considered as mature tools in the experimental mechanics circles. These techniques have enlarged considerably the field of optical metrology, featuring nanometric sensitivities in whole-field measurements of profile, shape and deformation of mechanical rough surfaces. Nonetheless, when we consider classical fringe processing techniques, e.g. phase-shifting methods, the deformation range is intrinsically limited to the correlation volume of the speckle field. In addition, the phase evaluation from such patterns is still computationally intensive, especially in the characterisation of dynamic regimes, for which there is a growing interest in a wide range of research and engineering activities. A promising approach lies in the pixel history analysis. We propose in this paper to implement the empirical mode decomposition (EMD) algorithm in a fast way, to put the pixel signal in an appropriate shape for accurate phase computation with the Hilbert transform.
©2008 Blackwell Publishing Ltd

We study in detail the optical forces generated by a plasmonic trap on a plasmonic nanoparticle. The permittivity of the trapped particle is tuned using a Drude model. The interplay between the plasmon resonances of the trap and of the particle can produce different regimes leading to attractive or repulsive forces. Hence a particle will be trapped or repulsed depending on its permittivity. Such a physical system should provide new functionalities for lab-on-the-chip applications.
©2008 Optical Society of America

In this paper, we study the multiple scattering by electrically small (the radius of the cylinder is much smaller than the wavelength) plasmonic nanocylinders near surface plasmon resonance. The cylinders are assumed to be identical in dimension and composition. The incident plane wave is assumed to be TE polarized so that the plasmon resonance of two-dimensional cylindrical structures (for both individual and group of cylinders) can be excited. It is found that multiple plasmonic cylinders enhance the near-field magnetic field intensity due to mutual coupling. When the electrical dimension q of the cylinders (q=k0R, where k0 is the wave number of the free space and R is the radius of the cylinder) is fixed, the magnitude of the field distribution primarily depends on the positions of the cylinders at normal incidence.
©2008 American Institute of Physics

An interactive Java applet for real-time simulation and visualization of the transmittance properties of multiple interference dielectric filters is presented. The most commonly used interference filters as well as the state-of-the-art ones are embedded in this platform-independent applet which can serve research and education purposes. The Transmittance applet can be freely downloaded from the site http://cpc.cs.qub.ac.uk.
©2008 Elsevier B.V.

We present an experimental and theoretical study on the optical properties of arrays of gold anoparticle in-tandem pairs (nanosandwiches). The well-ordered Au pairs with diameters down to 35 nm and separation distances down to 10 nm were fabricated using extreme ultraviolet (EUV) interference lithography. The strong near-field coupling of the nanoparticles leads to electric and magnetic resonances, which can be well reproduced by Finite-Difference Time-Domain (FDTD) calculations. The influence of the structural parameters, such as nanoparticle diameter and separation distance, on the hybridized modes is investigated. The energy and lifetimes of these modes are studied, providing valuable physical insight for the design of novel plasmonic structures and metamaterials.
©2008 Optical Society of America

We numerically study the effect of structural asymmetry in a plasmonic metamaterial made from gold nanowires. It is reported that optically inactive (i.e., optically dark) particle plasmon modes of the symmetric wire lattice are immediately coupled to the radiation field, when a broken structural symmetry is introduced. Such higher order plasmon resonances are characterized by their subradiant nature. They generally reveal long lifetimes and distinct absorption losses. It is shown that the near-field interaction strongly determines these modes.
©2008 American Chemical Society

The optical properties of plasmonic dipole and bowtie nanoan-tennas are investigated in detail using the Green's tensor technique. The influence of the geometrical parameters (antenna length, gap dimension and bow angle) on the antenna field enhancement and spectral response is discussed. Dipole and bowtie antennas confine the field in a volume well below the diffraction limit, defined by the gap dimensions. The dipole antenna produces a stronger field enhancement than the bowtie antenna for all investigated antenna geometries. This enhancement can reach three orders of magnitude for the smallest examined gap. Whereas the dipole antenna is monomode in the considered spectral range, the bowtie antenna exhibits multiple resonances. Furthermore, the sensitivity of the antennas to index changes of the environment and of the substrate is investigated in detail for biosensing applications; the bowtie antennas show slightly higher sensitivity than the dipole antenna.
©2008 Optical Society of America

The polarization sensitivity of optical resonant dipole antennas is investigated numerically using the Green’s tensor technique. The electric field-intensity in the feed-gap of the antenna is calculated as function of the polarization of the incident field. A simple analytical model is proposed that matches the numerical data very well. While a very strong polarization sensitivity can be achieved for specific wavelengths, our results also indicate that there are situations where the antenna is not sensitive at all to the polarization. The role played by different plasmon resonances in the system is illustrated.
©2008 European Optical Society

Amplitude and phase measurements of the near-field generated by isolated subwavelength apertures in a gold film are presented. The near-field distribution of such a structure is complex and the measured signal strongly depends on the electric field components effectively detected by the experimental setup.By comparing this signal with3Dvectorial calculations we are able to determine which electric field components are effectively measured. The sensitivity of the phase distribution is key to this measurement. The proposed characterization technique should prove extremely useful to calibrate a Scanning near-field optical microscopy (SNOM) beforehand in order to retrieve quantitative information on the polarization of the field distribution under study.
©2008 The Royal Microscopical Society

We demonstrate that it is possible to combine several small metallic particles in a very compact geometry without loss of their individual modal properties by adding a gold metallic film underneath. This film essentially acts as a ‘‘ground plane’’ which channels the optical field of each particle and decreases the interparticle coupling. The localization of the electric field can then be controlled temporally by illuminating the chain with a chirped pulse. The sign of the chirp controls the excitation sequence of the particles with great flexibility.
©2008 American Physical Society

Molecular fluorescence decay is significantly modified when the emitting molecule is located near a plasmonic structure. When the lateral sizes of such structures are reduced to nanometer-scale cross-sections, they can be used to accurately control and amplify the emission rate. In this contribution, we extend the Green dyadic method, to quantitatively investigate both radiative and non-radiative decay channels experienced by a single fluorescent molecule confined in an adjustable dielectric-metal nanogap. The technique produces data in excellent agreement with current experimental work.
©2008 American Physical Society

We report on the characterization of long-range surface plasmon waveguide bends at telecom wavelengths (λ=1550 nm). The structures consist of a thin Au stripe embedded in a transparent polymer film. When the polymer thickness is larger than the lateral extension of the plasmon, the stripe sustains a conventional long-range mode; in the opposite case, the mode is hybrid because its field distribution is confined by total internal reflection in the dielectric cladding. This hybridization increases the damping by absorption but dramatically reduces the radiation loss that occurs for curved geometries, such as bends. Our results are supported quantitatively by full-wave finite-element simulations.
©American Physical Society (2008)

We numerically study near-field induced coupling effects in metal nanowire-based composite nanostructures. Our multilayer system is composed of individual gold nanowires supporting localized particle plasmons at optical wavelengths, and a spatially separated homogeneous silver slab supporting delocalized surface plasmons. We show that the localized plasmon modes of the composite structure, forming so-called “magnetic atoms”, can be controlled over a large spectral range by changing the thickness of the nearby metal slab. The optical response of single wire and array-based metallic structures are compared. Spectral shifts due to wire-mirror interaction as well as the coupling between localized and delocalized surface plasmon modes in a magnetic photonic crystal are demonstrated. The presented effects are important for the optimization of metal-based nanodevices and may lead to the realization of metamaterials with novel plasmonic functionalities.
©2008 The Royal Microscopy Society

The surface polariton properties of TM or TE plane wave scattered by a coated cylinder are investigated in this paper. The coated cylinder (whose outer radius is much smaller than the wavelength) is assumed to be electrically small and low dissipative. Analytical formulas of the plasmonic resonances are derived and found to agree well with those obtained from exact expressions in the classical scattering theory. The behaviors of the scattering coefficients at resonances are also discussed and compared for different cases. While a single cylinder has the resonance at the relative permittivity of Ór = -1 (or relative permeability of ìr = -1) for the TE (or TM) polarization, the resonances of the coated cylinders change with different n values (where n denotes the series term or mode of the field), and also the inner and outer radii. It is shown that the scattered field in the near zone can be enhanced significantly compared to the incident wave. For the TE incident case, we take a silver coated nano-cylinder as an example to illuminate the near-field optical effect. Also, we have studied the peak values of the nth order scattered field for different n values and electrical parameter k0b (where k0 is the wavenumber of the free space and b denotes the outer radius of the cylinder) around the cylinder. The derived new formulas for total cross sections are given and they may provide us with some potential photonic applications such as surface cleaning and etching.
©2008 Optical Society of America

With its nearly 40 years of existence, speckle interferometry (SI) has become a complete technique, widely used in many branches of experimental mechanics. It is thus a challenging task to try to summarise in a couple of pages its principal characteristics from both theoretical and practical points of view. Admittedly, even though this goal is not met here, it appeared worth attempting to provide the photomechanics community with a discussion of the ins and outs of the technique. The necessity of a vocabulary free of ambiguity was a prerequisite, and hence the first section is a plea for a clearer definition of the discipline. Moreover, this section offers the opportunity to re-examine the basic aspects of SI. Then, the main features of the method are briefly considered following a strengths, weaknesses, opportunities and threats (SWOT) analysis. Endowed with a lot of specific advantages, compared with other whole-field methods, SI can play an increasing role in photomechanics.
©2008 Blackwell Publishing Ltd

We analyze the influence of near-field coupling on the formation of collective plasmon modes in a multilayer metallic nanowire array. It is shown that the spectral interference between super- and subradiant normal modes results in characteristic line shape modifications which are directly controlled by the spacing as well as the alignment of the stacked lattice planes. Moreover, a distinct near-field induced reversal of particle plasmon hybridization is reported. Our numerical findings are in excellent agreement with experimental results. [Note: This article has been selected for the December 3, 2007 issue of Virtual Journal of Nanoscale Science & Technology.]
©2007 The American Physical Society

We present a numerical study and analytical model of the optical near field diffracted in the vicinity of subwavelength grooves milled in silver surfaces. The Green’s tensor approach permits the computation of the phase and amplitude dependence of the diffracted wave as a function of the groove geometry. It is shown that the field diffracted along the interface by the groove is equivalent to replacing the groove by an oscillating dipolar line source. An analytic expression is derived from the Green’s function formalism, which reproduces well the asymptotic surface plasmon polariton
SPP wave as well as the transient surface wave in the near zone close to the groove. The agreement between this model and the full simulation is very good, showing that the transient “near-zone” regime does not depend on the precise shape of the groove. Finally, it is shown that a composite diffractive evanescent wave model that includes the asymptotic SPP can describe the wavelength evolution in this transient near zone. Such a semianalytical model may be useful for the design and optimization of more elaborate photonic circuits, whose behavior in a large part will be controlled by surface waves.
©2007 American Physical Society

We perform rigorous simulations of hybrid long-range modes guided by a central metal core and a twodimensional dielectric slab. We show that these modes are subject to fewer limitations than conventional long-range plasmon modes in terms of field confinement and guiding performance. These hybrid modes may offer substantial improvements for integrated plasmonic components, as illustrated here by the consideration of 90° bends.
©2007 Optical Society of America

The new optical concepts currently developed in the research field of plasmonics can have significant practical applications for integrated optical device miniaturization as well as for molecular sensing applications. Particularly, these new devices can offer interesting opportunities for optical addressing of quantum systems. In this article, we develop a realistic model able to explore the various functionalities of a plasmon device connected to a single fluorescing molecule. We show that this theoretical method provides a useful framework to understand how quantum and plasmonic entities interact in a small area. Thus, the fluorescence signal evolution from excitation control to relaxation control depending on the incident light power is clearly observed.
©2007 American Institute of Physics

We study the forces generated by an electromagnetic field on two coupled gold nanowires at the vicinity of the plasmon resonance wavelength. Two different regimes are observed, depending on the separation distance d between the wires. In the near field coupling regime, both attractive and repulsive forces can be generated, depending on d and the illumination wavelength. Furthermore, at the plasmon resonance, it is possible to create forces 100 times larger than the radiation pressure. In the far field coupling regime, both particles are pushed by the incident field. However, the force amplitude applied on each wire is modulated as a function of d , even for large separations. This indicates that the system behaves like a cavity and pseudo Fabry-Perot modes can be excited between the particles. The interaction of these modes with the plasmon resonances of the nanowires, determines the forces on the particles. Around the plasmon resonance wavelength, when the cavity is tuned to the incident light, forces are close to the average value corresponding to the radiation pressure of the incident field. On the other hand, when the cavity is detuned, the particles are retained or pushed anti-symmetrically. We finally study the forces applied on these nanowires in the centre of mass reference frame (CMRF) for the far field coupling regime. For any separation distance d we observe equilibrium positions in the CMRF for at least one illumination wavelength. The stability of these equilibrium positions is discussed in detail.
©2007 Optical Society of America

The tunneling of surface plasmon-polaritons (SPPs) across an interruption in the metallic film supporting them is numerically investigated in details. Both non-symmetrical and symmetrical geometries are considered. A very high tunneling efficiency is calculated for the long-range surface plasmon in the symmetrical geometry, with an amplitude transmission as high as 80% over a 5 ìm gap for a 40 nm thick gold film illuminated at ë=785nm. The transmission is somewhat lower in the nonsymmetrical geometry. The coupling between the different SPP modes (radiative and non-radiative) in that geometry is also investigated in detail. This coupling depends periodically upon the length of the gap. Overall, the results indicate that SPPs are not very sensitive to technological imperfections and can survive large waveguide interruptions.
©2007 Optical Society of America

The excitation of a surface plasmon polariton (SPP) wave on a metal-air interface by a diffraction grating under monochromatic normal illumination is investigated numerically. The influence of the different experimental parameters (grating thickness, period, and duty cycle) is discussed in detail for a semi-infinite metal and a thin film. Both engraved (grooves) and deposited (protrusions) gratings are considered. The most efficient coupling to the SPP is obtained for a groove grating which duty cycle is about 0.5. Furthermore a small grating depth of some tens of nanometers is sufficient to excite a SPP mode with a coupling efficiency higher than 16% in each direction. Implications for practical SPP experiments are discussed.
©2006 American Institute of Physics

The interplay between localized surface plasmon (LSP) and surface plasmon-polariton (SPP) is studied in detail in a system composed of a three-dimensional gold particle located at a short distance from a gold thin film. Important frequency shifts of the LSP associated with the particle are observed for spacing distances between 0 and 50 nm. Beyond this distance the LSP and SPP resonances overlap, although some cavity effects between the particle and the film can still be observed. In particular, when the spacing increases the field in the cavity decreases more slowly than one would expect from a simple image dipole interpretation. For short separations the coupling between the particle and the film can produce a dramatic enhancement of the electromagnetic field in the space between them, where the electric field intensity can reach 5000 times that of the illumination field. Several movies show the spectral and time evolutions of the field distribution in the system both in and out of resonance. The character of the different modes excited in the system is studied. They include dipolar and quadrupolar modes, the latter exhibiting essentially a magnetic response.
©2006 Optical Society of America

Suitably shaped metal nanostructures act as resonant optical antennas that efficiently collect light and confine it to a subwavelength volume. Vice versa, light emission from nano volumes can be enhanced by coupling to antenna structures. We give a short introduction to antenna theory and discuss recent experiments that show the feasibility of achieving strong field enhancement using resonant dipole antennas for near infrared wavelengths. By scanning an optical antenna fabricated at the apex of an aFM tip over individual quantum dots, we observe enhanced emission of the latter while it is in close proximity of the antenna feed gap. Resonant optical antennas hold promise to be applied for spectroscopic characterization of nano structures with high spatial resolutions and single-molecule sensitivity.
©2006 Schweizerische Chemische Gesellschaft

We present a numerical study of the tunability properties of a plasmonic subwavelength particle deposited on a metallic slab. The particle is composed of a metallic part, supporting a localized plasmon mode, separated from the slab by a dielectric spacer. It is shown that the position of the resonance wavelength can be modified over a large spectral range by changing either the spacer thickness by a few tens of nanometers or its susceptibility within the range of usual dielectrics. A linear relation is observed between the resonance wavelength and the spacer permittivity.
©2006 Optical Society of America

We experimentally and theoretically study the controlled coupling between localized and delocalized surface plasmon modes supported by a multilayer metallic photonic crystal slab. The model system to visualize the interaction phenomena consists of a gold nanowire grating and a spatially separated homogeneous silver film. We show that plasmon-plasmon coupling leads to drastic modification of the optical properties in dependence on the chosen geometrical parameters. Strong coupling and plasmon hybridization can be clearly observed. The numerical calculations reveal excellent agreement with the experiments.
©2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

For the observation of single molecule dynamics with fluorescence fluctuation spectroscopy (FFS) very low fluorophore concentrations are necessary. For in vitro measurements, this requirement is easy to fulfill. In biology however, micromolar concentrations are often encountered and may pose a real challenge to conventional FFS methods based on confocal instrumentation. We show a higher confinement of the sampling volume in the near-field of sub-wavelength sized apertures in a thin gold film. The gold apertures have been measured and characterized with fluorescence correlation spectroscopy (FCS), indicating light confinement beyond the far-field diffraction limit.We measured a reduction of the effective sampling volume by an order of magnitude compared to confocal instrumentation.
©2006 Optical Society of America

We address the general issue of resolving the wave vector in complex electromagnetic media including negative refractive media. This requires us to make a physical choice of the sign of a square root imposed merely by conditions of causality. By considering the analytic behavior of the wave vector in the complex plane, it is shown that there are a total of eight physically distinct cases in the four quadrants of two Riemann sheets.
©2005 Optical Society of America

Metallic Nanostructures are giving rise to a great deal of attention from a broad scientific community, ranging from physicist and electrical engineers to biologists. The interest is growing rapidly in finding novel devices for future applications that allow using metallic waveguides for optical signal transmission and processing. In this contribution, we investigate some of the fundamental phenomena that take place in these systems. Also the extraordinary transmission of light though sub-wavelength holes in a metal is investigated, keeping in mind various potential biophotonics applications. In this paper, we demonstrate an optical nano-imaging technique that is particularly well suited to characterize the near-field interaction of light with metallic nanostructures: coherent near-field microscopy. This technique allows the total characterization of the near-field by giving full access to its amplitude and its phase. Its application to the characterization and study of plasmonic nanostructures is illustrated using several systems, the coherent near-field optical measurements of light transmission though sub-wavelength holes drilled in a gold thin film and surface plasmons propagating on a metal film and its interaction at a metal-air interface.
©2005 SPIE

The numerical study of plasmonic optical objects is of great importance in the context of massive integration of light processing devices on a very small surface. A wide range of nanoobjects are currently under study in the scientific community like stripe waveguides, Bragg’s mirrors, resonators, couplers or filters. One important step is the efficient coupling of a macroscopic external field into a nanodevice, that is the injection of light into a subwavelength metallic waveguide. In this article we highlight the problem of the excitation of a surface plasmon polariton wave on a gold-air interface by a diffraction grating. Our calculations are performed using the Green’s function formalism. This formalism allows us to calculate the field diffracted by any structure deposited on the surface of a prism, or a multilayered system, for a wide range of illumination fields (plane wave, dipolar field, focused gaussian beam, ...). In the first part we optimize a finite grating made of simple objects deposited on or engaved in the metal with respect to the geometrical parameters. In order to optimize the performances of this device, we propose to use a pattern of resonant particles studied in the second part, and show that a composite dielectric/metallic particle can resonate in presence of a metallic surface and can be tuned to a specific wavelength window by changing the dielectric part thickness.
©2005 SPIE

We have fabricated nanometer-scale gold dipole antennas designed to be resonant at optical frequencies. On resonance, strong field enhancement in the antenna feed gap leads to white-light supercontinuum generation. The antenna length at resonance is considerably shorter than one-half the wavelength of the incident light. This is in contradiction to classical antenna theory but in qualitative accordance with computer simulations that take into account the finite metallic conductivity at optical frequencies. Because optical antennas link propagating radiation and confined/enhanced optical fields, they should find applications in optical characterization, manipulation of nanostructures, and optical information processing.
©2005 American Association for the Advancement of Science

By combining the field-susceptibility technique with the optical Bloch equations, a general formalism is developed for the investigation of molecular photophysical phenomena triggered by nanometer scale optical fields in the presence of complex environments. This formalism illustrate the influence of the illumination regime on the fluorescence signal emitted by a single molecule in a complex environment. In the saturated case, this signal is proportional to the optical local density of states, while it is proportional to the near-field intensity in the non-saturated case.
©2005 Elsevier B.V.

A classical electromagnetic calculation of the lifetime of an emitting electric dipole near a material slab is presented. The lifetime is deduced from the imaginary part of the electric field Green's tensor associated with the stratified medium. The method is applied not only to the well known case of metallic reflectors, but also to magnetic reflectors and to negative refractive index slabs. The frequency dependence of the nonradiative decay rate at small distances is analyzed and interpreted in terms of the surface polariton modes of the slab.
©2004 American Institute of Physics

The utilization of the Green’s tensor associated with a complex optical background (surface, cavity or stratified medium) leads to a dramatic reduction of the computation effort associated with scattering calculations in that background. This approach is illustrated with examples where a mere change of the background Green’s tensor makes possible the investigation of completely different physical situations. Two different discretization approaches are compared and similarities between the Green’s tensor technique and the Method of Lines are emphasized; in the latter, the utilization of analytic solutions in one specific direction also reduces the discretization of the system.
©2004 Elsevier

The accepted model for light emission and propagation in organic LEDs (OLED) which consists of several optically thin functional layers deposited on a thick substrate is a classical dipole located in the emitting layer. The propagation of the emitted light is commonly described by a Fourier expansion of the dipole field into plane waves which represent the various radiating and bound modes of the layered structure in k-space. To calculate the electric and magnetic fields inside and outside the LED an integration over the individual plane waves has to be performed. This entails numerical difficulties which can be overcome elegantly with the so-called Green’s tensor approach for stratified media recently developed by the second author. In our contribution we demonstrate the applicability of this method to the computation of electromagnetic field distributions in organic LED structures. Visualizations of typical field distributions arising from individual dipoles are presented and discussed thus allowing a more intuitive understanding of effects relating to dipole location and orientation and material absorption. Furthermore it is shown that scattering of bound modes by particle like inhomogeneities of the layer structure can be effectively modelled with the Green’s tensor approach. Visualizations are presented and discussed with regard to increased light extraction.
©2004 SPIE

We investigate numerically the spectrum of plasmon resonances for metallic nanowires with a non–regular cross-section in the 20–50 nm range. After briefly recalling the physical properties of metals at optical frequencies, we point out the intrinsic difficulties in the computation of the plasmon resonances for nanoparticles with a non–regular shape. We then consider the resonance spectra corresponding to nanowires whose cross-sections form different simplexes. The number of resonances strongly increases when the section symmetry decreases: A cylindrical wire exhibits one resonance, whereas we observe more than 5 distinct resonances for a triangular particle. The spectral range covered by these different resonances becomes very large, giving to the particle specific distinct colors. At the resonance, dramatic field enhancement is observed at the vicinity of non–regular particles, where the field amplitude can reach several hundred times that of the illumination field. This near–field enhancement corresponds to surface enhanced Raman scattering (SERS) enhancement locally in excess of 1012. The distance dependence of this enhancement is investigated and we show that it depends on the plasmon resonance excited in the particle, i.e. on the illumination wavelength. The average Raman enhancement for molecules distributed on the entire particle surface is also computed and discussed in the context of experiments in which large numbers of molecules are used. Finally we discuss the influence of the model permittivity which enters the calculation, as well as the resonances shift and broadening produced by a water background.
©2003 Springer Verlag

A novel local illumination scheme for optical lithography is proposed. It is based on the excitation of a surface plasmon on a metal film incorporated into a polymer light coupling mask for contact lithography. The electromagnetic field associated with the surface plasmon generates illumination volumes in the photoresist which are not limited by the diffraction (or Rayleigh) limit. Computer simulations indicate that the replication of 20 nm features using 630 nm illumination wavelength can be achieved with this technique.
©2003 Elsevier Science B.V.

©2003 American Physical Society

We study experimentally the response of three dimensional arrays of microscopic wires. Very good agreement is found with previous theoretical work indicating that such a system can be considered as an effective plasmonic medium with a specific plasma frequency. The sample size threshold where this effective behavior appears is shown to be relatively small.
©2002 American Institute of Physics

We study experimentally and numerically the electromagnetic resonances in split ring resonators (SRRs), around 1 GHz. For an individual SRR, we show that both electric and magnetic fields can induce resonances, the magnetic one being the strongest. The utilization of such resonant structures as efficient microwave filter is also demonstrated. The coupling between two or more SRRs can be quite complex and strongly depends on their geometrical arrangement. For small separation distances, very strong coupling, leading to sharp resonances with high quality factors are observed. In that case a magnetic field circulation which connects neighboring elements is established. The practical implications of these results for the fabrication of a left-handed metamaterial are discussed.
©2002 American Institute of Physics

We study experimentally and numerically a new three-dimensional magnetic resonator structure with high isotropy, so that its response becomes independent of the illumination direction in a specific plane. The utilisation of such elements to build a finite left--handed medium is discussed, as well as the physical constraints towards the realisation of a fully isotropic structure.
©2002 American Institute of Physics

We use the Green's tensor technique to study the optical processes taking place in configurations typically used for the replication and characterization of nanostructures. For the replication process we investigate light-coupling masks for optical contact lithography and for the characterization process the mode scattered by a defect or a short grating in a planar waveguide. Both configurations consist of structures embedded in a stratified background composed of a stack of material layers with different permittivities. We perform calculations for two-dimensional and three-dimensional structures and compare their optical behavior. Our results show that the additional material interfaces in three-dimensional systems can lead to significantly different field distributions and must be taken into account for a complete understanding of the electromagnetic properties of the systems.
©2003 American Geophysical Union

We describe a library to compute various types of Green's tensor for three dimensional electromagnetic scattering calculations. This library includes the retarded and non-retarded (quasi-static) Green's tensors for infinite homogeneous space and the non-retarded Green's tensor associated with a surface. Both standard and filtered Green's tensor can be computed. Filtered Green's tensor can be used to accurately investigate high permittivity scatterers with the coupled-dipole approximation.
©2002 Elsevier Science B.V.

We study the influence of metal roughness on the near-filed distribution generated by an aperture or an apertureless (scattering) probe. Different experimental parameters are investigated: roughness magnitude, aperture form, distribution of the roughness. Our results show that aluminum roughness has a dramatic impact on the emission characteristics of a near-field probe and in particular on its polarization sensitivity. Apertureless or scattering probes appear to be less sensitive to roughness and to provide a well confined field even with a somewhat rough probe.
©2002 The Royal Microscopical Society

We investigate numerically the spectrum of plasmon resonances for metallic nanowires with a non-regular cross-section, in the 20-50 nm range. We first consider the resonance spectra corresponding to nanowires whose cross-sections form different simplexes. The number of resonances strongly increases when the section symmetry decreases: A cylindrical wire exhibits one resonance, whereas we observe more than 5 distinct resonances for a triangular particle. The spectral range covered by these different resonances becomes very large, giving to the particle specific distinct colors. At the resonance, dramatic field enhancement is observed at the vicinity of non-regular particles, where the field amplitude can reach several hundred times that of the illumination field. This near-field enhancement corresponds to surface enhanced Raman (SERS) enhancement in excess of 10^(12). The distance dependence of this enhancement is investigated and we show that it depends on the plasmon resonance excited in the particle, i.e. on the illumination wavelength.
©2001 The American Physical Society

We present numerical experiments of light scattering by a circular dielectric cylinder embedded in a stratified background, using the Green's tensor technique. The stratified background consists of two or three dielectric layers, the latter forming an anti-reflection system. We show movies of the scattered field as a function of different parameters: polarization, angle of incidence, and relative position of the cylinder with respect to the background interfaces.
©2001 Optical Society of America

This paper is a contribution to the study of the optical properties of high permittivity nanostructures deposited on surfaces. We present a new computational technique derived from the coupled dipole approximation (CDA), which can accommodate high permittivity scatterers. The discretized CDA equations are reformulated using the sampling theory to overcome different sources of inaccuracy that arise for high permittivity scatterers. We first give the non-retarded filtered surface Green's tensor used in the new scheme. We then assess the accuracy of the technique by comparing it to the standard CDA approach and show that it can accurately handle scatterers with a large permittivity.
©2001 Optical Society of America

We compare three different approaches to high-resolution contact lithography with special emphasis on contrast mechanisms for subwavelength structures. Masks with protruding metal absorbers, masks with absorbers embedded in the transparent background, and masks with air gaps and recessed absorbers are studied. Using the Green's tensor technique we compute the light intensity distribution in the photoresist. The intensity and contrast functions are investigated for different mask geometries (absorber thickness, height of protruding elements), and the difference between chrome and gold as absorber material is discussed. Our results show that embedding the absorbers in a transparent mask material enhances the transmitted intensity and the contrast compared with a mask having protruding metal absorbers. A further improvement is achieved by a topographically patterned mask with air gaps and recessed absorbers. Optimized mask dimensions can be found for which the contrast and the depth of focus are increased.
©2001 Elsevier Science B.V.

We investigate the plasmon resonances for silver nanowires with a non-regular cross section. To study the relationship between the cross section and the spectrum of plasmon resonances, we consider cross sections evolving from a rectangular shape to a triangular one. In particular, we study the influence of the sharpness of the corner on the near field enhancement at the vicinity of the particle and discuss its implications for surface enhanced Raman scattering (SERS). We also investigate the influence of the absorption on the plasmon resonances spectrum and on the near field enhancement.
©2001 The American Physical Society

We study the coupling induced by retardation effects when two plasmon resonant nanoparticles are interacting. This coupling leads to an additional resonance which strength depends on a subtle balance between particles separation and sizes. The scattering cross section and the near-field associated with this coupled resonance are studied for cylindrical particles in air and in water. Implications for surface enhanced Raman scattering and nano-optics are discussed.
©2001 Optical Society of America

We investigate numerically the plasmon resonances of 10-50 (nm) nanowires with a non-elliptical section. Such wires have a much more complex behavior than elliptical wires and their resonances span a larger frequency range. The field distribution at the surface of these wires exhibits a dramatic enhancement, up to several hundred times the incident field amplitude. These strongly localized fields can provide an important mechanism for surface enhanced Raman scattering (SERS).
©2001 Elsevier Science B.V.

We study the interaction of a mode propagating in a planar waveguide with a three-dimensional rectangular defect (protrusion or notch) in the structure. The scattering by the defect disturbes the propagation of the mode and light is coupled out of the waveguide. To investigate these phenomena we compute electric field distributions with the Green's tensor technique and show movies with varying defect geometries and different mode polarizations. These calculations should be useful for optimizing specific elements in complex photonic circuits.
©2001 Optical Society of America

We investigate the plasmon resonances of interacting silver nanowires with a 50 nm diameter. Both non-touching and intersecting configurations are investigated. While individual cylinders exhibit a single plasmon resonance, we observe much more complex spectra of resonances for interacting structures. The number and magnitude of the different resonances depend on the illumination direction and on the distance between the particles. For very small separations, we observe a dramatic field enhancement between the particles, where the electric field amplitude reaches a hundredfold of the illumination. A similar enhancement is observed in the grooves created in slightly intersecting particles. The topology of these different resonances is related to the induced polarization charges. The implication of these results to surface enhanced Raman scattering (SERS) are discussed.
©2001 Optical Society of America

We present an accurate and self-consistent technique for computing the electromagnetic field in scattering structures formed by bodies embedded in a stratified background and extending infinitely in one direction (two-dimensional geometry). With this fully vectorial approach based on the Green's tensor associated with the background only the embedded scatterers must be discretized, the entire stratified background being accounted for by the Green's tensor. We derive the formulas for the computation of this dyadic and discuss in detail its physical substance. The utilization of this technique for the solution of scattering problems in complex structures is then illustrated with examples from photonic integrated circuits (waveguide grating couplers with varying periodicity). A further advantage of this approach lies in the fact that the boundary conditions at the edges of the computation window and at the different material interfaces are automatically and perfectly fulfilled.
©2001 The American Physical Society

We present a new technique for computing the electromagnetic field propagating and scattered in three-dimensional structures formed by bodies embedded in a stratified background. This fully vectorial technique is based on the Green's tensor associated with the stratified background. Its advantage lies in the fact that only the scatterers must be discretized, the stratified background being accounted for in the Green's tensor. Further, the boundary conditions at the different material interfaces, as well as at the edges of the computation window are perfectly and automatically fulfilled. Several examples illustrate the utilization of the technique for the modeling of photonic circuits (integrated optical waveguides), the study of the optics of metal (surface plasmons), and the development of new optical lithography techniques.
©2001 Optical Society of America

We study numerically two-dimensional nanoparticles with a non-regular shape and demonstrate that these particles can support many more plasmon resonances than a particle with a regular shape (e.g. an ellipse). The electric field distributions associated with these different resonances are investigated in detail in the context of near-field microscopy. Depending on the particle shape, extremely strong and localized near-fields, with intensity larger than 10^5 that of the illumination wave, can be generated. We also discuss the spectral dependence of these near-fields and show that different spatial distributions are observed, depending which plasmon resonance is excited in the particle.
©2001 The Royal Microscopical Society

We present a three-dimensional technique for computing light scattering and propagation in complex structures formed by scatterers embedded in a stratified background. This approach relies on the Green's tensor associated with the background and requires only the discretization of the scatterers, the entire stratified background being accounted for in the Green's tensor. Further, the boundary conditions at the edges of the computation window and at the different material interfaces in the stratified background are automatically fulfilled. Different examples illustrate the application of the technique to the modeling of photonic integrated circuits: waveguides with protrusions (single element "grating") and notches. Subtle effects, like polarization cross-talks in an integrated optics device are also investigated.
©2001 Kluwer Academic Publishers

We present a formalism based on the method of moment to solve the volume integral equation using tetrahedral (3D) and triangular (2D) elements. We introduce a regularization scheme to handle the strong singularity of the Green's tensor. This regularization scheme is extended to neighboring elements, which dramatically improves the accuracy and the convergence of the technique. Scattering by high permittivity scatterers, like semiconductors, can be accurately computed. Furthermore, plasmon-polariton resonances in dispersive materials can also be reproduced.
©2000 IEEE

We study the plasmon resonances for small two-dimensional silver particles (nanowires) with elliptical or triangular shapes in the 20 (nm) size range. While the elliptical particle has only two resonances, a well known fact, we demonstrate that the triangular particle displays a much more complex behavior with several resonances over a broad wavelength range. Using animations of the field amplitude and field polarization, we investigate the properties of these different resonances. The field distribution associated with each plasmon resonance can be related to the polarization charges on the surface of the particles. Implications for the design of plasmon resonant structures with specific properties, e.g., for nano-optics or surface enhanced Raman scattering (SERS) are discussed.
©2000 IOP Publishing and Deutsche Physikalische Gesellschaft

We present a technique for the computation of the Green's tensor in three-dimensional stratified media composed of an arbitrary number of layers with different permittivities and permeabilities (including metals with a complex permittivity). The practical implementation of this technique is discussed in detail. In particular we show how to efficiently handle the singularities occurring in Sommerfeld integrals, by deforming the integration path in the complex plane. Examples assess the accuracy of this approach and illustrate the physical properties of the Green's tensor, which represents the field radiated by three orthogonal dipoles embedded in the multilayered medium.
©2000 The American Physical Society

This work gives a detailed derivation of the non-retarded dyadic Green's tensor associated with surfaces in the quasi-static approximation. The derivation is made from a rigorous model where the dyadic is expressed as Sommerfeld integrals. We then assess the domain where this approximation can be used for scattering calculations on surfaces by comparing rigorous and non-retarded solutions. Implications of this work for scattering calculations in near-field optics are finally discussed.
©Elsevier Science B.V.

We study the plasmon resonances of 10 (nm)-100 (nm) two-dimensional metal particles with a non-regular shape. Movies illustrate the spectral response of such particles in the optical range. Contrary to particles with a simple shape (cylinder, ellipse) non-regular particles exhibit many distinct resonances over a large spectral range. At resonance frequencies, extremely large enhancements of the electromagnetic fields occur near the surface of the particle, with amplitudes several hundred-fold that of the incident field. Implications of these strong and localized fields for nano-optics and surface enhanced Raman scattering (SERS) are also discussed.
©2000 Optical Society of America

In this review we describe fundamentals of scanning near-field optical microscopy with aperture probes. After the discussion of instrumentation and probe fabrication, aspects of light propagation in metal-coated, tapered optical fibers are considered. This includes transmission properties and field distributions in the vicinity of subwavelength apertures. Furthermore, the near-field optical image formation mechanism is analyzed with special emphasis on potential sources of artifacts. To underline the prospects of the technique, selected applications like amplitude and phase contrast, fluorescence imaging and Raman spectroscopy as well as near-field optical desorption are presented. This demostrates that scanning near-field optical microscopy is no longer an exotic method but has matured into a valuable tool.
©2000 American Institute of Physics

We discuss the potential and limitations of light-coupling masks for high-resolution subwavelength optical lithography. Using a three-dimensional fully-vectorial numerical approach based on Green's tensor technique, the near-field distribution of the electric field in the photoresist is calculated. We study the dependence of the illuminating light and the angle of incidence on polarization. Furthermore, we investigate the replication of structures of various sizes and separations. It is predicted that the formation of features in the 60 [nm] range is possible using light with a 248 [nm] wavelength. However, with decreasing separation among the features, crosstalk limits the ultimate resolution.
©1999 American Vacuum Society

We present three-dimensional simulations of the image formation process in near-field optical microscopy. Our calculations take into account the different components of a realistic experiment: an extended metal coated tip, a subwavelength sample and its substrate. We investigate all possible detection (transmitted, reflected and collected field) and scanning (constant height, constant gap) modes. Our results emphasize the strong influence of the tip motion on the experimental signal. They also show that it is possible, by controlling the polarization of both the illumination and the detected field, to strongly reduce these artifacts.
©1999 The Royal Microscopical Society

We investigate the properties of photonic band gap structures of finite size and arbitrary geometry using the density of states deduced from scattering calculations. We first demonstrate this procedure on a finite 2D array of cylinders, and then study at optical frequencies a system formed by a finite array of finite height cylinders positioned on a substrate and illuminated with an evanescent field.
©1999 The American Physical Society

We illustrate the propagation of light in a new type of coupling mask for lensless optical lithography. Our investigation shows how the different elements comprising such masks contribute to the definition of an optical path that allows the exposure of features in the 100-nm-size range in the photoresist.
©1998 Optical Society of America

We describe an approach to optical lithography using light-scattering contact masks with protruding elements that couple light into a photoresist. This method differs from conventional contact lithography in two important ways. First, because portions of the light-coupling mask (LCM) are made from a polymer, intimate contact with the resist occurs over large areas without additional load. This contact is readily reversible and causes no observable damage or contamination to the LCM or substrate. Second, the structure formed by the protruding parts of the LCM in contact with the resist defines local optical modes that impart directionality to the light propagating through the LCM and amplify its intensity. We provide experimental realization and theoretical description of the method, demonstrating its use in the formation of 100 nm features with light of 256 nm.
©1998 American Vacuum Society

We develop a fully vectorial formalism for the investigation of electromagnetic scattering in polarizable backgrounds, i.e. where the scatterers are not in vacuum but situated in a medium with a dielectric permittivity different from unity. Our approach is based on the Green's tensor technique and the corresponding Green's tensors for two-dimensional (2D) and three-dimensional (3D) systems are developed. The analysis of 2D systems is not restricted to the case where transverse electric (TE) and transverse magnetic (TM) modes are decoupled, but treated in a general manner. Practical examples illustrate the application of the method: scattering by a microcavity for 2D and color formation in opal for 3D.
©1998 The American Physical Society

We show that it is possible to increase the performance of the coupled-dipole approximation (CDA) for scattering by using concepts from the sampling theory. In standard CDA the source in each discretized cell is represented by a point dipole and the corresponding scattered field given by Green's tensor. In the present approach, the source has a certain spatial extension and the corresponding Green's tensor must be redefined. We derive these so-called filtered Green's tensors for 1D, 2D and 3D systems; which forms the basis of our new scheme: the filtered coupled-dipole technique (FCD). By reducing the aliasing phenomena related to the discretization of the scatterer, we obtain with FCD a more accurate description of the original scatterer. The convergence and accuracy of FCD is assessed for 1D, 2D and 3D systems and compared to CDA results. In particular we show that, for a given discretization grid, the scattering cross-section obtained with FCD is more accurate (up to a factor of 100). Furthermore, the computational effort required by FCD is similar to that of CDA.
©1998 IEEE

Light-coupling masks (LCMs) based on structured organic polymers that make conformal contact with a substrate can constitute an amplitude mask for light-based lithographies. The LCM is exposed through its backside, from where the light is differentially guided by the structures towards the substrate. Images of arbitrarily shaped features having dimensions much smaller than that of the vacuum wavelength of the exposing light are formed in the resist in a 1:1 correspondence to their size in light-guiding portions of the mask. LCMs allow pattern replication at high resolution and densities over large areas in photoresist without the need for elaborate projection optics.
©1998 American Institute of Physics

This paper presents an extension of the generalized multipole technique (GMT) for 2D anisotropic scatterers. New expansions similar to the Bessel multipole expansion are derived for arbitrary anisotropic media. Numerical simulations prove the accuracy and the rapid convergence of these expansions. As the results obtained are extremely accurate, this technique is most helpful for the evaluation of reference solutions and for the understanding of the physical interaction of light with arbitrary anisotropic media.
©Elsevier Science B.V.

New expansions are derived for the simulation of three-dimensional anisotropic scatterers with the generalized multipole technique (GMT). This extension of the GMT makes possible the investigation of subtle phenomena such as the interaction of light with realistic crystals or magneto-optic materials.
©1998 Optical Society of America

Scanning near-field optical microscopy (SNOM) is an optical microscopy whose resolution is not bound to the diffraction limit. It provides chemical information based upon spectral, polarization and/or fluorescence contrast images. Details as small as 20nm can be recognized. Photophysical and photochemical effects can be studied with SNOM on a similar scale. This article reviews a good deal of the experimental and theoretical work on SNOM in Switzerland.
©1997 Neue Schweizerische Chemische Gesellschaft

Recently, local probes used in optical experiments added a new dimension to the study of the optical properties of small particles lying on a surface. Until now, several theoretical frameworks, developped to understand the interaction of optical fields with mesoscopic and nanoscopic objects, emphasized mainly the prediction of the electric near-field distributions generated by these structures. This paper demonstrates how such subwavelength dielectric surface structures also produce a particular confinement of the optical magnetic near-field when the sample is illuminated by a surface wave.
©1997 The American Physical Society

We show that strong optical field gradients can be created at the tip apex of a local probe microscope illuminated by an external light source. We demonstrate that these confined fields can be easily, precisely and continuously tuned by changing the polarization and the incidence of the external field. We also investigate the topology of the field intensity in the tip-surface junction.
©1997 American Institute of Physics

This paper discusses recent theoretical efforts to develop a general and flexible method for the calculation of the field distributions around and inside complex optical systems involving both dielectric and metallic materials. Starting from the usual light-matter coupling Hamiltonian, we derive a self-consistent equation for the optical field in arbitrary optical systems composed of N different subdomains. We show that an appropriate solving procedure based on the real-space discretization of each subdomain raises the present approach to the rank of an accurate predictive numerical scheme. In order to illustrate its applicability, we use this formalism to address challenging problems related to nonradiative energy transfers in near-field optics. In particular, we investigate in detail the detuning of a microresonator probed by a near-field optical probe.
©1996 American Physical Society

Some 15 years ago, optical topographic signals with subwavelength resolution were obtained independently by several experimental teams. Since this exploratory period, a growing number of experimental configurations have been proposed and continuously developed. Simultaneously, this research field was supported by different theoretical works, aimed at developing our understanding of the interaction of optical fields with mesoscopic objects. Over the past three years, several theoretical frameworks have been proposed (Green's functions, field susceptibility, boundary conditions methods, multiple multipoles expansions, etc.). In this paper, an attempt at a careful comparison between two classes of numerical models is presented. Using the same test object, we discuss and compare the numerical solutions issued from a reciprocal-space perturbative method (Rayleigh approximation) and the solution originating from a direct-space integral approach (Green's function or field susceptibility). The discussion is given for different values of the relevant experimental parameters. The convergence of both approaches is investigated.
©1996 American Physical Society

Using a fully vectorial three-dimensional numerical approach (generalized field propagator, based on the Green's tensor technique), we investigate the near-field images produced by subwavelength objects buried in a dielectric surface. We study the influence of the object index, size and depth on the near-field. We emphasize the similarity between the near-field spawned by an object buried in the surface (dielectric contrast) and that spawned by a protrusion on the surface (topographic contrast). We show that a buried object with a negative dielectric contrast (i.e., with a smaller index than its surrounding medium) produces the reversed near-field image from that of an object with a positive contrast.
©1996 Optical society of America

©(1996) American Physical Society

We present a new practical scheme to study the spectroscopic properties of molecules embedded in optically complex surroundings. The response function accounting for the modification of the spectroscopic behavior of the molecules is derived self-consistently in direct space through the numerical solution of a Dyson's equation. We apply this scheme to investigate near-field optical effects due to fluorescence phenomena. Experimentally relevant examples show that the dramatic decay of the molecular lifetime upon approaching a surface defect could achieve well resolved imaging of subwavelength structures.
©1995 The American Physical Society

The design of tips is of outstanding importance to obtain maximal efficency in scanning near-field microscopy. As a support towards this optimization, this work presents the computation of the electromagnetic field penetrating into two-dimensional tips. The investigated geometries consist of elongated two-dimensional dielectric slabs (glass) terminated by pointed tips. Distributions of the electric field amplitude inside and outside two-dimensional models of tips are computed and from those solutions, far-field differential cross-sections are obtained.
©1995 Elsevier Science B.V.

The concepts of near-field optical microscopy and experimental ans theoretical work carried out in Switzerland over the last 10 years are reviewed. After a description of the pioneering experiments if the mid-1980s, we focus onthe recent efforts of the three Swiss laboratories currently working in the field in close collaboration. This newly refreshed initiative in near-field optics is supported by the Swiss Priority Program Optique.
©1995 Society of Photo-Optical Instrumentation Engineers

Optical scanning probe devices offer an extremely efficient way of collecting complex structure of optical field lying near a surface. This paper discusses recent theoretical efforts to develop an efficient method for calculation of the field distributions in experimentally relevant near-field and integrated optics systems. In order to overcome the obstacles inherent in the matching of the electromagnetic boundary conditions on the surface of complex objects, the discussion is presented in the framework of the integral-equation formalism. This treatment is based on the field-susceptibility Green-function technique applied in real space. Two original numerical schemes, both based on a different discretization procedure, are discussed, and several numerical applications on systems of experimental interest are presented. Particularly, the problem of near-field distribution around three-dimensional objects of various sizes and shapes is investigated as a function of experimental parameters.
©1995 The American Physical Society

By applying an appropriate voltage between an STM tip and a metallic substrate, it is possible to induce highly localized electrostatic fields. In this paper, it is shown that the apex of the STM probe, responsible for the resolution, confines an electric field of small lateral extension inside the junction. The change in the potential energy of an adsorbate submitted to such a field is calculated with a self-consistent scheme. A microscopic description of both the tip-apex and the adsorbates is used and the correlations between each polarizable center are accounted for with a discretized Lippmann-Schwinger equation. Several applications show that our real space approach is extremely attractive for studying electrostatic field distributions in low symmetry systems. Field induced manipulation processes of C6O molecules are discussed in this context.
©1995 Elsevier Science B.V.

We present a new theoretical and numerical framework for the study of the optical properties of micro- and nanometric three-dimensional structures of arbitrary shape. We show that the field distribution induced inside and outside such a structure by different external monochromatic sources can be obtained from a unique generalized field propagator expressed in direct space. An application of the method to the confinement of optical fields due to the scattering of subwavelength objects is presented.
©1995 The American Physical Society

The optical imaging of micro- and nanometric objects requires the detection of non-radiative field components confined at the vicinity of their surface. Since 1984, numerous experimental applications of this concept in Near Field Optics (NFO) have been demonstrated and a broad variety of Scanning Near Field Optical Microscopes (SNOM) have been elaborated and continuously improved. In order to guide the ongoing development of this new subwavelength optical probing method, as well as to refine the understanding of the contrast mechanisms involved in NFO, several theoretical frameworks have already been proposed and considerable modelling work has been performed. The present paper will be devoted to a detailed analysis of the NFO image formation mechanisms of three-dimensional (3D) objects. In order to circumvent the obstacles inherent to the matching of the electromagnetic boundary conditions on the surface of complex objects, this analysis will be presented in the framework of the Integral Equation Formalism (IEF). Two original numerical schemes, both based on a different discretization procedure, will be discussed; and several numerical applications on systems of experimental interest will be presented. Particularly, the problem of near field distributions around 3D-objects of various sizes and shapes will be discussed as a function of experimental parameters.
©(1995) Kluver

The detailed imaging process of subwavelength objects deposited on a planar surface is studied within the framework of a three-dimensional model of scanning near-field optical microscope. The model consists of a truncated pointed fiber approaching a planar surface on which a three-dimensional protrusion is deposited. For this geometry, Maxwell's equations are solved exactly by applying the field susceptibility method in the direct space. The technique provides precise evaluations of the physically relevant near and far fields. In order to refine the understanding of the imaging process of subwavelength objects, we present simulated images of low-symmetry protrusions for two different modes of polarization and as a function of the approach distance. These simulations show clearly that subwavelength surface defects induce confined optical near-field distributions that are directly related to the shapes of the objects. We conclude that the central problem of near-field optical microscopy is the optimal detection of the confined fields that are set up by the objects themselves.
©1994 The American Physical Society

After the discovery of C60, a large fam1family of fullerene molecules was also identified. Among them, elongated fullerenes are formed by the tubular assembly of carbon atoms. The van der Waals bonds between fullerene molecules are due to the correlations between fluctuating charge densities inside the molecules. The interaction is then dominated by collective excitations which are sensitive to the shape of the molecules. Therefore, van der Waals attraction is expected to be modified when considering successively spherical C60, C70 and more elongated fullerenes (tubules). This paper presents self-consistent computations of the van der Waals interaction between a (111) diamond probe tip and various fullerene molecules adsorbed on a gold surface. Relative to spherical C60, the dependence law of the force experienced by the probe tip as a function of the tip-sample distance decreased when approaching fullerene tubules. Simulations of scanning force microscope scans of carbon tubules next to C60 molecules show that the shape of the molecules affects the interpretation of scanning force microscopy imaging. Particularly, information about the height of the various molecules deposited on the surface must be considered with some care since carbon tubules with the same radius as C60 interact more strongly with the probe tip.
©1994 American Institute of Physics

©

When two objects of subwavelength size interact in the presence of a light beam, a spatially confined electromagnetic field arises in a small spatial region located at the immediate proximity of the particles. In scanning probe microscopy, such induced short-range interactions change the magnitude of the forces interacting between the probe tip and the substrate. Depending on the frequency of light excitation with respect to those of the gap modes associated with the tip-sample junction, these inductive forces act to pull the probe toward the surface. Such an effect can be used to record optical adsorption of various samples with an atomic-force microscope. In this paper we show that the accurate description of the physical processes responsible for these forces can be analyzed within the framework of the localized field-susceptibility method. Practical solutions for the light-inductive force were found by discretization of the probe apex in real space. All multiple interactions including reflections with a substrate of arbi- trary profile were accounted for by self-consistent procedures. We can therefore present simulations performed on systems of experimental interest.
©1994 The American Physical Society

When a light beam impinges on two interacting objects of subwavelength size, a spatially confined electromagnetic field arises in the immediate proximity of the particles. In scanning probe microscopy, short-range forces induced by this electromagnetic near-field change the magnitude of the probe tip-substrate interaction. In this letter we analyse the physical process responsible for these forces in the context of the localized field susceptibility method.
©(1994) IOP

We present a new approach to the computation of electrical field propagating in a dielectric structure. We use Green's function technique to compute an exact solution of the wave equation. No paraxial approximation is made and our method can handle any kind of dielectric medium (air, semiconductor, metal, etc.). An original iterative numerical scheme based on the parallel use of Lippman-Schwinger and Dyson's equations is demonstrated. The influence of the numerical parameters on the accuracy of the results is studied in detail and the high precision and stability of the method are assessed. Examples for one and two dimensions establish the versatility of the method and its ability to handle structures of arbitrary shape. The application of the method to the computation of eigenmode spectra for dielectric structures is illustrated.
©1994 Optical Society of America

The capability of atomic-force microscopy (AFM) to localize both individual adsorbates and aggregates of adsorbed molecules was demonstrated a few years ago. More recently submonolayers of fullerene molecules deposited on a gold substrate have been imaged using such devices. In this paper, simulations of the atomic force between a thin probe tip and a set of adsorbed molecules is presented. The long-range part of the interaction is determined from a whole self-consistent procedure in which many-body effects are accounted for at all orders. In this description the probe tip interacts with the molecules and the surface through many-body dispersion forces. Short-range interactions are included by using an atom-atom semi empirical pairwise potential. Simulations of AFM images of C60 adsorbed molecules are presented in two different modes of imaging: the constant-tip-height mode and the constant-force mode.
©1993 The American Physical Society

The growing interest in the study of natural or artificial nanoscale structures stabilized by a corrugated surface calls for specific models adapted to the awkward symmetry of such systems. In this work the field susceptibility of a system composed of a finite number of microsystems interacting with a solid surface is derived from a Dyson's type equation. The many-body character of the interactions between each particle, including reflection with solid surface, is taken into account by a self-consistent procedure. We show tht the calculaton of this field susceptiility provides good basis to obtain the van der Waals dispersion energy inside a finite line of physisorbed atoms. We also discuss the possibility of applying this method to study optical energy transfer in complex systems.
©1993 Elsevier Science Publishers B.V.

©

The thermal behaviour of visible AlGaInP-GaInP ridge laser diodes is investigated numerically and experimentally. It is shown that various parameters critically influence the thermal resistance R of such devices. R is inversely proportional to the heat sink thermal conductivity, although the effect of a poorly conducting heatsinking material is not dramatic. However a substantial improvement - quite larger than in the AlGaAs system - is achieved for junction side down mounting compared to junction side up. R depends strongly on the width w of the ridge and this effect is different for junction side up or down mounting. In the first case R is proportional to log(w) and in the second R is proportional to the inverse of w. The thickness of the soldering material is a sensitive parameter which may add up to 15 K/W to R. On the other hand, for junction side up mounted devices, the top metallization layer has a very favorable effect : a 1 micrometer thick gold layer reduces R already by 30%. The dynamic of thermal phenomena is also studied. It is shown that when a laser is switched on, the heating speed of the active region rises up to 0.1 K/ns while the steady state of the device is reached in the ms time range. Finally, our numerical data show good agreement with experimental results.
©1992 IEEE

©

We report the temperature dependence of resistivity and the Hall coefficient Rh of a series of YBa_{2}(Cu_{1-x}Zn_x)_{3}O_{7} ceramics, with x up to 0.075'. In all our samples we have observed linear temperature dependence of (eRh)^{-1}. The slope d(eRh)^{-1}/dT decreases with the increase of Zn concentration, while the intercept (eRh)^{-1} at T->0 increases. These changes, together with a decrease of alpha due also to the Cu substitution, provoke the flattening in the temperature dependence of the Hall mobility. The analysis of the data shows that on cannot consider Zn as a simple charge donor for YBCO, but more complex changes in the mechanism of transport seem to be introduced by the Cu substitution.
©1989 Elsevier Science Ltd