Nanophotonics and Metrology laboratory NAM

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 ç 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 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

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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