Optical characterization

At the NAM, we utilize a variety of techniques for the optical characterization of plasmonic nanostructures. Many of these setups have been custom made and are continuously improved to explore new facets of nanophotonics. Note that this page is still under construction and we are curently adding the descrition of specific techniques.

Near-field microscopy

Principles of SNOM In a conventional optical microscope, light is concentrated onto the sample. The object is then imaged and magnified by a lens system. The image is a low pass filtered representation of the original object. The high spatial frequencies are lost during propagation through the objective. Hence, there is always a loss of information during propagation from near- to far-field and only structures with lateral dimensions larger than approximately the wavelength used. To achieve sub-wavelength resolution in optical fields, it is crucial to probe high spatial frequencies contained in the evanescent waves. To access the evanescent waves, the probe must be brought close to the surface, i.e. in the near-field of the object.

The Scanning Near-field Optical Microscopy (SNOM or NSOM) uses atomic force microscope (AFM) techniques for the approach of a probe close to a surface of a sample and scans the sample in order to investigate the optical near-field. The SNOM probe, which detects the light has been the subject of a lot of technological effort. It is a crucial part of the SNOM because it is the principal sensor of the detection or the main source for the illumination.

Near-field of a surface plasmon-polariton

The previous figure shows on the left one of our SNOMs. On the right we report the amplitude and phase measured for a surface plasmon-polariton propagating on a thin gold film. This surface wave is completely bound to the metal surface and would not reach the far-field; hence, it would not be visible in a conventional microscope, where it would be completely dark. The near-field images however, reveal that the plasmon decays rapidly as it propagates (the near-field amplitude phases off toward the right of the figure). To simulatneously measure the amplitude and the phase of the surface plasmon, we simply put our near-field microscope inside an interferometer. The measurement of the phase provides valuable information on the wave character of the surface plasmon.

Nonlinear plasmonics

It is well known that plasmonic antennas exhibit so-called hot spots in their gap, where the incident intensity can be enhanced by several orders of magnitude. Since optical nonlinear processes depend on a high power of the field intensity, the local enhancement of the field provided by plasmonic structures can greatly enhance these nonlinear processes. Moreover, when the pump light is delivered in the form of ultrashort pulses, considerably higher conversion efficiencies can be achieved with limited thermal problems. Thus, mode-locked lasers are ideally suited for nonlinear optical experiments as these sources exhibit very high peak power with limited average power. In this setup, we use a kerr-lens mode-locked Ti:sapphire laser that can deliver up to 800mW of average output power at 800 nm with pulse durations of roughly 150 fs at a repetition rate of 80 MHz. This allows us to achieve peak intensity up to roughly 100GW/cm2.

Setup for nonlinear measurements

Typically, lock-in detection is performed with a mechanical chopper as the modulation unit and a high sensitivity PMT for the detection. For spectral measurement, a commercial spectrometer is used. Additionally, an intensity autocorrelator was build in order to measure the pulse durations of the Ti:sapphire oscillator. A non-collinear configuration is chosen and the second-harmonic is generated in a nonlinear BBO crystal (see picture). This allows the measurement of ultrashort pulses down to 80 fs at a wavelength around 800nm.