Microabsorption, Modulation and Dark Field Spectroscopy
In our working group, we investigate a wide variety of plasmonic structures, from simple dimers to complex crystals made of individual nanoparticles. For the excitation of the dark and bright plasmons as well as for the characterisation of its far field, such as the determination of the resonance frequency or the coupling strength of light and matter, we have different measurement methods at our disposal.
Spatial Modulation Spectroscopy and Structured Light
Modulation spectroscopy can measure the tiny changes nanosystems make to light by absorption and scattering. By using modulation, we can measure for example the extinction of single carbon nanotubes. Together with structured light we are even able to measure weak signals like dark plasmon modes that usually are not able to be excited with linear polarized light.
The modulation setup is equipped with a supercontinuum white light source (FIU 15) from NKT Photonics and a fine tunable laser line filter (LLTF) form Photon etc. Filtering out single wavelengths in the spectral range between 400 nm and 1000 nm allows us to measure full resonance profiles. To create the structured light, we use a Q-plate from ARCoptix and a spatial light modulator (SLM) from Holoeye. With these two instruments we are able to change the phase and the polarization of the light. Doing so we can generate many different types of laser profiles.
Microabsorption and Reflection
Without using the LLTF and modulation, this setup can also be used for measuring the reflection and transmission of transparent samples, and thus for measuring absorption. In this case, the transmitted or reflected light is coupled into one of two Avantes spectrometers using another beam splitter by means of an optical fibre, which enables us to take measurements in the range between 450 nm and 1600 nm. The measurements can be used, for example, to determine resonance frequencies, the type of excited plasmon (dark or bright plasmon) or the strength of the coupling between light and matter.
Dark Field Spectroscopy
For further characterisation, especially of plasmonic structures, we have a setup for dark-field spectroscopy. The heart of the setup is a Nikon Eclipse LV100 darkfield microscope. The white light scattered by the sample is collected by the special darkfield objectives (Nikon - LU Plan Fluor) and guided by an optical fibre into a Horiba Scientific iHR320 spectrometer with nitrogen-cooled CCD camera. In addition to classical dark field images, we can measure spectra of the scattered light from our samples in the range from 300 nm to 1100 nm, which we can use to determine the resonance frequency of the plasmonic far field.