Optical spectroscopy and microscopy of metallic nanostructures

Abstract

In this work metallic nanostructures are studied using optical spectroscopy and microscopy. The most striking feature of the optical properties of metallic nanostructures is their capability to support plasmons, coupled oscillations of the conduction electrons and the electromagnetic field. When a plasmon is excited, the optical field can be locally enhanced by orders of magnitude. Also scattering and absorption cross sections increase significantly. These properties open up many interesting applications as well as give information about light-matter interactions on the nanoscale. Optical microscopy is the natural tool to study the optical properties of nanostructures. The spatial resolution of conventional microscopy is limited to approximately half the wavelength of light. However, by a suitable choice of contrast mechanism, e.g., polarization, wavelength, or degree of polarization, a wealth of information can often be extracted about the nanometer scale details of a structure although they cannot be directly resolved. In this thesis, optical spectroscopy is combined with microscopy to investigate two types of metallic structures: nanoparticles and slits in metallic thin films. A new method of optical microscopy is developed to make it possible to detect and spectroscopically study individual gold nanoparticles smaller than 10 nm in diameter. The plasmon resonances of such particles are researched using the developed technique. The light transmission properties of narrow slits fabricated in gold thin films are also investigated using methods of optical microscopy. It is shown that the transmission spectrum exhibits resonances whose properties sensitively depend on the dimensions of the structure. Furthermore, the influence of an external disturbance on the transmittance spectrum is studied, an important question for applications in sensing and optical switching. Results of numerical calculations are compared with the experimental data and a good agreement is found. The central element of any microscopy system is the focusing optic. In this thesis the focusing of partially polarized light is studied theoretically. It is observed that, strikingly, light may become locally fully polarized even when the incident optical field is unpolarized. This counter intuitive effect is confirmed by experiments.