Metallic nanostructures for optoelectronic and photovoltaic applications
The optical properties of metallic nanostructures are of interest to scientists and engineers. They allow the manipulation of light on a microscopic level, which has applications in photonics, optoelectronics and energy conversion. In this dissertation, we investigate the surface plasmon resonances, near fields and scattering properties of these structures and their interaction with semiconductors. The optical properties of metallic nanoparticles are analyzed by EM theory and finite element simulation. We perform experiments where the scattered fields of the nanoparticles are coupled to semiconductor photodiodes. From the results we study how the near fields and scattering properties can influence the optical absorption response spectrum in such devices. We find that the quasi-static behavior of the nanoparticles can explain the observed modulation in the absorption spectra. We also apply the same methods of simulation and experiment to Silicon-on-Insulator photodetectors. The coupling between the nanoparticles and waveguide modes in the silicon film leads to large increases in absorption, particularly in the infrared wavelengths, where silicon is a poor absorber. We find that we can engineer the absorption enhancement spectra by controlling the nanoparticle distribution. Another application that we investigate is in UV multi-spectral imaging. We study dielectric overlayer transmission grating structures and exploit the surface plasmon and dielectric waveguide properties of our structures to engineer UV band reject filters. The exploitation of metallic nano-structures is necessary as traditional materials for resonating optical structures do not work well in the UV. Finally, we investigate metallic scattering backside reflectors for optimal light trapping in ultrathin film solar cells. Such solar cells suffer from poor quantum efficiency at certain wavelengths due to long photon absorption length. Therefore coupling of incident light into waveguide modes is an effective way to increase the quantum efficiency. By numerical analysis we study the performance of random vs periodic texturing of the backside reflector. The results are useful for designing optimal solar cells with very thin absorber layers.