Silicon photonic crystals and spontaneous emission

Abstract

Photonic crystals, i.e. materials that have a periodic variation in refractive index, form an interesting new class of materials that can be used to modify spontaneous emission and manipulate optical modes in ways that were impossible so far. This thesis is divided in three parts. Part I discusses the design and fabrication of two-dimensional photonic crystals in silicon using deep anisotropic etching with a SF6/O2 plasma. The etching process was optimized for the fabrication of two-dimensional photonic crystals by tuning the main parameters of the etching process, i.e. temperature, bias voltage and O2 flow. Vertical confinement in these structures is provided by integrating the structures in a dielectric waveguide. For this purpose, amorphous silicon, silicon-on-insulator and SiGe structures were considered. Fabrication of structures in both amorphous silicon and silicon-on-insulator was successfully demonstrated. The incorporation of luminescent species, such as laser dyes, was demonstrated using a new wet chemical coating technique that forms thin silica layers on a substrate. Part II discusses the modification of spontaneous emission in one dimensional systems by studying the decay rate of luminescing Cr ions close to a dielectric interface. The decay rate of the Cr ions can be changed by bringing the samples into contact with a range of liquids with different refractive indices. The change in radiative decay rate can be calculated by calculating the local density of states. To explain the experimental results additional non-radiative decay channels have to be introduced and yields a quantum efficiency of ~50% for the Cr R-line luminescence. This concept was further extended to a thin silica layer on silicon implanted with erbium ions and resulted in the radiative rate of erbium in pure silica: 54 s-1. This number was used to analyze the decay rate of erbium ions in silica colloidal spheres that can be used as building block for three-dimensional photonic crystals by self-assembly. Finally, Part III discusses the optical properties and modified spontaneous emission from a three-dimensional silicon photonic crystal of finite (5-layers) thickness. The crystals are made in a layer-by-layer approach using lithographic tools and show near 100% reflection in the 1.4-1.7 mu m wavelength range indicative of a photonic stopgap. A direct comparison with the calculated reflectivity reveals that some features in the reflectivity can be ascribed to the finite thickness of the crystal, while other features can be explained in terms of a superstructure that leads to zone folding of the photonic bandstructure. The collected spontaneous emission from erbium implanted crystals is strongly reduced for wavelengths in the stopgap from 1.4-1.7 mu m. The changes in collected luminescence intensity are explained in terms of a rate equation model that takes into account the effect of Bragg scattering, the local density of states and the quantum efficiency of the emitters inside the crystal. Using this model a spectral attenuation of 5 dB per unit cell at 1.539 mu m wavelength is obtained from the experimental data, which is in perfect agreement with existing theory and transmission data.