Propagació i generació de llum en nanoestructures fotòniques

Author: Muriel Botey Cumella
Title: Propagació i generació de llum en nanoestructures fotòniques
Director: J. Martorell
Presentation date: July 24, 2009
Link to text: http://www.tdx.cat/handle/10803/6595

Abstract: Photonic periodic nanostructures have offered, in the last two decades, a new framework for the study of the interaction between electromagnetic radiation and matter. Such structures can engineer the electromagnetic properties of materials and have become a powerful tool used to confine, route, suppress, localize, split, disperse, and filter light. The scope of the electromagnetic radiation control can be extended to light propagation and generation. Photonic crystals have successfully been used as host materials to suppress or enhance light generation mechanisms such as spontaneous emission or nonlinear processes. The aim of this thesis is to investigate finite-size effects and to what extent the properties of ideal infinite or infinitely periodic structures hold for structures that are finite in size. Until recently, experimental as well as theoretical developments in the field of photonic crystals have been based, mostly, on calculations that consider ideal structures with perfectly periodic boundary conditions. Deviations from the behavior predicted form such assumptions were already observed when the field was born. However, some of them remained, for the most part, unexplained. In the present work, we tackle some of these aspects related to light propagation and generation in the real finite photonic crystals that can be fabricated. With this purpose, we perform such study from, both, an experimental as well as a theoretical perspective. We study finite-size effects in the region of the first order Bragg reflection band as well as in the high energy range where the wavelength of light is on the order or smaller than the lattice parameter. To be more specific, part of the work is devoted to the study of colloidal crystals at the range of low energy. We develop 3D full wave vector calculation in the Rayleigh-Gans approximation to simulate low index contrast structures. This model accounts for relevant real crystal's aspects such as boundary conditions, a slight dispersion in the spheres diameter and includes an effective absorption accounting for Rayleigh scattering and inelastic diffusion due to imperfections. This model is used to study light propagation within real photonic nanostructures, and to determine their dispersive properties. The predictions of the model are contrasted with experimental polarization dependent measurements of the partial band structure of an actual colloidal crystal. We also determine the experimental photon's lifetime, at the edge of the first order pseudogap, by means of the pulse reshaping induced by changes in the group velocity accompanied by the photon trapping. To explain light propagation in the high energy range, we use a vector KKR calculation that we apply to understand the group velocity of light propagating in artificial opals slabs. We show that for certain frequencies, the group velocity can either be superluminal, positive or negative or approach zero depending on the crystal size and absorption. Such behavior can be attributed to the finite character of the structure and accounts for previously reported experimental observations. The same propagation at anomalous group velocity may explain the experimental observation of second harmonic generation enhancement of light from a nonlinear opal film. Indeed, the group velocity slowing-down provides an enhancement mechanism for nonlinear processes. We finally consider another configuration such that the quadratic nonlinear interaction occurs within a sub-wavelength layer. In the presence of a nearby reflecting surface we demonstrate that the contribution of terms that do not conserve light momentum, and that would vanish in an infinitely long medium, is the most relevant one.