Physics-based multiscale modeling of III-nitride light emitters

The application of computer simulations to scientific and engineering problems has evolved to an established phase over the last decades. In the field of semiconductor device physics, Technology CAD (TCAD) has been regarded as an indispensable tool for the interpretation and prediction of device behavior. More specifically, TCAD modeling and simulation of nanostructured III-nitride light emitters still have challenging problems and is currently a topic under active research. This thesis devotes to the theoretical and numerical investigations of III-nitride bulk and quantum structures, following a bottom-up approach aimed at modeling and understanding photoluminescence and electroluminescence in these structures. In the first part, the calculation of electronic bandstructure is addressed, where a novel k · p model derived from Non-local Empirical Pseudopotential method(NL-EPM) is presented. Optical properties are then calculated employing both Poisson-k · p and a density-matrix based approach, gain and luminescence spectra can be extracted by solving the semiconductor-Bloch equation numerically. The last part of this thesis deals with the microscopic quantum transport, within the framework of the quantum-statistical nonequilibrium Greens function formalism(NEGF). While classical drift-diffusion models assume that bound carriers hold their coherence in the confined direction and unbound carriers are completely incoherent, NEGF does not distinguish between bound and unbound states and treats them on equal footing. In addition, NEGF also provides intuitive insights into energy-resolved carrier distributions, currents and coherence loss mechanisms. The numerical computations alongside this thesis can be computationally very involved, some code developed along with this thesis is deployed on the clusters and able to scale up to more than 1000 CPU cores, thanks to the parallel implementation technique such as OpenMP and MPI, as well as HPC infrastructures available at CINECA computing center

Fluctuation-induced interactions and nonlinear nanophotonics

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 293-329).We present theoretical and numerical methods for studying Casimir forces and nonlinear frequency conversion in nanophotonic media consisting of arbitrary geometries and materials. The first section of the thesis focuses on the study of various geometry-enabled resonant effects leading to strong nonlinear interactions. The starting point of this work is a coupled-mode theory framework for modeling a wide range of resonant nonlinear frequency-conversion processes in general geometries, ameliorating the need for repeated and expensive finite-difference time-domain simulations. We examine the predictions of the theory for two particular nonlinear processes: harmonic generation and difference-frequency generation. Our results demonstrate strong enhancement of nonlinear interactions at a "critical" input power leading to 100% frequency conversion, among many other interesting dynamical effects. Using a quantum-mechanical description of light, based on cavity quantum electrodynamics, similar enhancement effects are demonstrated at the single-photon level, leading to the possibility of achieving all-optical switching of a single signal photon by a single gating photon in a waveguide-cavity geometry consisting of pumped four-level atoms embedded in a cavity. Finally, we describe how one may tailor the geometry of certain materials to enhance their nonlinear susceptibilities by exploiting a consequence of the Purcell effect. The second section of the thesis, the main contribution of this work, presents a new formulation for studying Casimir forces in arbitrary geometries and materials that directly exploits efficient and well-developed techniques in computational electromagnetism. To begin with, we present the step-by-step conceptual development of our computational method, based on a well-known stress tensor formalism for computing Casimir forces. A proof-of- concept finite-difference frequency-domain implementation of the stress-tensor method is described and checked against known results in simple geometries. Building on this work, we then describe the basic theoretical ingredients of a new technique for determining Casimir forces via antenna measurements in tabletop experiments. This technique is based on a (derived) correspondence between the complex-frequency deformation of the Casimir frequency-integrand for any given geometry and the real-frequency classical electromagnetic response of the same geometry, but with dissipation added everywhere. This correspondence forms the starting point of a numerical Casimir solver based on the finite-difference time-domain method, which we describe and then implement via an off-the-shelf time-domain solver, requiring no modifications. These numerical methods are then used to explore a wide range of geometries and materials, of various levels of complexity: First, a four-body piston-like geometry consisting of two cylinders next to adjacent walls, which exhibits a non-monotonic lateral Casimir force (explained via ray optics and the method of images); Second, a zipper-like, glide-symmetric structure that leads to a net repulsive force arising from a competition between attractive interactions. Finally, we examine a number of geometries consisting of fluid-separated objects and find a number of interesting results. These include: stable levitation and suspension of compact objects, dispersion-induced orientation transitions, and strong non-zero temperature Casimir effects.by Alejandro Rodriguez-Wong.Ph.D

Optical and non-equilibrium properties of graphene = Optische und Nichtgleichgewichtseigenschaften von Graphen

Die vorliegende Dissertation befasst sich mit der theoretischen Beschreibung von Nichtgleichgewichts-Eigenschaften von Graphen. Dazu wird eine quantenkinetische Beschreibung der Dynamik der elektronischen Quasiteilchen in Graphen unter dem Einfluss externer optischer Felder und Coulomb-Wechselwirkung entwickelt. Als Anwendungen wird die Relaxation optisch angeregter Elektronen untersucht und eine effektive hydrodynamische Beschreibung des elektronischen Transports in Graphen abgeleitet