957 research outputs found
Multi-dimensional modeling and simulation of semiconductor nanophotonic devices
Self-consistent modeling and multi-dimensional simulation of semiconductor nanophotonic devices is an important tool in the development of future integrated light sources and quantum devices. Simulations can guide important technological decisions by revealing performance bottlenecks in new device concepts, contribute to their understanding and help to theoretically explore their optimization potential. The efficient implementation of multi-dimensional numerical simulations for computer-aided design tasks requires sophisticated numerical methods and modeling techniques. We review recent advances in device-scale modeling of quantum dot based single-photon sources and laser diodes by self-consistently coupling the optical Maxwell equations with semiclassical carrier transport models using semi-classical and fully quantum mechanical descriptions of the optically active region, respectively. For the simulation of realistic devices with complex, multi-dimensional geometries, we have developed a novel hp-adaptive finite element approach for the optical Maxwell equations, using mixed meshes adapted to the multi-scale properties of the photonic structures. For electrically driven devices, we introduced novel discretization and parameter-embedding techniques to solve the drift-diffusion system for strongly degenerate semiconductors at cryogenic temperature. Our methodical advances are demonstrated on various applications, including vertical-cavity surface-emitting lasers, grating couplers and single-photon sources
On the convexity of the entropy along entropic interpolations
Convexity properties of the entropy along displacement interpolations are
crucial in the Lott-Sturm-Villani theory of lower bounded curvature of geodesic
measure spaces. As discrete spaces fail to be geodesic, an alternate analogous
theory is necessary in the discrete setting. Replacing displacement
interpolations by entropic ones allows for developing a rigorous calculus, in
contrast with Otto's informal calculus. When the underlying state space is a
Riemannian manifold, we show that the first and second derivatives of the
entropy as a function of time along entropic interpolations are expressed in
terms of the standard Bakry-\'Emery operators and . On the
other hand, in the discrete setting new operators appear. Our approach is
probabilistic; it relies on the Markov property and time reversal. We
illustrate these calculations by means of Brownian diffusions on manifolds and
random walks on graphs. We also give a new unified proof, covering both the
manifold and graph cases, of a logarithmic Sobolev inequality in connection
with convergence to equilibrium
Controlled merging and annihilation of localized dissipative structures in an AC-driven damped nonlinear Schrödinger system
We report studies of controlled interactions of localized dissipative structures in a system described by the AC-driven damped nonlinear Schrödinger equation (equivalent to the Lugiato-Lefever model). Extensive numerical simulations reveal a variety of interaction scenarios that are governed by the properties of the system driver, notably its gradients. In our experiments, performed with a nonlinear optical bre (Kerr) resonator, the phase prole of the driver is used to induce interactions of the dissipative structures on demand. We observe both merging and annihilation of localized structures, i.e. interactions governed by the dissipative, out of-equilibrium nature of the system. These interactions fundamentally dier from those typically found for conventional conservative solitons
How good is the generalized Langevin equation to describe the dynamics of photo-induced electron transfer in fluid solution?
The dynamics of unimolecular photo-triggered reactions can be strongly
affected by the surrounding medium. An accurate description of these reactions
requires knowing the free energy surface (FES) and the friction felt by the
reactants. Most of theories start from the Langevin equation to derive the
dynamics, but there are few examples comparing it with experiments. Here we
explore the applicability of a Generalized Langevin Equation (GLE) with an
arbitrary potential and a non-markovian friction. To this end we have performed
broadband fluorescence measurements with sub-picosecond time resolution of a
covalently linked organic electron donor-acceptor system in solvents of
changing viscosity and dielectric permittivity. In order to establish the FES
of the reaction we resort to stationary electronic spectroscopy. On the other
hand, the dynamics of a non-reacting substance, Coumarin 153, provide the
calibrating tool for the friction over the FES, which is assumed to be solute
independent. A simpler and computationally faster approach uses the Generalized
Smoluchowski Equation (GSE), which can be derived from the GLE for pure
harmonic potentials. Both approaches reproduce the measurements in most of the
solvents reasonably well. At long times, some differences arise from the errors
inherited from the analysis of the stationary solvatochromism and at short
times from the excess excitation energy. However, whenever the dynamics become
slow the GSE shows larger deviations than the GLE, the results of which always
agree qualitatively with the measured dynamics, regardless of the solvent
viscosity or dielectric properties. The here applied method can be used to
predict the dynamics of any other reacting system, given the FES parameters and
solvent dynamics are provided. Thus no fitting parameters enter the GLE
simulations, within the applicability limits found for the model in this work.Comment: 30 pages, 22 figures, 5 tables, 97 reference
Quantum Chemical Investigation of the Interaction of Hydrogen with Solid Surfaces
In dieser Arbeit werden die Wechselwirkungen von Wasserstoff mit festen Materialien
und Oberflächen untersucht. Zunächst wird der Kontext unserer Untersuchung durch
eine kurze Einordnung in die Geschichte der Naturwissenschaften im Allgemeinen, und
der Oberflächenforschung im Speziellen, hergestellt. Anschließend wird der quanten-
mechanische Apparat, welcher nötig ist um die betrachteten Systeme zu beschreiben,
eingeführt um dann detailliert die Potentialhyperfläche der Entstehung von Wasser
durch Adsoprtion von Wasserstoff auf einer teilweise oxidierten Ruthenium(0001)
Metalloberfläche zu studieren. Zudem wird das gleiche System betrachtet, wenn die
Metalloberfläche zusätzlich von einer biatomaren, kristallinen Lage Siliziumdioxid
(SiO2 ) bedeckt ist, wodurch eine räumliche Beengung eintritt. Wir verwenden unsere
Ergebnisse zusammen mit experimentellen Beobachtungen und mathematischen Metho-
den um ein vollständig theoretisches Modell zu entwerfen und das System grundlegend
verstehen zu können. In einem weiteren Schritt werden die chemischen Änderungen der
Siliziumdioxid Doppellage untersucht, wenn das System Wasserstoffplasma ausgesetzt
wird. Es werden diverse mögliche Defektstrukturen diskutiert und mithilfe experi-
menteller Befunde die wahrscheinlichste Struktur isoliert. Im letzten Kapitel werden
die typischen Näherungen untersucht, welche notwendig sind um quantenmechanische
Methoden mit Hilfe von Computern durchfĂĽhrbar zu machen. Wir verwenden den
sogenannten embedded-fragment Ansatz um die Diffusionsbarriere von Wasserstoff
auf Aluminiumoxid mit chemischer Genauigkeit zu berechnen. Unsere Ergebnisse
auf dem coupled-cluster with singles, doubles and perturbative triples (CCSD(T))-
Niveau können sowohl als Referenz für experimentelle Untersuchungen, als auch für
andere quantenmechanische Methoden wie z.B. die Dichtefunktionaltheorie, angesehen
werden.The present thesis aims at investigating the interactions of hydrogen with solid surfaces
and materials. We first offer a brief historical context for surface science, as well as
quantum mechanics and science is general, before deriving the mathematical appa-
ratus necessary to investigate our systems of interest. We then move on to explore
the potential energy surface of the water-formation-reaction on a partially oxidized
ruthenium(0001) surface when confined under a two-atom thick sheet of silica (SiO2 ).
We further employ our findings in conjunction with experimental observations and
mathematical modeling to set up a fully theoretical model of the system in order to
explain its behavior. In the second chapter we investigate the chemical alteration of
the ultra-thin silica bilayer by means of exposing it to hydrogen plasma. We elucidate
possible defects formed during the process and pin-point the most likely structure
found. In the last chapter, we investigate the possible error sources that are inherent in
quantum mechanical modeling and employ the so called embedded fragment approach
to lift the approximations up to the coupled cluster singles and doubles with perturba-
tive triples (CCSD(T)) level of theory. We then apply this methodology to the diffusion
of hydrogen on aluminum oxide to obtain a diffusion barrier of chemical accuracy that
may both be used to benchmark other approaches such as density functional theory, as
well as experimental findings
Non-Linear Lattice
The development of mathematical techniques, combined with new possibilities of computational simulation, have greatly broadened the study of non-linear lattices, a theme among the most refined and interdisciplinary-oriented in the field of mathematical physics. This Special Issue mainly focuses on state-of-the-art advancements concerning the many facets of non-linear lattices, from the theoretical ones to more applied ones. The non-linear and discrete systems play a key role in all ranges of physical experience, from macrophenomena to condensed matter, up to some models of space discrete space-time
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