Computational electromagnetics for nanowire solar cells

This review article provides an overview of various novel nanowire array solar cells and highlights the aspects of electromagnetic simulations that are a valuable tool for understanding the optical processes leading to their distinct properties. As the computational methods commonly used for the task are well established, we focus on the question how numerical modeling can be used to assess the performance of a design and reveal the working principle of the devices. We conclude that scientific literature identifies numerical simulations as paramount for design and interpretation of experimental dat

Solcore: A multi-scale, python-based library for modelling solar cells and semiconductor materials

Computational models can provide significant insight into the operation mechanisms and deficiencies of photovoltaic solar cells. Solcore is a modular set of computational tools, written in Python 3, for the design and simulation of photovoltaic solar cells. Calculations can be performed on ideal, thermodynamic limiting behaviour, through to fitting experimentally accessible parameters such as dark and light IV curves and luminescence. Uniquely, it combines a complete semiconductor solver capable of modelling the optical and electrical properties of a wide range of solar cells, from quantum well devices to multi-junction solar cells. The model is a multi-scale simulation accounting for nanoscale phenomena such as the quantum confinement effects of semiconductor nanostructures, to micron level propagation of light through to the overall performance of solar arrays, including the modelling of the spectral irradiance based on atmospheric conditions. In this article we summarize the capabilities in addition to providing the physical insight and mathematical formulation behind the software with the purpose of serving as both a research and teaching tool.Comment: 25 pages, 18 figures, Journal of Computational Electronics (2018

Quantum Transport in Semiconductor Nanostructures

I. Introduction (Preface, Nanostructures in Si Inversion Layers, Nanostructures in GaAs-AlGaAs Heterostructures, Basic Properties). II. Diffusive and Quasi-Ballistic Transport (Classical Size Effects, Weak Localization, Conductance Fluctuations, Aharonov-Bohm Effect, Electron-Electron Interactions, Quantum Size Effects, Periodic Potential). III. Ballistic Transport (Conduction as a Transmission Problem, Quantum Point Contacts, Coherent Electron Focusing, Collimation, Junction Scattering, Tunneling). IV. Adiabatic Transport (Edge Channels and the Quantum Hall Effect, Selective Population and Detection of Edge Channels, Fractional Quantum Hall Effect, Aharonov-Bohm Effect in Strong Magnetic Fields, Magnetically Induced Band Structure).Comment: 111 pages including 109 figures; this review from 1991 has retained much of its usefulness, but it was not yet available electronicall

Infrared Radiation

This book represents a collection of scientific articles covering the field of infrared radiation. It offers extensive information about current scientific research and engineering developments in this area. Each chapter has been thoroughly revised and each represents significant contribution to the scientific community interested in this matter. Developers of infrared technique, technicians using infrared equipment and scientist that have interest in infrared radiation and its interaction with medium will comprise the main readership as they search for current studies on the use of infrared radiation. Moreover this book can be useful to students and postgraduates with appropriate specialty and also for multifunctional workers

Theory of spin-orbit interactions in electronic transport and light scattering at the mesoscale.

89 p.En esta Tesis, estudiamos los fundamentos teóricos de las interacciones de espín-órbita en dos de las ramas más prometedoras de la física de hoy en día, la espintrónica y la nanofotónica. Nos enfocamos en entender las posibles analogías entre ambas ramas y en la búsqueda de un posible lenguaje y modelo común para describir las interacciones de espín-órbita. El punto de partida común son las interacciones de espín-órbita que aparecen en problemas de dispersión tanto de electrones como de radiación electromagnética en impurezas aisladas. A partir de este estudio inicial, pasamos a profundizar en las interacciones en ambas ramas por separado.En la parte de nanofotónica, la accesibilidad de los experimentos de dispersión de impurezas aisladas y la falta de estudio básico en las interacciones de espín-órbita en luz, hacen que hoy en día siga siendo un campo con intensa investigación. Concretamente, estudiamos los efectos de la interacción entre el momento dipolar eléctrico y magnético en la dispersión de onda plana de una partícula dieléctrica con índice de refracción alto. Como consecuencia de la transferencia entre el momento angular de espín y orbital, demostramos la aparición de errores de localización divergentes en la detección de campo lejano.En la rama de espintrónica, en cambio, el concepto de acoplo de espín-órbita y sus consecuencias están bien establecidos. Los estudios hoy en día, por tanto, se basan en transporte electrónico, en los que los efectos de espín-órbita de dispersión con impurezas han de sumarse a los del acoplo espín-órbita intrínseco o los del desorden. Concretamente, en esta tesis nos enfocamos en dispositivos híbridos, con regiones con acoplo espín-órbita adyacentes a regiones sin tal acoplo. En este sentido, ofrecemos la descripción de sistemas híbridos en los que las ecuaciones de difusión de espín y carga han de resolverse con ayuda de condiciones de frontera.Gracias al estudio teórico desarrollado en esta tesis en ambas ramas de la física, hemos tenido la oportunidad de contribuir con diferentes publicaciones que forman parte de esta Tesis por compendio de artículos

Quasi-periodic and random THz photonic resonators

In the last decades the fields of photonics and nanotechnology have led to some impressive scientific and technological achievements. Among them, the exploration of yet unexploited spectral regions, such as the Terahertz (THz) range, i.e. wavelengths of 60micron-300 micron, has been a major breakthrough. On the one hand, this was possible after the novel theoretical concept of light amplification in multiple quantum wells and superlattices was proposed in the Seventies, introducing the groundbreaking quantum cascade laser (QCL) idea. On the other hand, the development of new nanofabrication technologies and crystal growth techniques, such as the molecular beam epitaxy (MBE), allowed an unprecedented control over the material structure, down to the deposition of nanometer-thick semiconductor layers. This paved the way to the practical realization of electrically pumped multi-stage gain media, the QCL, and to the successful demonstrations of their operation in a broad frequency range, from the mid-IR to the far-infrared. Apart from the purely scientific interest, Terahertz photonics has now a fundamental role in many applications, like metrology, spectroscopy, biomedical and pharmaceutical imaging, quality and process control, communications and security. Nowadays, a lot of effort is made to improve the performance of Terahertz QCL in terms of optical power, efficiency, beam pattern, frequency control and thermal management. Some of these crucial issues can be addressed by the use of photonic structures, i.e. specially designed patterns of dielectric scatterers superimposed to the active region. Such structures can be implemented in one- (1D), two (2D)- or three (3D)-dimensional architectures, to provide a tight control of the frequencies and far-field emission pattern of the laser. Periodic photonic crystals have been studied for long time, providing intriguing insights. More recently, aperiodic patterns have attracted increasing attention due to their greater flexibility and the possibility to study and explore novel physical phenomena. The aim of the present thesis is to design, fabricate and investigate the transport and optical behavior of THz QCLs exploiting distributed feedback, achieved through the use of 2D quasi-periodic and random resonators. The main goal is to demonstrate multimode emission over a broad frequency bandwidth, centered around 3.1 THz. Unlike perfect photonic crystals, quasi-crystal geometries do not possess discrete translational invariance, yet they do possess long-range order which gives rise to a rich spectrum. After developing a simulation code based on the generation algorithm called "Generalized Dual Method", we designed the following quasi-crystal geometries: i) a 7-fold pattern with a perfect symmetry under 2π/7 rotations around a central axis, ii) an imperfect 7-fold geometry where small defect points are introduced. This allowed to compare the effects of introducing a small amount of disorder in the design of the photonic structures. In order to understand the effect of a further increase of disorder, a third type of random structures was also studied, whose scatterers positions were extracted from a uniform pseudo-random distribution. We then simulated these photonic structures using the numerical approach of finite elements analysis, to understand how light propagation is affected by the size, the number and the arrangement of the scatterrers. A set of devices for each geometry was selected among those with the largest number of electromagnetic modes with predicted high quality factors Q. They were then nano-fabricated with the same QCL active region in a cleanroom facility, using a combination of UV optical lithography, plasma-assisted etching, metal deposition, chemical processes and ultrasonic wedge bonding. Finally, all lasers were characterized electrically and optically to study how the different physical and geometrical parameters affect the lasing threshold, the slope efficiency, the emitted power and the far-field intensity profile. The emission spectra were probed via Fourier Transform Infrared Spectroscopy (FT-IR), demonstrating the predicted multimode emission in most devices. In a future perspective, such multimode emission could be used to mode-lock radiation in a THz QCL, for example using passive optical components. An interesting possibility is the future integration of graphene in QCL to exploit its saturable absorption in the THz region. To this end, the transmission of THz radiation through a few layers of graphene transferred on an intrinsic silicon substrate was measured, reporting saturable absorption in the THz

Theory of spin-orbit interactions in electronic transport and light scattering at the mesoscale.

89 p.En esta Tesis, estudiamos los fundamentos teóricos de las interacciones de espín-órbita en dos de las ramas más prometedoras de la física de hoy en día, la espintrónica y la nanofotónica. Nos enfocamos en entender las posibles analogías entre ambas ramas y en la búsqueda de un posible lenguaje y modelo común para describir las interacciones de espín-órbita. El punto de partida común son las interacciones de espín-órbita que aparecen en problemas de dispersión tanto de electrones como de radiación electromagnética en impurezas aisladas. A partir de este estudio inicial, pasamos a profundizar en las interacciones en ambas ramas por separado.En la parte de nanofotónica, la accesibilidad de los experimentos de dispersión de impurezas aisladas y la falta de estudio básico en las interacciones de espín-órbita en luz, hacen que hoy en día siga siendo un campo con intensa investigación. Concretamente, estudiamos los efectos de la interacción entre el momento dipolar eléctrico y magnético en la dispersión de onda plana de una partícula dieléctrica con índice de refracción alto. Como consecuencia de la transferencia entre el momento angular de espín y orbital, demostramos la aparición de errores de localización divergentes en la detección de campo lejano.En la rama de espintrónica, en cambio, el concepto de acoplo de espín-órbita y sus consecuencias están bien establecidos. Los estudios hoy en día, por tanto, se basan en transporte electrónico, en los que los efectos de espín-órbita de dispersión con impurezas han de sumarse a los del acoplo espín-órbita intrínseco o los del desorden. Concretamente, en esta tesis nos enfocamos en dispositivos híbridos, con regiones con acoplo espín-órbita adyacentes a regiones sin tal acoplo. En este sentido, ofrecemos la descripción de sistemas híbridos en los que las ecuaciones de difusión de espín y carga han de resolverse con ayuda de condiciones de frontera.Gracias al estudio teórico desarrollado en esta tesis en ambas ramas de la física, hemos tenido la oportunidad de contribuir con diferentes publicaciones que forman parte de esta Tesis por compendio de artículos

Semi-analytic modeling of stacked metasurfaces

Ziel dieser Arbeit war die Entwicklung eines semi-analytischen Models mehrschichtiger nano- strukturierter Oberflächen. Einzelne Schichten werden hierbei in Forschungsgemeinschaft als “Metasurface” bezeichnet. In Folge nennt man Schichtsysteme aus Metasurfaces “Metasurface Stacks” oder “Stacked Metasurfaces”. Das besondere an Metasurfaces liegt an einer speziellen Art der Licht-Materie-Wechselwirkung. Im Gegensatz zu herkömmlichen, natürlich vorkommenden optischen Materialien, welche im Wesentlichen durch ihre atom- und molekularphysikalischen Eigenschaften wechselwirken, besitzen Metasurfaces mesoskopische Strukturen. Diese haben Größen, die der von Lichtwellen entsprechen. Dadurch entstehen zum einen Streuphänomene die komplexe Feldwechselwirkungen erzeugen. Darüberhinaus sorgen evaneszente Felder, die auf der Oberfläche der Nano-Strukturen angeregt werden können, für ein geändertes Resonanzverhalten, welches sich durch verschiedene Reflektions- und Absorptionseigenschaften auszeichnet. Sind die Strukturen einer Metasurface periodisch angeordnet lassen sich die dort angeregten Felder durch sogenannte Bloch-Moden beschreiben. Diese sind periodische Feldlösungen der Maxwell-Gleichungen. Betrachtet man nun die Gesamtheit aller Bloch-Moden der Metasurface, kann man eine dominante Mode mit, im Vergleich zu allen anderen, maximalem Energietransport in das Fernfeld identifizieren. Diese nennt man in der Literatur Fundamentalmode. Ist die Metasurface so beschaffen, dass bei Wechselwirkung mit Licht einer bestimmten Wellenlänge diese Fundamentalmode signifikant alle anderen Moden dominiert und letztere stark dämpfen, das heißt evaneszent abfallen, so kann das betreffende Medium als homogen gedeutet werden. Darauf basierend wurde in der vorliegenden Arbeit ein semi-analytisches Model von Stacked Metasurfaces entwickelt, welches verschiedenen experimentellen Tests stand hielt. Ein besonderer Erfolg liegt in der Erweiterung des Modells zur Untersuchung von Feynman-Pfaden

SCEE 2008 book of abstracts : the 7th International Conference on Scientific Computing in Electrical Engineering (SCEE 2008), September 28 – October 3, 2008, Helsinki University of Technology, Espoo, Finland

This report contains abstracts of presentations given at the SCEE 2008 conference.reviewe