Microscopic approach for intersubband-based thermophotovoltaic structures in the terahertz and mid-infrared

This paper describes microscopic calculations of photocurrent generation spectra due to intersubband transitions in semiconductor heterostructures that can extract energy from photons in the THz and Mid Infrared Ranges. As expected in the mid infrared the interconduction conduction band transitions dominate the photocurrent. However, the numerical results presented here show that in the far infrared there is a range in which valence-band-based transitions dominate the photocurrent and these can be sustained under perpendicular incidence. This would lead to devices that do not need prisms and couplers in contrast with conduction-band based intersubband absorbers. Examples for different quantum well structures and different thermal source temperatures are compared and contrasted numerically. It is further demonstrated that many body effects, so far ignored in simulations of materials for photovoltaic and thermophotovoltaic applications, are shown to be of relevance for both conduction (TM Mode) and valence-band based (TE Mode) configurations

Superlattice nonlinearities for Gigahertz-Terahertz generation in harmonic multipliers

Semiconductor superlattices are strongly nonlinear media offering several technological challenges associated with the generation of high-frequency Gigahertz radiation and very effective frequency multiplication up to several Terahertz. However, charge accumulation, traps and interface defects lead to pronounced asymmetries in the nonlinear current flow, from which high harmonic generation stems. This problem requires a full non-perturbative solution of asymmetric current flow under irradiation, which we deliver in this paper within the Boltzmann-Bloch approach. We investigate the nonlinear output on both frequency and time domains and demonstrate a significant enhancement of even harmonics by tuning the interface quality. Moreover, we find that increasing arbitrarily the input power is not a solution for high nonlinear output, in contrast with materials described by conventional susceptibilities. There is a complex combination of asymmetry and power values leading to maximum high harmonic generation.Comment: 13 pages, 7 figures, Accepted for Nanophotonics (De Gruyter

Controlling the harmonic conversion efficiency in semiconductor superlattices by interface roughness design

In semiconductor superlattices, when Bragg oscillating electrons interact with an input electromagnetic field, frequency multiplication is possible. An ideal superlattice has a purely antisymmetric voltage current response and can thus produce only odd harmonics. However, real world superlattices can also have even harmonic response and that increases the range of possible output frequencies. These effects have been recently explained with a predictive model that combines an Ansatz solution for the Boltzmann Equation with a Nonequilibrium Green's Functions approach. This predictive tool, coupled with recent progress on GHz input sources, support the growing interest in developing compact room temperature devices that can operate from the GHz to the THz range. The natural question to ask is what efficiencies can be expected. This paper addresses this issue by investigating power-conversion efficiency in irradiated semiconductor superlattices. Interface imperfections are consistently included in the theory and they strongly influence the power output of both odd and even harmonics. Good agreement is obtained for predicted odd harmonic outputs with experimental data for a wide frequency range. The intrinsic conversion efficiency used is based on the estimated amplitude of the input field inside the sample and thus independent of geometrical factors that characterize different setups. The method opens the possibility of designing even harmonic output power by controlling the interface quality

TERA-MIR radiation : materials, generation, detection and applications II

The main objective of MPNS COST ACTION MP1204 is to advance novel materials, concepts and device designs for generating and detecting THz (0.3–10 THz) and Mid Infrared (10–100 THz) radiation using semiconductor, superconductor, metamaterials and lasers. This special edition summarizes part of the progress achieved in the first year of our network by covering a wide range of related topics, e.g. fundamental studies of intervalence THz polaritons and entangled excitons to the design and development of THz and MIR Quantum Cascade Lasers and novel antennas, filters and metamaterials. Spectroscopy and modelling of the interaction of THz radiation with biomatter are investigated. Mid infrared detection and emission are modelled and develped including supercontinuum studies. Relevant results for nitrides, bismides and graphene are discussed covering industrial and academic applications, theory and experiments, illustrating the impact of a synergistic approach to THz and MIR within our TERA-MIR concept

The linewidth enhancement factor of intersubband lasers: from a two-level limit to gain without inversion conditions

The linewidth enhancement (α factor) due to fluctuations in the refractive index induced by carrier fluctuations of intersubband lasers was initially expected to be zero. However, values ranging from -0.5 to 3 have been found experimentally. This paper resolves this controversy showing that counter rotating terms, usually ignored in simulations are the actual fundamental origin of nonzero α at peak gain even without inclusion of nonparabolicity and manybody effects, which are however needed to explain negative values. For laser without inversion conditions, significant as a potential out of the box solution for the elusive room temperature operation of terahertz lasers, α is found to be larger, but still at the same order of magnitude of conventional inverted medium lasers, thus ensuring their applicability to a huge number of spectroscopic applications which require sharp laser linewidths

Estudio de óxidos ferroicos complejos por simulaciones de primer principio a gran escala

RESUMEN: El objetivo principal de la tesis de D. Mauro Antonio Pereira Gonçalves ha sido el estudio de las propiedades topológicas no-triviales en sistemas ferroeléctricos mediante simulaciones atomísticas con modelos efectivos de segundos-principios (descritos en el capítulo 2 de la memoria). Desde hace más de una década se conoce la existencia de texturas magnéticas con una topología no trivial (skyrmiones, merones, hopfiones, etc). Estas estructuras poseen unas propiedades muy llamativas: están protegidas por topología (cuesta una energía destruirlas), y se pueden mover fácilmente con corrientes eléctricas externas lo que las hace candidatas ideales para la fabricación de memorias tipo “pista de carreras” (“race-track memories”). La fuerza motriz para que aparezcan estas fases no triviales es la presencia de interacciones relativistas quirales tipo Dzyaloshinskii-Moriya, presentes en láminas o interfases magnéticas en la que hay ausencia o ruptura de la simetría de inversión. Como esta interacción no tiene análogo en sistemas ferroeléctricos, el pensamiento general era que no se podían estabilizar estas texturas con topología no trivial en sistemas polares, caracterizados por la presencia de una polarización espontánea e invertible. Solo en los últimos años se habían mostrados indicios de su existencia en compuestos complejos (nanocolumnas de un material ferroeléctrico de pequeño diámetro embebidas en una matriz dieléctrica), difíciles de caracterizar experimentalmente. Sin embargo, en la presente tesis se predice teóricamente la existencia de skyrmiones en dos sistemas ferroeléctricos distintos, mucho más sencillos que los anteriores y, por lo tanto, experimentalmente verificables: • En el caso de un único material (PbTiO3) en el que se puedan escribir nanodominios ferroeléctricos tipo columna con una polarización opuesta a la de la matriz circundante (Capítulo 3 de la memoria). La rotación de la polarización en la pared de dominio entre las dos polarizaciones es la clave para obtener una estructura con una topología no trivial tipo skyrmion. Este hallazgo constituyó la primera predicción de un skyrmion eléctrico en un único material sencillo. Además de predecir su estabilización, en la tesis se dan las claves para poder modificarlo de manera controlada mediante campos eléctricos, tensiones impuestas por un substrato o por efectos térmicos (temperatura) (Capítulo 4 de la memoria). Se observan distinto tipos de transiciones, tanto skyrmion-skyrmion, como transiciones topológicas entre skyrmiones y sistemas dipolares triviales. Determinadas funciones respuesta (susceptibilidad) divergen en los puntos de la transición. • En el caso de superredes ferroeléctrico/dieléctrico en las que se intercalan capas de PbTiO3 y SrTiO3 (Capítulo 5 de la memoria). Dependiendo de la tensión epitaxial pueden estabilizarse estructuras tipo vórtice o skyrmiones. En la presente tesis se detallan las condiciones para obtener estas últimas, se procede al análisis riguroso de las propiedades topológicas, y sus propiedades funcionales (susceptibilidades y constantes dieléctricas). Este estudio se ha realizado en colaboración con grupos experimentales de la Universidad de California-Berkeley y la Universidad de Cornell. La constante retroalimentación entre teoría y experimento ha dado lugar al primer trabajo en el que se observa la estabilización de skyrmiones polares a temperatura ambiente en materiales ferroeléctricos, siendo los cálculos realizados en la presente tesis, una parte integral de los mismos.ABSTRACT: The main goal of this thesis was to explore the possibility that ferroelectric materials, characterized by a spontaneous and switchable electric polarization, may present topologically non-trivial structures akin to the skyrmions that occur in their ferromagnetic counterparts. The main tool used in the investigation was atomistic simulation based of first-principles effective models (“second-principles methods”), applied to two model systems: ferroelectric PbTiO3 and ferroelectric/paraelectric superlattices made of PbTiO3 and SrTiO3. More precisely, the simulations were used to analyze multidomain configurations in these compounds, motivated by previous reports that they may present non-trivial structural features. The main finding of the thesis is that, indeed, a simple multidomain configuration in PbTiO3 - namely, a columnar nanodomain with polarization opposed to that of its surrounding matrix – is sufficient to generate a dipole texture - associated to the rotation of the polarization at the domain wall between nanodomain and matrix - with the topology of a skyrmion. This constitutes the first prediction of an electric skyrmion in a simple ferroelectric material. Further, it is shown that the properties and topology of this skyrmion can be tuned by external electric and elastic fields, as well as by temperature, obtaining novel effects such as topological and iso-topological phase transitions. Finally, the investigation of the PbTiO3/SrTiO3 superlattices reveals that the skyrmion structures can be obtained as the ground state solution for such systems. This latter study was developed in the context of a collaboration with experimental groups at UC Berkeley and elsewhere, which led to the first experimental confirmation of electric skyrmions. Hence, in conclusion, the theoretical work in this thesis has been an integral part of the discovery of electric skyrmions in ferroelectric materials

The Impacts of MERCOSUR on Brazil

International Relations/Trade,

Numerical study of high impedance T-match antennas for terahertz photomixers

This paper outlines an efficient numerical method to design terahertz photomixers. The simulations are benchmarked using measured power levels from results published in the literature. Next, the method is applied to two new photomixer designs based on the high impedance T-match antenna with bias supply DC-blocking structures for either a uniplanar layout or a multilayer structure for improved device reliability. Manufacturability is favoured by avoiding the use of airbridges, substrate thinning or under-etching. The estimated output power of the improved design is 9.0 (Formula presented.), which is an improvement of three times over the reference photomixer

THz intervalence band antipolaritons

THz polaritons and antipolaritons have strong potential for device applications and are a challenging field of fundamental studies. In this paper, we start from a numerically exact nonequilibrium many body solutions and adjust it to a simplified nonlinear dielectric constant approach to the optical susceptibility. The resulting expression is inserted in the wave equation to describe the coupling of TE-polarized THz radiation with an intervalence band transition in GaAs/Al0.3Ga0.7As multiple quantum wells embedded in microcavities. The energy dispersions relations leading to THz polaritons are investigated. Here we focus on the impact of dephasing and scattering processes for different structures and excitation conditions in an inverted medium leading to antipolaritons