Sequential multi-photon strategy for semiconductor-based terahertz detectors

A semiconductor-based terahertz-detector strategy, exploiting a bound-to-bound-to-continuum architecture, is presented and investigated. In particular, a ladder of equidistant energy levels is employed, whose step is tuned to the desired detection frequency and allows for sequential multi-photon absorption. Our theoretical analysis demonstrates that the proposed multi-subband scheme could represent a promising alternative to conventional quantum-well infrared photodetectors in the terahertz spectral region.Comment: Submitted to Journal of Applied Physic

Terahertz detection schemes based on sequential multi-photon absorption

We present modeling and simulation of prototypical multi bound state quantum well infrared photodetectors and show that such a detection design may overcome the problems arising when the operation frequency is pushed down into the far infrared spectral region. In particular, after a simplified analysis on a parabolic-potential design, we propose a fully three-dimensional model based on a finite difference solution of the Boltzmann transport equation for realistic potential profiles. The performances of the proposed simulated devices are encouraging and support the idea that such design strategy may face the well-known dark-current problem.Comment: 3 pages, 2 figures; submitted to Applied Physics Letter

Monte Carlo Kinetic Modeling of the Combined Carrier-Phonon Nonequilibrium Dynamics in Semiconductor Heterostructure Devices

Electron-phonon interaction is a key mechanism for charge and heat transport in both bulk materials as well as in state-of-the-art electronic and optoelectronic solid-state devices. Indeed, that of an effective heat dissipation, at the diverse design levels, has always been a primary issue in device operation and performances. In various circumstances, the charge carrier subsystem happens to be coupled to a significant nonequilibrium optical phonon population. This regime may be particularly pronounced in new-generation quantum emitters based on semiconductor heterostructures and operating both in the mid-infrared as well as in the terahertz region of the electromagnetic spectrum. In this chapter, we review a global kinetic approach based on a Monte Carlo simulation technique that we have recently proposed for the modeling of the combined carrier-phonon nonequilibrium dynamics in realistic unipolar multisubband device designs. Results for the case of a prototypical resonant-phonon terahertz emitting quantum cascade laser are shown and discussed

Design and Simulation of THz Quantum Cascade Lasers

Strategies and concepts for the design of THz emitters based on the quantum cascade scheme are analyzed and modeled in terms of a fully three-dimensional Monte Carlo approach; this allows for a proper inclusion of both carrier-carrier and carrier-phonon scattering mechanisms. Starting from the simulation of previously published far-infrared emitters, where no population inversion is achieved, two innovative designs are proposed. The first one follows the well-established chirped-superlattice scheme whereas the second one employs a double-quantum well superlattice to allow energy relaxation through optical phonon emission. For both cases a significant population inversion is predicted at temperatures up to 80 K.Comment: 4 pages, 2 figures, 2 table

Improving the operation temperature of semiconductor-based Terahertz photodetectors: A multiphoton design

We propose and theoretically investigate a semiconductor-based terahertz-detector design exploiting a multiphoton absorption strategy through a bound-to-bound-to-continuum scheme. Our results demonstrate that such a multisubband architecture may access values of the background-limited infrared photodetection temperature, significantly higher than those of conventional quantum well infrared photodetectors operating at the same frequency, and therefore could represent a better alternative to the latter in the terahertz spectral region

Modeling of open quantum devices within the closed-system paradigm

We present an alternative simulation strategy for the study of nonequilibrium carrier dynamics in quantum devices with open boundaries. We propose to replace the usual modeling of open quantum systems based on phenomenological injection/loss rates with a kinetic description of the system-reservoir thermalization process. In this simulation scheme the partial carrier thermalization induced by the device spatial boundaries is treated within the standard Boltzmann-transport approach via an effective scattering mechanism between the highly nonthermal device electrons and the thermal carrier distribution of the reservoir. Applications to state-of-the-art semiconductor nanostructures are discussed. Finally, the proposed approach is extended to the quantum-transport regime; to this end, we introduce an effective Liouville superoperator, able to describe the effect of the device spatial boundaries on the time evolution of the single-particle density matrix

Quantum-Information Processing with Semiconductor Macroatoms

An all optical implementation of quantum information processing with semiconductor macroatoms is proposed. Our quantum hardware consists of an array of semiconductor quantum dots and the computational degrees of freedom are energy-selected interband optical transitions. The proposed quantum-computing strategy exploits exciton-exciton interactions driven by ultrafast sequences of multi-color laser pulses. Contrary to existing proposals based on charge excitations, the present all-optical implementation does not require the application of time-dependent electric fields, thus allowing for a sub-picosecond, i.e. decoherence-free, operation time-scale in realistic state-of-the-art semiconductor nanostructures.Comment: 11 pages, 5 figures, to be published in Phys. Rev. Lett., significant changes in the text and new simulations (figure 3

Tight-binding approach to excitons bound to monolayer impurity planes: strong radiative properties of InAs in GaAs

A theory of Wannier-Mott excitons bound to monolayer (ML) impurity planes in semiconductors, which is based on Green's function tight-binding calculations of the single-particle states, is presented. Binding energies and oscillator strengths for one and two MLs of InAs in GaAs are predicted to be much larger than in the usual InxGa1-xAs/GaAs thick quantum wells. The reason is the increase of effective mass of both carriers due to folding of the InAs bands along the growth direction. The results suggest that ML insertions can be used as intense light sources in light-emitting devices

Crossover from strong to weak confinement for excitons in shallow or narrow quantum wells

We present a theoretical study of the crossover from the two-dimensional (2D, separate confinement of the carriers) to the three-dimensional (3D, center-of-mass confinement) behavior of excitons in shallow or narrow quantum wells (QW's). Exciton binding energies and oscillator strengths are calculated by diagonalizing the Hamiltonian on a large nonorthogonal basis set. We prove that the oscillator strength per unit area has a minimum at the crossover, in analogy with the similar phenomenon occurring for the QW to thin-film crossover on increasing the well thickness, and in agreement with the analytic results of a simplified δ-potential model. Numerical results are obtained for GaAs/Alx Ga1-xAs and InxGa1-xAs/GaAs systems. Our approach can also be applied to obtain an accurate description of excitons in QW's with arbitrary values of the offsets (positive or negative) and also for very narrow wells. In particular, the crossover from 2D to 3D behavior in narrow GaAs/AlxGa1-xAs QW's is investigated: the maximum binding energy of the direct exciton in GaAs/AlAs QW's is found to be ∼26 meV and to occur between one and two monolayers

Hyperoxemia and excess oxygen use in early acute respiratory distress syndrome : Insights from the LUNG SAFE study

Publisher Copyright: © 2020 The Author(s). Copyright: Copyright 2020 Elsevier B.V., All rights reserved.Background: Concerns exist regarding the prevalence and impact of unnecessary oxygen use in patients with acute respiratory distress syndrome (ARDS). We examined this issue in patients with ARDS enrolled in the Large observational study to UNderstand the Global impact of Severe Acute respiratory FailurE (LUNG SAFE) study. Methods: In this secondary analysis of the LUNG SAFE study, we wished to determine the prevalence and the outcomes associated with hyperoxemia on day 1, sustained hyperoxemia, and excessive oxygen use in patients with early ARDS. Patients who fulfilled criteria of ARDS on day 1 and day 2 of acute hypoxemic respiratory failure were categorized based on the presence of hyperoxemia (PaO2 > 100 mmHg) on day 1, sustained (i.e., present on day 1 and day 2) hyperoxemia, or excessive oxygen use (FIO2 ≥ 0.60 during hyperoxemia). Results: Of 2005 patients that met the inclusion criteria, 131 (6.5%) were hypoxemic (PaO2 < 55 mmHg), 607 (30%) had hyperoxemia on day 1, and 250 (12%) had sustained hyperoxemia. Excess FIO2 use occurred in 400 (66%) out of 607 patients with hyperoxemia. Excess FIO2 use decreased from day 1 to day 2 of ARDS, with most hyperoxemic patients on day 2 receiving relatively low FIO2. Multivariate analyses found no independent relationship between day 1 hyperoxemia, sustained hyperoxemia, or excess FIO2 use and adverse clinical outcomes. Mortality was 42% in patients with excess FIO2 use, compared to 39% in a propensity-matched sample of normoxemic (PaO2 55-100 mmHg) patients (P = 0.47). Conclusions: Hyperoxemia and excess oxygen use are both prevalent in early ARDS but are most often non-sustained. No relationship was found between hyperoxemia or excessive oxygen use and patient outcome in this cohort. Trial registration: LUNG-SAFE is registered with ClinicalTrials.gov, NCT02010073publishersversionPeer reviewe