Lasing from HgCdTe microdisc cavities is demonstrated at wavelengths as long as 22 — 25 μm. The optical threshold and operation temperature are far from being limited by intrinsic recombination processes. The employed ion etching technology appears to introduce additional defects in the vicinity of the microdiscs, degrading figures of merit as the height of the cavity increases. However, a watt-level mid-infrared pumping source should suffice for lasing in microdiscs with moderate height and ~100 μm diameter.

Гетероструктуры на основе CdHgTe с двойными квантовыми ямами (КЯ) являются сложными объектами, обладающими бóльшим числом топологических фаз, чем в гетероструктурах с одиночными КЯ [1].Последние известны прежде всего тем, что они являются первыми экспериментально обнаруженными двумерными топологическими изоляторами [2].Разнообразие возможных состояний в двойных КЯ связано с большим количеством параметров (составы и толщины КЯ и туннельного барьера и т.п.) и, не в последнюю очередь, с тем, что разделяющий КЯ барьер может являться туннельно-прозрачным для электронов, но не для дырок.Многообразие параметров приводит к необходимости проверки их номинальных значений, закладываемых при росте структур, независимыми методами.Одним из таких методов может являться исследование магнитопоглощения.В слабых магнитных полях оно дает информацию об эффективных массах на уровне Ферми, а в сильных (квантующих) -информацию о переходах между уровнями Ландау.В конечном итоге это позволяет восстанавливать реальный зонный спектр исследуемых объектов, определять фактические параметры структур, а также обнаруживать явления, выходящие за рамки «простых» моделей.В настоящем докладе будут представлены результаты исследований магнитопоглощения в квантующих магнитных полях в гетероструктурах HgTe/CdHgTe с двойными квантовыми ямами.Будут продемонстрированы спектры магнитопоглощения, измеренные в магнитных полях до 30 Тл, выявлены многочисленные спектральные особенности и выполнена их интерпретация в рамках четырехзонной модели Кейна [3].Кроме того, будет продемонстрировано дополнительное расщепление основных линий магнитопоглощения, связанных с переходами с «нулевых» уровней Ландау, которое позволит конкретизировать природу структурной асимметрии в двойных квантовых ямах HgTe/CdHgTe, а также определить вклады, обусловленные встроенным электрическим полем, различием толщин квантовых ям и порядка их расположения в структуре [4].Наконец, в докладе будут представлены результаты исследований магнитопоглощения при различных температурах.Будет продемонстрирован фазовый переход между различными топологическими фазами, а также рассмотрена тонкая структура антикроссинга «нулевых» уровней Ландау в образце с «двойной инверсией».

HgTe/CdHgTe quantum well (QW) heterostructures have attracted a lot of interest recently due to insights they provided towards the physics of topological insulators and massless Dirac fermions. Our work focuses on HgCdTe QWs with the energy spectrum close to the graphene-like relativistic dispersion that is supposed to suppress the non-radiative Auger recombination. We combine various methods such as photoconductivity, photoluminescence and magneto-optical measurements as well as transmission electron microscopy to retrofit growth parameters in multi-QW waveguide structures, designed for long wavelengths lasing in the range of 10–22 μm. The results reveal that the attainable operating temperatures and wavelengths are strongly dependent on Cd content in the QW, since it alters the dominating recombination mechanism of the carriers.

The lack of radiation sources in the frequency range of 7-10 THz is associated with strong absorption of the THz waves on optical phonons within the GaAs Reststrahlen band. To avoid such absorption, we propose to use HgCdTe as an alternative material for THz quantum cascade lasers thanks to a lower phonon energy than in III-V semiconductors. In this work, HgCdTe-based quantum cascade lasers operating in the GaAs phonon Reststrahlen band with a target frequency of 8.3 THz have been theoretically investigated using the balance equation method. The optimized active region designs, which are based on three and two quantum wells, exhibit the peak gain exceeding 100 cm-1 at 150 K. We have analyzed the temperature dependence of the peak gain and predicted the maximum operating temperatures of 170 K and 225 K for three- and two-well designs, respectively. At temperatures exceeding 120 K, the better temperature performance has been obtained for the two-well design, which is associated with a larger spatial overlap of weakly localized lasing wavefunctions, as well as, a higher population inversion. We believe that the findings of this work can open a pathway towards the development of THz quantum cascade lasers featuring a high level of optical gain due to the low electron effective mass in HgCdTe.

A challenge of bridging the terahertz gap with semiconductor lasers faces an inevitable problem of enhanced non-radiative Auger recombination with reduction of photon energy. We show that this problem can be mitigated in mercury-cadmium-telluride quantum wells (HgCdTe QWs) wherein the Auger process is suppressed due to formation of quasi-relativistic electron-hole dispersion imposing strong energy-momentum restrictions on recombining carriers. Such dispersion is formed upon interaction of topological states at the two QW interfaces. We characterize the lasing properties of HgCdTe QWs quantitatively by constructing a microscopic theory for recombination, absorption, and gain, and show the feasibility of lasing down to ~ 50 $\mu$m at liquid nitrogen temperature with threshold currents two orders of magnitude lower than in existing lasers. Our findings comply with recent experimental data on stimulated far-infrared emission from HgCdTe QWs and show the directions toward achievement of maximum possible lasing wavelength.

We review recently proposed concepts of infrared and terahertz photodetectors based on graphene van der Waals heterostructures and HgTe-CdHgTe quantum well heterostructures and demonstrate their potential. (C) 2019 Association of Polish Electrical Engineers (SEP). Published by Elsevier B.V. All rights reserved.

Interband photoluminescence (PL) and stimulation emission (SE) from HgTe/HgCdTe quantum well (QW) heterostructures are studied in 5–20 µm wavelength range in regard to long‐wavelength lasing applications. The authors obtain carrier lifetimes using time‐resolved photoconductivity measurements and show that the dominating mechanism of carrier recombination changes from the radiative process to the non‐radiative one as the bandgap is decreased, limiting the “operating” temperature for SE. The authors suggest that decreasing the QW width should reverse the balance in carrier recombination in favor of radiative processes and demonstrate 75 K improvement in the “operating” temperature in structure with narrower QW.

The paper presents the results of the application of MOCVD growth technique for formation of the GaAs/AlAs laser structures with InGaAs quantum wells on Si substrates with a relaxed Ge buffer. The fabricated laser diodes were of micro-striped type designed for the operation under the electrical pumping. Influence of the Si substrate offcut from the [001] direction, thickness of a Ge buffer and insertion of the AlAs/GaAs superlattice between Ge and GaAs on the structural and optical properties of fabricated samples was studied. The measured threshold current densities at room temperatures were 5.5 kA/cm2 and 20 kA/cm2 for lasers operating at 0.99 μm and 1.11 μm respectively. In order to obtain the stimulated emission at wavelengths longer than 1.1 μm, the InGaAs quantum well laser structures with high In content and GaAsP strain-compensating layers were grown both on Ge/Si and GaAs substrates. Structures grown on GaAs exhibited stimulated emission under optical pumping at the wavelengths of up to 1.24 μm at 300 K while those grown on Ge/Si substrates emitted at shorter wavelengths of up to 1.1 μm and only at 77 K. The main reasons for such performance worsening and also some approaches to overcome them are discussed. The obtained results have shown that monolithic integration of direct-gap A3B5 compounds on Si using MOCVD technology is rather promising approach for obtaining the Si-compatible on-chip effective light source.

The authors discuss the photon-assisted resonant radiative intergraphene layer transitions for applications of double-graphene-layer (D-GL) heterostructures for THz/IR lasers and photodetectors (PDs). The main element of both lasers and PDs under consideration is a D-GL core-shell heterostructure with the independently contacted GLs separated by the thin transparent tunnel-barrier layer. The bias voltage applied between the GL's contacts induces the electron and hole gases in the opposing GLs. The electron and hole densities in GLs are controlled by the gate voltage.

Magnetoabsorption in HgCdTe-based structures in quantizing magnetic fields have been measured. The results were fit with numerical calculations made within four-band model.

We report on a magnetospectroscopy study of a set of four HgTe quantum wells of different thickness. In quantizing magnetic fields, intraband and interband transitions have been observed. The obtained results are compared with the allowed transition energies calculated using the 8 × 8 Kane model.

Recently, a new class of materials, so-called topological insulators, has emerged. These are systems characterized by the inversion of the electronic band structure and also by a certain strength of the spin-orbit interaction. HgTe/CdxHg1-xTe quantum wells represent a prominent example. They can change to the topological insulator phase from the conventional insulator phase when the thickness of the quantum well is increased over the critical thickness d(c) = 6.3 nm. Here, we report on a far-infrared magnetospectroscopy study of a set of HgTe/CdxHg1-xTe quantum wells with different thicknesses from below to above the critical value d(c). In quantizing magnetic fields up to 16 T, both intraband and interband transitions have been clearly observed. In the widest quantum well with inverted band structure, we confirm the avoided crossing of the zero-mode Landau levels observed earlier in similar structures. In both noninverted quantum wells close to the critical thickness, we report unambiguously on the square root dependence of the transition energy on the magnetic field, as expected in the single-particle model of massless Dirac fermions. The obtained results are compared with the allowed transition energies between Landau levels in the valence and conduction bands calculated using the 8 x 8 Kane model.

This chapter considers new terahertz (THz) device concepts based on graphene layers (GLs), utilizing the specific features of the energy spectrum of graphene, namely, the zero energy gap and the constant velocity of electrons and holes. It reviews and analyzes several proposals of following devices: GL lasers with optical pumping and cascade emission of optical phonons; and GL injection lasers and tunneling transit-time devices with electrically induced lateral pin junctions. In the device structures based on the electrically induced pin junctions, such as injection lasers and GL tunneling transit-time (G-TUNNETT) devices, the highly conducting split gates can serve as the waveguides. The chapter demonstrates that novel THz sources can generate the electromagnetic radiation with the frequencies starting from about 1 THz at room temperatures. Controlled Vocabulary Terms graphene; optical pumping; semiconductor lasers; terahertz waves; transit time devices

A possibility to develop the so called TASER (Terahertz-Amplification-by-the-Stimulated-Emission-of-Radiation) by using two-dimensional (2D) electron transport in quantum well (QW) structures is investigated by Monte Carlo simulation of the optical-phonon-emission assisted transit-time resonance (OPTTR) of 2D electrons in momentum space under the low lattice temperature. A considerable extension of the frequency region for THz radiation generation (upto 5 times) when going from 3D- to 2D-case is predicted.

Hole cyclotron resonance lines due to transitions between different electric subbands were discovered in the magnetoabsorption spectra of strained Ge/GeSi heterostructures with wide (similar to 80 nm) QWs. The measurements were performed on the undoped samples in the frequency range from 0.35 to 1.25 THz at T = 4.2 K with optical band-gap excitation.

New shallow acceptor magnetoabsorption lines in THz range have been discovered under bandgap photoexcitation in strained Ge/GeSi quantum well (QW) heterostructures. The magnetoabsorption lifetime was as high as 10−4 s, indicating that the absorption is due to the transitions from the ground states of very shallow acceptors. The observed absorption resonances are shown to result from the photoexcitation of A+‐centers and 1s→2p+ type transitions from the ground state of the barrier‐situated A0‐centers into excited states in the 1st and 2nd electric subbands. The shallowest discovered ground acceptor states (EB ⩽ 0.5 meV) are attributed to the “barrier‐spaced” acceptors (a hole bound with an acceptor ion in the neighboring Ge QW).

The model of a novel laser using the GaAs/AlGaAs lattice nonlinearity for mid-infrared emission due to nonlinear frequency conversion is proposed. An active region, designed to generate two-frequency near-infrared simultaneous oscillations, is embedded in a vertical-cavity surface-emitting structure. Double Bragg mirrors, tuned to both the mid- and near-infrared wavelengths, are used to enhance emission. The power density of mid-infrared emission, at a wavelength of about 13 /spl mu/m, is shown to be of the order of 10/sup -3/ /spl mu/W//spl mu/m/sup 2/ under a pumping current of 5 kA/cm/sup 2/.