Nanowire terahertz quantum cascade lasers

Abstract

Quantum cascade lasers made of nanowire axial heterostructures are proposed. The dissipative quantum dynamics of their carriers is theoretically investigated using non-equilibrium Green functions. Their transport and gain properties are calculated for varying nanowire thickness, from the classical-wire regime to the quantum-wire regime. Our calculation shows that the lateral quantum confinement provided by the nanowires allows an increase of the maximum operation temperature and a strong reduction of the current density threshold compared to conventional terahertz quantum cascade lasers.Quantum cascade lasers (QCLs) 1 are semiconductor unipolar devices emitting coherent infrared radiation. Their active regions consist of stacked planar quantum wells (QWs) forming a superlattice along the growth direction. At first sight, a large part of the physical properties of QCLs can be qualitatively understood in a onedimensional picture, in particular for quantum confinement, tunneling processes and radiative transitions. Nevertheless, the free in-plane motion of the carriers provides a continuous energy reservoir (i.e. subbands) which plays a major role in the energy and phase relaxation processes. For QCLs operating in the terahertz (THz) range 2,3 , due to the relatively small energy separation between the subbands associated with the lasing levels, the non-radiative scattering processes are very efficient and are responsible for intrinsic limitations in terms of quantum efficiency and maximum operation temperature.Reducing the dimensionality of electronic systems is generally a way to increase the carrier lifetimes 4-7 . A strong magnetic field applied along the growth axis of a QCL quantizes the lateral motion in terms of Landau levels. Their energy tuning with magnetic field has been shown to be responsible for modulation of the level lifetimes 8,9 , allowing to demonstrate lasing up to higher temperature (225 K in ref. 10 ) than in usual THz QCLs (199 K in ref. 11 ). However, Landau levels remain highly degenerate so that their ideal discrete density of states is strongly broadened by disorder effects 12 . On the other hand, QCLs based on 0D nanostructures such as self-assembled or lithographically defined quantum dots (QDs) have been proposed [13][14][15][16] in order to achieve low threshold current and high operation temperature. However, a direct comparison of the device performances with respect to conventional THz QCLs is lacking, and the intermediate regime between the limiting cases coupled 0D QDs and coupled 2D QWs remains to be investigated.In this letter we propose QCLs based on nanowire (NW) superlattice heterostructures. Using the formalism of non-equilibrium Green functions (NEGF), we calculate charge transport and THz gain in laterally-quantized quantum cascade (QC) structures. We investigate the dynamics of charge carriers in QC structures having lateral dimensionality ranging from 2D to 0D, providing a direct comparison of NW QCLs and conventional QCLs. Quantum cascade lasers made of nanowire axia...

Density matrix model for polarons in a terahertz quantum dot cascade laser

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“…[26][27][28][29]. In these works, the electron-phonon interaction is accounted for by a phonon Greens function which enters into the electron self-energy; it is therefore represented as an average field which is assumed to remain at thermal equilibrium.…”

“…The most detailed approach involves nonequilibrium Green's function (NEGF) methods, which provide motivation for the lateral confinement approach, but are extremely computationally intensive and also tedious for the nonexpert [24][25][26][27][28][29]. Density matrix models are attractive since they allow intuitive use of quantum-cascade wave functions as a basis and have been shown to capture signatures of coherent electron tunneling as well as coherent response to the optical field [30][31][32][33][34].…”

Intersubband absorption in Si‐ and Ge‐doped GaN/AlN heterostructures in self‐assembled nanowire and 2D layers

“…Semiconductor nanowires (NWs) have already demonstrated a wide range of promising applications in optoelectronics, electronics, sensing, energy, etc. There is an interest in NWs to improve the performance of intersubband (ISB) devices motivated by: Their low electrical cross‐section implies low electrical capacitance, thus a larger operation bandwidth than planar devices. Owing to antenna effects, this comes without degradation of light absorption.…”

Mid-Infrared Quantum-Dot Quantum Cascade Laser: A Theoretical Feasibility Study

“…In addition to QDs, nanowires have also been studied in this context theoretically recently [13,14]. Early proposals with QDs [15] involved transitions between QDs in different layers, but the idea of using transitions inside a single QD was already suggested in Reference [16].…”