Light-enhanced incoherence of electronic transport in quantum cascade lasers

Andrzej Kolek

doi:10.1038/s41598-020-66302-4

Abstract

Since their invention in the middle of the 1990s, quantum cascade lasers (QCLs) attract increasing theoretical interest stimulated by their widening applications. One of the key theoretical issues is the optimization of electronic transport which in most of these devices is governed by the injection barrier of QCL heterostructure. In the paper, the nonequilibrium Green’s function formalism is used to study electronic transition through the injection barrier as a function of laser field in the cavity; for the increasing field, a crossover is observed from the strong coupling regime, in which electronic transport through the barrier is coherent, to the weak coupling regime, in which electronic transport gets incoherent. This crossover is characterized by gain recovery time, τrec, which takes sub-picosecond values for mid-IR QCLs operating at room temperature. This time is also important for the performance of devices under steady-state conditions; the maximum output power is obtained when the figure of merit, FOM = (g(0)/gth − 1)/gcτrec [g(0) is the linear response gain, gth is the threshold gain needed to compensate all losses, gc is the gain cross-section], reaches maximum. It is shown that the use of this optimization criterion can result in the structures essentially different from those which can be obtained when the optimized quantity is the linear response gain, g(0).

Highlights

Since their invention in the middle of the 1990s, quantum cascade lasers (QCLs) attract increasing theoretical interest stimulated by their widening applications
The nonequilibrium Green’s functions (NEGF) method is applied to the number of real QCL structures emitting light of ~5.2 μm wavelength that were tailored for the use in the systems of nitric oxide detection
While the “selfenergy” approach has already been used for interband devices[34,35,36], it is new for lasers relying on intersubband transitions

Summary

Introduction

Since their invention in the middle of the 1990s, quantum cascade lasers (QCLs) attract increasing theoretical interest stimulated by their widening applications. The QCL emitting in the mid-IR range usually works in the so-called strong-coupling regime[2] This description refers to the coupling of electronic states located on the opposite sides of the injection barrier; namely: the injector ground state and the upper laser state. Examples that use the density matrix (DM) method were proposed by Dupont et al.[9] for THz QCL and by Khurgin et al.[10] and Dinh et al.[11] for mid-IR devices In these attempts, the optimization of the injection barrier www.nature.com/scientificreports aims at the maximization of the gain peak value. Where ρij are the elements of the density matrix, τtr is the effective transport time, and 1 + Δ2τ 2

Methods

Results

Conclusion