Coherence of resonant-tunneling transport in terahertz quantum-cascade lasers

TL;DR

This paper develops density-matrix models to analyze coherence effects in resonant-tunneling transport of terahertz quantum-cascade lasers, revealing new insights into current behavior and gain spectra, including the impact of coherent coupling and anticrossing phenomena.

Contribution

It introduces analytical density-matrix models for RT transport in QCLs, providing new understanding of current characteristics and gain spectra influenced by coherence effects.

Findings

Current-voltage slope discontinuity measures population inversion at threshold.

Broad gain linewidth results from coherent RT-assisted depopulation.

Double-peaked gain spectra occur at specific bias conditions, matching experiments.

Abstract

We develop simple density-matrix models to describe the role of coherence in resonant-tunneling (RT) transport of quantum-cascade lasers (QCLs). Specifically, we investigate the effects of coherent coupling between the lasing levels with other levels on the transport properties and gain spectra. In the first part of the paper, we use a three-level density-matrix model to obtain useful analytical expressions for current transport through the injector barrier in a QCL. An expression for the slope discontinuity in the current-voltage characteristics at the lasing threshold is derived. This value is shown to be a direct measure of the population inversion at threshold, and contradicts the previously held belief of it being indicative of ratio of the laser level lifetimes. In the second part of the paper, we use density matrices to compute the gain spectrum for a resonant-phonon terahertz QCL design. The large anticrossing of the doublet of lower radiative levels is reflected in a broad gain linewidth due to a coherent RT assisted depopulation process. At certain bias conditions, the gain spectrum exhibits double peaks which is supported by experimental observations.