Contrasting influence of charged impurities on transport and gain in terahertz quantum cascade lasers

TL;DR

This paper investigates how charged impurities differently affect transport and optical gain in terahertz quantum cascade lasers, revealing that impurity scattering significantly impacts gain but only weakly influences current density, due to momentum-dependent impurity interactions.

Contribution

It demonstrates the contrasting effects of impurity scattering on transport and gain, highlighting the importance of momentum dependence and suggesting doping profile engineering for improvements.

Findings

Current density is weakly affected by impurity scattering.

Optical gain is highly sensitive to impurity scattering.

Small-momentum impurity scattering causes incoherent tunneling regimes.

Abstract

Transport and gain properties of a resonant-phonon terahertz quantum cascade laser are calculated using nonequilibrium Green's functions. Impurity scattering is shown to be responsible for contrasting nonlinear effects in the transport and the gain properties. For typical doping concentrations, the current density is found to be weakly sensitive to the impurity scattering strength. In contrast, the calculated gain is found to be very sensitive to the impurity scattering strength. This difference is attributed to the strong momentum dependence of the long-range coupling to charged impurities. Small-momentum impurity scattering is shown to be responsible for an incoherent regime of resonant tunneling processes. These new insights into the crucial role of impurity scattering open a new route of improvement of terahertz quantum cascade lasers by engineering of the doping profile.