Density-matrix theory of the optical dynamics and transport in quantum cascade structures: The role of coherence

TL;DR

This paper investigates how quantum coherence influences optical response and stationary transport in quantum cascade lasers, revealing that coherence causes observable effects in different parameter regimes, emphasizing the need for a fully coherent model.

Contribution

It introduces a microscopic density-matrix approach to analyze the role of coherence in quantum cascade structures, highlighting its impact on both optical and transport phenomena.

Findings

Coherence affects stationary current and gain oscillations.

Observable coherent effects occur in regimes with different level broadening and tunnel coupling.

A fully coherent model is essential for accurate description of optical and transport properties.

Abstract

The impact of coherence on the nonlinear optical response and stationary transport is studied in quantum cascade laser structures. Nonequilibrium effects such as pump-probe signals, the spatio-temporally resolved electron density evolution, and the subband population dynamics (Rabi flopping) as well as the stationary current characteristics are investigated within a microscopic density-matrix approach. Focusing on the stationary current and the recently observed gain oscillations, it is found that the inclusion of coherence leads to observable coherent effects in opposite parameter regimes regarding the relation between the level broadening and the tunnel coupling across the main injection barrier. This shows that coherence plays a complementary role in stationary transport and nonlinear optical dynamics in the sense that it leads to measurable effects in opposite regimes. For this reason, a fully coherent consideration of such nonequilibrium structures is necessary to describe the combined optical and transport properties