Nonlinear response of quantum cascade structures

TL;DR

This paper analyzes the nonlinear optical response of terahertz quantum cascade lasers using a Green's functions approach, revealing higher harmonics and providing a method to reconstruct internal field strengths, with results matching experimental data.

Contribution

It introduces a non equilibrium Green's functions method to study nonlinear phenomena in quantum cascade lasers and proposes a new iterative technique for internal field reconstruction.

Findings

Higher harmonics of the response function are retrievable.

Gain under strong laser fields differs from linear response predictions.

Experimental data indicates the system operates beyond linear response.

Abstract

The gain spectrum of a terahertz quantum cascade laser is analysed by a non equilibrium Green's functions approach. Higher harmonics of the response function were retrievable, providing a way to approach nonlinear phenomena in quantum cascade lasers theoretically. Gain is simulated under operation conditions and results are presented both for linear response and strong laser fields. An iterative way of reconstructing the field strength inside the laser cavity at lasing conditions is described using a measured value of the level of the losses of the studied system. Comparison with recent experimental data from time-domain-spectroscopy indicates that the experimental situation is beyond linear response.