Negative free carrier absorption in terahertz quantum cascade lasers

TL;DR

This paper investigates how negative free carrier absorption, caused by LO phonon-assisted transitions, can act as a gain mechanism in terahertz quantum cascade lasers, especially when electronic temperatures exceed lattice temperatures, leading to potential laser development.

Contribution

It demonstrates the existence of negative free carrier absorption as a gain source and reports a terahertz quantum cascade laser operating under these conditions.

Findings

Negative free carrier absorption can provide gain in quantum cascade lasers.

A gain of a few cm$^{-1}$ is achieved at 200K.

Development of a terahertz quantum cascade laser in this regime.

Abstract

We analyze the peculiar case where the free carrier absorption arising from LO phonon absorption-assisted transitions becomes negative and therefore turns into a gain source for quantum cascade lasers. Such an additional source of gain exists when the ratio between the electronic and the lattice temperatures is larger than one, a condition that is usually fulfilled in quantum cascade lasers. We find a gain of few cm$^{-1}$'s at 200K. We report the development of a terahertz quantum cascade laser operating in the negative free carrier absorption regime.