Free carrier absorption in quantum cascade structures

TL;DR

This paper demonstrates that free carrier absorption in quantum cascade lasers is minimal and differs significantly from classical models due to quantum effects, with a detailed quantum mechanical analysis of various absorption mechanisms.

Contribution

It provides a quantum mechanical calculation of free carrier and inter-subband absorption in QCLs, highlighting the dominance of electron-phonon interactions at room temperature.

Findings

Free carrier absorption is very small in QCLs.

Quantum effects cause a radical difference from classical Drude predictions.

Optical phonon interactions dominate at room temperature.

Abstract

We show that the free carrier absorption in Quantum Cascade Lasers is very small and radically different from the classical Drude result on account of the orthogonality between the direction of the carrier free motion and the electric field of the laser emission. A quantum mechanical calculation of the free carrier absorption and inter-subband oblique absorption induced by interface defects, coulombic impurities and optical phonon absorption/emission is presented for QCL's with a double quantum well design. The interaction between the electrons and the optical phonons dominates at room temperature.