Lineshape in Quantum Cascade Lasers - Temperature, Screening and Broadening

Sammanfattning: This dissertation deals with the optical properties of Quantum Cascade Lasers (QCLs). Studies are performed using transport calculations employing the theory of Non-Equilibrium Green's Functions, and screening is studied within the Random Phase Approximation. Focus is put on the temperature dependent effects in THz QCLs. After a popular science summary and a presentation and discussion of the theory and the methods used in this work, five original papers are presented. Paper I investigates the evolution of the optical spectra as a function of bias for a mid-infrared QCL. It is found that most spectral features can be explained by a shift of the electrons from the active region to the injection region of the laser. Paper II investigates the gain spectra of a THz QCL for increasing temperature. It is found that increased broadening, with constant population inversion, decrease the peak gain at low temperatures. The increased broadening is addressed to enhanced scattering due to reduced screening at elevated temperatures. Paper III investigates the impact of acoustic phonon scattering on the gain spectra for THz QCLs. It is found that at the temperatures of interest, acoustic phonon scattering has a marginal effect on the gain properties of THz QCLs. In Paper IV different aspects of the gain spectra are summarized with focus on the theoretical description using Non-Equilibrium Green's Functions and density matrix theory. Here, the focus lies on dispersive gain and correlations effects. Paper V investigates how the subband structure and the different subband temperatures affect screening. If is found that screening has a strong impact on scattering for increasing temperature, and that isotropic Debye screening is an excellent approximation for THz QCLs at temperatures of interest.