The Quantum Cascade Laser (QCL) is a semiconductor diode laser operating in the infra-red and THz region and was first realized at Bell Labs in 1994 [1]. The operating principle of QCLs differs fundamentally from standard diode lasers as light emission occurs from transitions between electronic states in quantum wells formed in the conduction band of laser material. This thesis investigate the microscopic mechanisms determining the spectra of the lasing material. Focus is put on the mechanisms that reduce peak gain with temperature. The main finding is that the electronic temperature strongly affect scattering strengths via screening which then is reflected in increased spectral broadening.

[1] Faist, J., Capasso, F., et. al., Science, 1994, 264, 553.