Relaxation kinetics in mid-infrared quantum cascade lasers

Non-equilibrium coupled electron-phonon system in active region of the quantum cascade intersubband laser is investigated. Distribution functions and gain spectra of a three-subband double-quantum-well active region are obtained as a function of temperature and injection current. The important role of the non-equilibrium phonons at losing threshold is shown and discussed in details. It is shown that the threshold current is strongly dependent of the power dissipated in the active region in steady state. The numerical calculations for an 8.5-μm laser illustrate the general issues of relaxation kinetics in quantum cascade lasers. Temperature dependence of the threshold current is obtained in a good agreement with the experiment. We also present the results of fabrication of high-T₀ quantum cascade lasers operating in 5- and 8-μm bands.