Theory of gain spectra for quantum cascade lasers and temperature dependence of their characteristics at low and moderate carrier concentrations

We have developed a theory describing the operation of lasers based on intersubband transitions in a quantum well. The theory combines a first-principles description of the intersubband lineshape and the optical gain with kinetic models for carrier heating. The theory is consistent with the experimental data available and suggests new ways of improving the laser design for room-temperature operation with high output power. At low carrier concentrations, it is possible to achieve positive values of the gain at room temperature even in the absence of an overall population inversion between quantum-well subbands. For higher (but still moderate) concentrations, the theory predicts a peculiar dependence of the output wavelength on the pump current, including a regime where the lasing wavelength switches ''digitally'' between two stable values.