Quantum cascade lasers without intersubband population inversion

The fundamental limits of the operation of quantum cascade intersubband lasers are investigated. Band nonparabolicities combined with the nonthermal electron distribution in the active region make laser action possible even in the absence of global k-space population inversion between subbands, i.e., when the lifetime of the lower subband exceeds that of the upper one. A laser based on local k-space population inversion with single-quantum-well active regions is demonstrated and its performance compared to that of quantum cascade lasers with double-quantum-well active regions.