Bloch gain in quantum cascade lasers

Laser action by a weak to vanishing population inversion is observed in mid-infrared (7 μm) quantum cascade lasers. The origin of the optical gain is the same than for the Bloch oscillator predicted in superlattices. This special gain does not only relax the threshold condition for lasing, but also underlines the major role played by coherent processes in semiconductor heterostructures, as it is based on scattering assisted optical transitions.