Many body theory of THz intervalence gain in quantum wells

This paper presents calculations of THz gain in III-V quantum wells based on transitions between the valence bands. The system is out of equilibrium and the optical susceptibility leading to absorption and gain is calculated after a Keldysh nonequilibrium Greens' functions approach. The relevance of many body and nonresonant contribution to the spectrum is discussed as a function of local population inversion in k-space for a system where the injected holes are distributed within the two lowest subbands without global population inversion.