Theory of non-Markovian optical gain in excited semiconductors

The optical gain of a quantum-well laser is studied taking into account the valence-band mixing, non-Markovian relaxation and the many-body effects. Conventional gain spectra calculated with the Lorentzian line shape function show two anomalous phenomena: unnatural absorption region below the band-gap energy and mismatch of the transparency point in the gain spectra with the Fermi-level separation, the latter suggesting that the carriers and the photons are not in thermal (or quasi) equilibrium. It is shown that the non-Markovian gain model with many-body effects removes the two anomalies associated with the Lorentzian line shape function with the proper choice of the correlation time.