Coulomb effects on the gain spectrum of semiconductors

We have investigated the effect of Coulomb interactions on the gain spectra of highly excited semiconductors, specifically GaAs. It is shown that at low temperatures there is an enhancement of the direct excitation of approximately a factor of 2 and a sideband due to particle-hole pair excitations that extends into the gap. As the temperature increases, it is argued by analogy to the electron-phonon problem that the sideband changes into an Urbach tail with gain of the order of 10-100 cm-1.