Gain saturation in photoexcited semiconductors

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Abstract
A model is presented which provides a satisfactory explanation for the spectral shape of the stimulated emission in highly photoexcited (direct-gap) semiconductors at low temperatures. The model assumes the existence of an electron-hole plasma which fills a fraction of the excited volume depending on the excitation intensity. The processes in the plasma are electron and hole generation by the external excitation, recombination by stimulated emission, and particle relaxation within the plasma by collisions. The last one is described by a cross section which depends on the particle-energy separation from the Fermi surface. The coupled rate equations for the electrons (holes) and the radiation field are solved under steady-state conditions. The calculated stimulated-emission spectra show a red shift as a function of a parameter which is proportional to the product of the excitation intensity and stripe length. The result explains well the red shift and saturation of stimulated-emission spectra observed in GaAs by Shaklee et al.