Saturation of interband absorption in semiconductors

The nonlinear absorption factors in a direct-gap semiconductor are calculated both for the pump and probe waves using the density matrix formalism. The key feature of the calculation is the consideration of the carrier dispersion. It is shown for the first time that the dispersion law defines the saturation behavior of the absorption factor. The intensity and frequency dependences of the absorption factors calculated differ significantly from those obtained previously. It is also shown that the conventional theory of nonlinear interband absorption based on the kinetic equations for carriers leads to the correct results only in the limit of small pump intensities.