A recently developed quantum kinetic description for the electrons in a two-band polar semiconductor coupled to LO phonons and excited by a femtosecond laser pulse is extended to include excitonic effects. Our numerical treatment yields a description of the relaxation of the electron-hole pairs which is valid on a time scale where the semiclassical Boltzmann description is no longer applicable. Oscillations with the period of the LO lattice oscillation are superimposed on the resulting polarization decay under excitation close to the band edge. The polarization oscillations are quantum beats between the direct interband transitions and their LO-phonon sidebands. The non-Markovian quantum kinetics is shown to yield, in contrast to the corresponding nonretarded kinetics for weak, nonresonant, stationary excitation, an Urbach absorption tail which is approximately exponential over several orders of absorption as universally observed in polar semiconductors.

• Received 24 July 1992

DOI:https://doi.org/10.1103/PhysRevB.47.3574