Abstract
The Keldysh-Schwinger nonequilibrium Green's function theory is used to derive on the level of a screened Hartree-Fock approximation the relevant equations which determine the optical properties of laser-excited semiconductors. The equation for the optical interband polarization is derived for four different cases: (1) For situations in which the resonantly excited carriers are in quasi-equilibrium and in an (a) spatially homogeneous, or (b) spatially inhomogeneous state. (2) For ultrashort pulse excitation in which the excitations are in a coherent (unrelaxed) state; (a) for nonresonant excitation of the exciton, and (b) for resonant band-to-band excitation. In case (1a) a Bethe-Salpeter equation in the screened ladder approximation results, in case (2b) we obtain a generalization of Stahl's coherent band edge equation. In case (2a) our result agrees with the polarization equation of Schmitt-Rink and Chemla and explains the optical Stark shift of the exciton, while in case (2b) our result describes the spectral hole-burning (light induced gap) with an unrelaxed electron-hole population.
Published Version
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