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We consider the ultrafast kinetics of evolution of optical phonons in a photoinjected highly excited plasma in semiconductors. The state of the nonequilibrium ͑''hot''͒ phonon system is described in terms of the concept of a nonequilibrium temperature, referred to as quasitemperature, per mode, which can be experimentally characterized and measured. The phonon emission time shows that optical phonons are preferentially produced, well in excess of equilibrium, in a reduced off-center region of the Brillouin zone. The phonons in this region are responsible for the phenomenon referred to as ''hot-phonon temperature overshoot.'' Most of the phonons, namely, those outside such a region, are only weakly to moderately excited, and mutual thermalization of the nonequilibrium carriers and optical phonons follows, typically, in the tenfold picosecond scale. All these results are influenced by the experimental conditions, which we discuss on the basis of calculations specialized for GaAs. Comparison with experimental data is presented. ͓S0163-1829͑96͒04840-0͔

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We consider effects of diffusion in the photogenerated carrier system in highly photoexcited polar semiconductors. We develop a quantum quasihydrodynamic description of the system based on the nonequilibrium statistical operator formalism. We derive a generalized Fick's diffusion equation for the charge density of the carriers, with the ambipolar diffusion coefficient obtained at the microscopic level and depending on the evolving macroscopic (nonequilibriium thermodynamic) state of the sample. A detailed numerical calculation for the case of GaAs is done, obtaining good agreement with experimental data.

The nonequilibrium thermodynamics and kinetics of evolution of relaxation processes in polar semiconductors under high levels of optical excitation is studied. This is done using a first-principles theory that allows for the determination of the nonlinear transport equations which describe the irreversible processes that develop in the media in typical pump-probe experiments. Numerical calculations are presented which permit a comprehensive discussion of measurements of ultrafast-time-resolved optical spectra of GaAs.