Collision duration for polar optical and intervalley phonon scattering

The use of fs laser pulses to excite plasmas in semiconductors has become a major method for studying fast processes. The transition times from the Gamma valley to the satellite X and L valleys are comparable to the reciprocal of the frequency of the phonons involved, bringing into question the use of the standard perturbation-theory approaches. Our aim is to evaluate the time required to emit a phonon, either the intravalley LO or the intervalley, by a nearly-free electron in semiconductors. The leading idea of our work is that the so-called ''collision duration'' is related to the time required to build up correlation between the initial and final state, and then to destroy this correlation as the collision is completed. The calculations are developed using e nonequilibrium Green's function formalism, which allows us to evaluate explicitly the effects of the correlations in time.