Collision retardation and its role in femtosecond-laser excitation of semiconductor plasmas

The use of femtosecond laser pulses to excite electron-hole plasmas in semiconductors has become a major method of studying fast processes. The transition times from the central {Γ} valley in GaAs to the satellite X and L valleys are comparable to the reciprocal of the frequency of the phonon involved, bringing into question the use of standard perturbation-theory approaches. We investigate the role of retardation arising from the collision duration required for such a transition to occur, through the use of a path-integral form of the quantum kinetic equation. A secondary self-scattering process and a probability of completed collisions are used to cast this equation in a semiclassical form.