Observation of memory effects in electron-phonon quantum kinetics

After a short-pulse optical excitation, the initial relaxation of electrons via emission of longitudinal optical phonons cannot be described using the standard Boltzmann equation, which assumes the scattering to be instantaneous. Rather, memory effects have to be taken into account. Within density matrix theory, we derive a set of kinetic equations which go beyond the selfconsistent second-Born (2B) approximation. The results compare favourably with an exactly solvable quantum kinetic model. Spectrally resolved pump-probe transmission experiments on bulk GaAs are reported using femtosecond pulses. In agreement with the calculations, the observed initially broad phonon replica sharpens on a timescale of 100 fs, which gives clear evidence of the quantum kinetic nature of electron-phonon scattering. (C) 1998 Elsevier Science B.V. All rights reserved.