Raman scattering in the presence of photoexcited nonequilibrium carriers in semiconductors: theoretical considerations

Abstract

A theory of Raman scattering from a semiconductor in the presence of nonequilibrium phonons and carriers, both generated and probed by very short subpicosecond laser pulses, is presented. Because of (1) the short duration of the probe pulse, (2) the generation of nonequilibrium phonons by the optically injected carriers, and (3) the presence of nonequilibrium ("hot") carriers, the spontaneous Raman scattering signal from both electronic and ionic excitations can show a distinct temporal evolution. We explicitly derive this time-varying signal, and contrast it with the usual case of long (i.e., cw) probe pulses and equilibrium carriers in polar semiconductors like GaAs. We find that the use of a short probe pulse simply broadens the observed spectral features. For hot carriers, the concept of an average time-dependent longitudinal dielectric function ε(q,ω,t) is introduced to determine the coupled LO-phonon-plasmon modes at a given time t, while carriers are relaxing towards the band extrema. When the carriers are very hot, a plasmalike collective excitation does not exist because of heavy Landau damping, and the electronic Raman scattering is only due to the single-particle excitations. The phononlike collective mode can shift in frequency as the carriers cool.