Subthreshold carrier-LO phonon dynamics in semiconductors with intermediate polaron coupling: A purely quantum kinetic relaxation channel

Femtosecond transmission spectra of highly polar CdTe are compared to more covalent GaAs contrasting semiclassical kinetics with two-time quantum kine tics based on the Dyson equation. Nonequilibrium heavy holes in CdTe show u ltrafast energy redistribution via the Frohlich mechanism even if photoexci ted below the LO phonon energy. This subthreshold relaxation is a genuine q uantum kinetic effect. It gains importance if the polaron self-energy is co mparable to the phonon energy. Conservation of the free-particle energies i s not required under these conditions.