Terahertz-frequency-resolved transient conductivity of nonthermal electrons photoexcited in GaAs

We calculate the terahertz-frequency dependence of the transient linear conductivity {σ}(t,{ω}) of electrons photoinjected in GaAs by a subpicosecond optical pulse. The electrons are assumed to relax to the conduction-band edge mainly by emitting longitudinal-optical phonons. We find that the occurrence of a nonthermal electron distribution, in combination with the time dependence of the mean effective momentum relaxation rate, leads to a non-Drude-like frequency response. In particular, the conductivity at large frequencies ({ω}/2{π}>1 THz) immediately after photoexcitation exceeds the corresponding conductivity after the electrons relax to the conduction-band edge. We also discuss how the Pines-Nozieres conductivity sum rule has to be modified under the nonequilibrium conditions.