Carrier-carrier scattering rates within nonequilibrium optically injected semiconductor plasmas

The rate at which athermal, nearly monoenergetic free-carrier distributions initially scatter out of optically injected states in bulk GaAs is studied experimentally and theoretically. Pump-probe absorption bleaching measurements done with 2-eV 75-fs pulses are reported for several injected densities ranging from <1×${10}^{16}$ ${\mathrm{cm}}^{\mathrm{-}3}$ to >5×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$. By using a technique in which the amplitude of the peak bleaching is measured as a function of the injected carrier density at a fixed (∼30 fs) pump-probe delay, the density-dependent carrier-carrier scattering (CCS) component of the signal is revealed. Calculations of the CCS rate, which include the effects of dynamical screening within Gaussian distributions are also reported, and compared with the results obtained using the static-screening approximation. In both cases the CCS rate is found to depend weakly on the density, N; approximately as ${\mathit{N}}^{0.35\pm 0.05}$. A heuristic model is described that provides insight as to why CCS rates within athermal distributions are relatively insensitive to the density and dynamic screening effects.