Transient electrical conductivity of nonequilibrium carriers excited by subpicosecond optical pulses in GaAs

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Abstract
We calculate the time-dependent electrical conductivity σ(t) for the nonequilibrium carriers excited by a subpicosecond laser pulse in a polar semiconductor like GaAs. The photoexcited carriers are assumed to relax towards equilibrium by emitting longitudinal optical (LO) phonons in a cascade via the strong Fröhlich interaction. We show that σ(t), obtained in the picosecond time domain, has a highly nonlinear time evolution. It develops a pronounced dip, with its value becoming negative, whenever the generated low density carrier distribution is sharply peaked initially at energy ε≂mℏωLO, m=integer, ℏωLO being the long wavelength LO phonon energy.