Ultrafast carrier dynamics in GaAs-like semiconductors, following carrier injection by subpicosecond laser pulses, has been of much interest in recent years. In this presentation, we focus attention on the nature of the transient, frequency-resolved complex linear electrical conductivity σ(t, ω)of these carriers. Our study reveals that such a nonequilibrium conductivity response has several features significantly different from those shown by near-equilibrium carrier systems. In particular, we find that the low-frequency conductivity has an oscillatory behavior when the excitation photon energy is varied and can take absolute negative values for a short time immediately after photoexcitation. Furthermore, the terahertz frequency-resolved conductivity can have an oscillatory behavior in frequency, significantly different from the Crude behavior. Also, a terahertz electrical pulse can be amplified on reflection from the photoexcited GaAs surface under certain conditions.

© 1992 Optical Society of America

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