Terahertz Negative Differential Conductivities in Bulk GaAs

Intrinsic dynamical conductivity spectra of GaAs have been determined from time-domain terahertz waveforms emitted from undoped bulk GaAs photoexcited by femtosecond laser pulses under strong bias electric fields. The cutoff frequency for the negative differential conductivities due to intervalley transfer gradually increases with increasing electric field and reaches ~800 GHz at 50 kV/cm. The temperature dependence of the cutoff frequency indicates that it is governed by the energy relaxation process of electrons from the L-to the Gamma-valley via successive optical phonon emission.