Ultrafast electron dynamics in GaAs and InP studied by time-resolved terahertz emission spectroscopy

We studied the ultrafast dynamics of electrons generated by tunable femtosecond optical pulses having positive and negative excess energies in GaAs and InP by observing the temporal waveform of THz radiation emitted from biased photoconductive antennas. Sub-picosecond intraband relaxation was observed when the excess energy was positive. When excited by optical pulses having negative excess energies, it was observed that the THz waveform had a picosecond decay, which was attributed to the transition from the Urbach state to the free carrier state of electrons on the picosecond time scale. This dynamical behavior was found to be very sensitive to the applied electric field in the range of several kV/cm. The largest THz signal was obtained by pumping the emitter at the band-gap energy.