Ultrafast intraband electron dynamics of GaAs and InP observed by terahertz emission

We have studied the ultrafast dynamics of electrons generated by femtosecond optical pulses with positive and negative excess energies in GaAs and InP by observing the waveform of the emitted terahertz radiation. Subpicosecond intraband relaxation was observed with positive excess energies. With negative excess energies, a picosecond transition from the Urbach state to free carrier states was observed. This dynamical behaviour was found to be very sensitive to the applied field magnitude in the range of several kV/cm.