Terahertz response of ultrafast spin polarization in semi-insulating GaAs

Due to its high sensitivity and time-resolved ability, terahertz time-domain spectroscopy is a powerful tool for investigating ultrafast carrier dynamics in semiconductors. In addition to charges, spins of ultrafast carriers provide an alternate degree of freedom to design modern electronic devices but are rarely studied by terahertz time-domain spectroscopy. Here, ultrafast spin polarization in semi-insulating GaAs is studied by optical-pump terahertz-probe experiments at room temperature. We used circularly and linearly polarized femtosecond laser pulses to inject nonequilibrium carriers in GaAs and observed that both the transmitted and reflected terahertz signals exhibited different dynamical evolutions under the excitations of linearly and circularly polarized laser pulses, which are ascribed to the generation and relaxation of spin-polarized electrons. The lifetime of the ultrafast spin polarization was obtained from our experiments, highlighting the potentialities of terahertz spectroscopy for the investigation of spin relaxation in semiconductors.