Pure spin current gratings in semiconductors generated by quantum interference

Abstract

We demonstrate that the quantum mechanical interference between the probability amplitudes for the two-photon absorption of a fundamental (1.55 μm)∼150 fs pulse and for the one-photon absorption of a noncollinearly propagating second-harmonic (775 nm) pulse can create transient, ballistic, purely spin-polarized current gratings in bulk GaAs at room temperature. For fundamental and second-harmonic pulses having orthogonal linear polarizations, two periodically modulated ballistic spin-polarized current gratings are injected that have opposite spins and opposite propagation directions at each point along the grating. Consequently, there is no initial modulation of the charge current, carrier population, or net spin. Before the carrier momentum relaxes, the transport associated with these spin currents forms two oppositely spin-polarized population gratings that are exactly out of phase spatially and that decay by electronic spin diffusion in a time of 3.2 ps. In addition, charge density gratings are directly produced by the quantum interference process, and they decay by ambipolar diffusion and recombination (∼17.6 ps). The polarization selection rules and sample orientation are used to separate the contributions of the current and density gratings.