Injection of ballistic pure spin currents in semiconductors by a single-color linearly polarized beam

We demonstrate the injection and control of pure spin currents in [110]-oriented GaAs quantum wells at room temperature by one-photon absorption of a single linearly polarized optical pulse. These currents result from the interference of absorption processes associated with the right and left circularly polarized components of the pulse, with the current direction determined by their relative phase. The current generation process differs from the circular photogalvanic effect, which relies only on the intensity of circularly polarized beams. By using spatially resolved pump-probe techniques, we obtain signatures for the currents by measuring the resulting spin separations of 1-4 nm. The separation decreases with increasing excitation fluence, consistent with a reduction in the momentum relaxation time with increasing carrier density.