Photoinduced pure spin-current injection in graphene with Rashba spin-orbit interaction

We propose a photoexcitation scheme for pure spin-current generation in graphene subject to a Rashba spin-orbit coupling. Although excitation using circularly polarized light does not result in optical orientation of spins in graphene unless an additional magnetic field is present, we show that excitation with linearly polarized light at normal incidence yields spin-current injection without magnetic field. Spins are polarized within the graphene plane and are displaced in opposite directions, with no net charge displacement. The direction of the spin current is determined by the linear polarization axis of the light, and the injection rate is proportional to the intensity. The technique is tunable via an applied bias voltage and is accessible over a wide frequency range. We predict a spin-current polarization as high as 75% for photon frequencies comparable to the Rashba frequency. Spin-current injection via optical methods removes the need for ferromagnetic contacts, which have been identified as a possible source of spin scattering in electrical spin injection in graphene.