All-optical injection and control of spin and electrical currents in quantum wells

We show that quantum interference between one- and two-photon absorption can be used to inject spin currents, with or without an accompanying electrical current, in unbiased semiconductor quantum well structures. The directions in which the electrical and spin currents are injected can be coherently controlled, with a relative phase parameter of the optical fields as the control parameter. We characterize the currents for an unstrained quantum well and a quantum well under biaxial compressive strain using the Luttinger-Kohn model; we work out particular examples. If compressive strain is used to appropriately rearrange the subbands, then a degree of spin polarization of the spin currents higher than possible in bulk GaAs can be achieved and maintained even for photon energies well above the band gap.