Coherent control of optical injection of spin and currents in topological insulators

Topological insulators have surface states with a remarkable helical spin structure, with promising prospects for applications in spintronics. Strategies for generating spin-polarized currents, such as the use of magnetic contacts and photoinjection, have been the focus of extensive research. While several optical methods for injecting currents have been explored, they have all focused on one-photon absorption. Here we consider the use of both a fundamental optical field and its second harmonic, which allows the injection of spin-polarized carriers and current by a nonlinear process involving quantum interference between one- and two-photon absorption. General expressions are derived for the injection rates in a generic two-band system, including those for one and two-photon absorption processes as well as their interference. Results are given for carrier, spin density, and current injection rates on the surface of topological insulators, for both linearly and circularly polarized light. We identify the conditions that would be necessary for experimentally verifying these predictions.