Optical spin-orbit torque in Rashba ferromagnets

We study the optically induced torques on thin film ferromagnetic layers under excitation by circularly polarized light. We include Rashba spin-orbit coupling and assume an out-of-plane magnetization, and consider incident light with an in-plane component of angular momentum (e.g. light with an oblique angle of incidence). Solving the semiconductor Bloch equations, we find simple expressions for the torque per optical absorption rate in the limits of weak and strong spin-orbit coupling. For weak spin-orbit coupling, the torque is bounded by the optical angular momentum injection rate. For strong spin-orbit coupling, the torque can exceed the angular momentum injection rate by a factor $\Delta\tau/\hbar$, where $\Delta$ is the magnetic exchange splitting and $\tau$ is the carrier scattering time.