Strong magneto-optical effects due to surface states in three-dimensional topological insulators

We show that a thin film of a three-dimensional topological insulator such as Bi$_2$Se$_3$ or Bi$_2$Te$_3$ can exhibit strong linear and nonlinear magneto-optical effects in a transverse magnetic field. In particular, one can achieve an almost complete circular polarization of an incident terahertz or mid-infrared radiation and an efficient four-wave mixing.