Topological insulators can exhibit strong magnetoelectric effects when their time-reversal symmetry is broken. In this Letter we consider the magneto-optical Kerr and Faraday effects of a topological insulator thin film weakly exchange-coupled to a ferromagnet. We find that its Faraday rotation has a universal value at low-frequencies, $\theta_{ \mathrm{ F}} = \mathrm{ tan}^{ -1}\,\alpha$ where $\alpha$ is the vacuum fine structure constant, and that it has a giant Kerr rotation $\theta_{ \mathrm{ K}} = \pi/2$. These properties follow from a delicate interplay between thin-film cavity confinement and the surface Hall conductivity of a topological insulator's helical quasiparticles.
Categories: cond-mat.mes-hall