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Giant Magneto-optical Kerr Effect and Universal Faraday Effect in Thin-film Topological Insulators

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, $θ_{\mathrm{F}} = \mathrm{tan}^{-1}\,α$ where $α$ is the vacuum fine structure constant, and that it has a giant Kerr rotation $θ_{\mathrm{K}} = π/2$. These properties follow from a delicate interplay between thin-film cavity confinement and the surface Hall conductivity of a topological insulator's helical quasiparticles.