Giant Magneto-Optical Kerr Effect and Universal Faraday Effect in Thin-Film Topological Insulators

Topological insulators can exhibit strong magneto-electric 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_F=tan⁡^-1α, where α is the vacuum fine structure constant, and that it has a giant Kerr rotation θ_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.