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 theta(F) = tan(-1 alpha), where alpha is the vacuum fine structure constant, and that it has a giant Kerr rotation theta(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.