Magneto-optical Third Harmonic Generation in Topological Insulator Films*

The discovery of materials where interactions with spin, lattice, and orbital degrees of freedom are linked with the electronic kinetic energy has led to a heightened interest in their nonlinear optical (NLO) response. Examples of these materials in three dimensions are topological insulators (TIs), which have a gapped bulk spectrum while hosting metallic helical states on surfaces. They exhibit strong magneto-optical responses due to the topologically nontrivial bulk and surface states, making them attractive candidates for probing enhanced NLO responses in a magnetic field. In this talk, we present our theory for magneto-optical third-harmonic generation (THG) in topological insulator films in a quantizing magnetic field. We obtain the THG NLO conductivity arising from one-, two-, and three-photon processes and investigate the effects of changing magnetic field, chemical potential, and film thickness. Comparing to the zero-field case, the THG nonlinear conductivity exhibits strong enhancements due to multiphoton cyclotron resonances and bulk states contribution. We address the key differences of these enhancements in TI from the THG in graphene.