Quantized Faraday rotation of surface states in 3D topological insulator thin films

Axion electrodynamics of topological surface states have been predicted a while ago, but the experimental observation has not been realized yet. One of the consequences of axion electrodynamics is a topological magneto-electric effect. In my talk, I will talk about utilizing a charge-transfer-doping method to lower the chemical point near the Dirac point in thin films of the topological insulator Bi$_2$Se$_3$. Using time-domain THz spectroscopy, we observed a crossover from semi-classical cyclotron resonances from topological surface states in the low field regime to a quantum regime at higher fields. A quantized Faraday rotation in the units of the fine structure constant was observed at high fields, which may provide the evidence for the topological magneto-electric effect.