  Magnetic topological insulator is a fertile platform to study the interplay
between magnetism and topology. The unique electronic band structure can induce
exotic transport and optical properties. However, a comprehensive optical study
in both near-infrared frequency and terahertz frequency has been lacking. Here,
we report magneto-optical effects from a heterostructure of Cr-incorporated
topological insulator, CBST. We use 800 nm magneto-optical Kerr effect to
reveal a ferromagnetic order in the CBST film with a high transition
temperature at 160 K. We also use time-domain terahertz polarimetry to reveal a
terahertz Faraday rotation of 1.5 mrad and Kerr rotation of 5.1 mrad at 2 K.
The calculated terahertz Hall conductance is 0.42 $e^2/h$. Our work shows the
optical responses of an artificially layered magnetic topological insulator,
paving the way towards high-temperature quantum anomalous Hall effect via
heterostructure engineering.
