THz spectroscopy of ultralow carrier density topological insulator thin films

Sb₂Te₃ is one of the earliest known examples of a 3D topological insulator with a single Dirac cone on the surface. Pure Sb₂Te₃ is generally p-doped with the Fermi level embedded in the bulk valence band which makes it challenging to study surface state dynamics through transport and optical measurements. Only recently, bulk insulating Sb₂Te₃ thin films have been grown using molecular beam epitaxy via chemical doping and buffer layer engineering. In addition, the carrier densities in these films are exceptionally low, nearly an order of magnitude lower than the best results obtained in Bi₂Se₃. Here we report the magneto-optical response of regular and bulk insulating (p-type and n-type) Sb₂Te₃ films at THz frequencies using time-domain polarimetry. While undoped, bulk conducting samples exhibit usual semiclassical behaviour, the bulk insulating samples show unusual optical properties and quantized response in presence of magnetic fields.