  We have investigated the THz range magneto-optical response of ultralow
carrier density films of Sb$_2$Te$_3$ using time-domain THz polarimetry.
Undoped Sb$_2$Te$_3$ has a chemical potential that lies inside the bulk valence
band. Thus its topological response is masked by bulk carriers. However, with
appropriate buffer layer engineering and chemical doping, Sb$_2$Te$_3$ thin
films can be grown with extremely low electron or hole densities. The ultralow
carrier density samples show unusual optical properties and quantized response
in the presence of magnetic fields. Consistent with the expectations for Dirac
fermions, a quantized Hall response is seen even in samples where the zero
field conductivity falls below detectable levels. The discontinuity in the
Faraday angle with small changes in the filling fraction across zero is
manifestation of the parity anomaly in 2D Dirac systems with broken time
reversal symmetry.
