We have investigated the THz range magneto-optical response of ultralow carrier density films of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ using time-domain THz polarimetry. Undoped ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ 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, ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ 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 a manifestation of the parity anomaly in 2D Dirac systems with broken time reversal symmetry (TRS).

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• Dirac fermions
• Landau levels
• Quantum Hall effect
• Topological insulators
• Thin films
• Topological materials
• Terahertz spectroscopy