THz spectroscopy of electric field modulated Dirac surface states of topological insulator thin films

Topological insulators (TIs) exhibit symmetry protected conducting surface states with Dirac like dispersion and novel spin textures. TIs can also be described as bulk magnetoelectric materials with quantized magneto-optical response functions. However, observing such phenomena in real materials can be challenging because of the finite bulk contributions which tend to screen the surface state effects. Here, we present a novel ionic gel gating technique that can be used to tune the chemical potential while simultaneously measuring the optical spectra. We demonstrate that the Fermi level for naturally n-doped Bi₂Se₃ films can be lowered down to ~10 meV above the Dirac point, resulting in a reduction in the carrier concentration by an order of magnitude. At high magnetic fields, quantization of Faraday angle is observed which provides evidence for the topological magnetoelectric effects. Furthermore, we also demonstrate that the filling factor in the surface states can be altered by changing the applied bias voltage.