Electric field modulated topological magnetoelectric effect in Bi2Se3

Topological insulators have been predicted to exhibit a variety of interesting phenomena including a quantized magnetoelectric response and novel spintronics effects due to spin textures on their surfaces. However, experimental observation of these phenomena has proved difficult due to the finite bulk carrier density which may overwhelm the intrinsic topological responses that are expressed at the surface. Here, we demonstrate an ionic gel gating technique to tune the chemical potential of ${\mathrm{Bi}}₂{\mathrm{Se}}₃$ thin films while simultaneously performing THz spectroscopy. We can tune the carrier concentration by an order of magnitude and shift the Fermi energy ${E}_F$ to as low as $\ensuremath{\simeq}10$ meV above the Dirac point. At high-bias voltages and magnetic fields, we observe a quantized Faraday angle consistent with the topological magnetoelectric effect that can be tuned by ionic gel gating through a number of plateau states.