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Topological insulators are commonly described as bulk insulators that exhibit symmetry protected conducting surface states with Dirac like dispersion and novel spin textures. A better description perhaps would be to understand them 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 and only recently, such topological magnetoelectric effects have been observed in Bi2Se3 films. 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. This allows us to tune the surface state contribution to the topological magnetoelectric effects which manifest as quantized Faraday and Kerr rotations at high magnetic fields. The technique, in principle, can be applied to a wide array of samples beyond topological insulators and is a versatile method of gating that is also compatible with various forms of low-energy optical spectroscopy.
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