We report mid-infrared spectroscopy measurements of an electrostatically gated topological insulator, in which we observe several percent modulation of transmittance and reflectance of (Bi1-xSbx)2Te3 films as gating shifts the Fermi level. Infrared transmittance measurements of gated (Bi1-xSbx)2Te3 films were enabled by use of an epitaxial lift-off method for large-area transfer of topological insulator films from infrared-absorbing SrTiO3 growth substrates to thermal oxidized silicon substrates. We combine these optical experiments with transport measurements and angle-resolved photoemission spectroscopy to identify the observed spectral modulation as a gate-driven transfer of spectral weight between both bulk and topological surface channels and interband and intraband channels. We develop a model for the complex permittivity of gated (Bi1-xSbx)2Te3, and find a good match to our experimental data. These results open the path for layered topological insulator materials as a new candidate for tunable infrared optics and highlight the possibility of switching topological optoelectronic phenomena between bulk and spin-polarized surface regimes.
@article{arxiv.1607.03844,
title = {Gate-Variable Mid-Infrared Optical Transitions in a $(Bi_{1-x}Sb_x)_2Te_3$ Topological Insulator},
author = {William S. Whitney and Victor W. Brar and Yunbo Ou and Yinming Shao and Artur R. Davoyan and D. N. Basov and Ke He and Qi-Kun Xue and Harry A. Atwater},
journal= {arXiv preprint arXiv:1607.03844},
year = {2017}
}