  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.
