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The temperature dependence of the complex optical properties of thethree-dimensional topological insulator Bi2Te2Se is reported for lightpolarized in the a-b planes at ambient pressure, as well as the effects ofpressure at room temperature. This material displays a semiconducting characterwith a bulk optical gap of 300 meV at 295 K. In addition to the two expectedinfrared-active vibrations observed in the planes, there is additional finestructure that is attributed to either the removal of degeneracy or theactivation of Raman modes due to disorder. A strong impurity band located at200 cm^{-1} is also observed. At and just above the optical gap, severalinterband absorptions are found to show a strong temperature and pressuredependence. As the temperature is lowered these features increase in strengthand harden. The application of pressure leads to a very abrupt closing of thegap above 8 GPa, and strongly modifies the interband absorptions in themid-infrared spectral range. While ab initio calculations fail to predict thecollapse of the gap, they do successfully describe the size of the band gap atambient pressure, and the magnitude and shape of the optical conductivity.

Optical properties of Bi2Te2Se at ambient and high pressure