Band Structure and Polarization Effects in Photothermoelectric Spectroscopy of a Bi₂Se₃ Device

Bi₂Se₃ is a prototypical topological insulator which has a small band gap (~0.3 eV) and topologically protected conducting surface states. Here we show in a quasi-bulk nanoflake device that we can measure the energy dependent optical absorption through the photothermoelectric effect. Spectral signatures are seen for several optical transitions between the valence and conduction bands, as well as the 1.5 eV optical transition between the two topologically protected conducting surface states. We also observe a surprising linear polarization dependence in the response of the device which reflects the influence of the metal contacts.