  Bi2Se3 is a prototypical topological insulator which has a small band gap
(~0.3 eV) and topologically protected conducting surface states. This material
exhibits quite strong thermoelectric effects. Here we show in a mechanically
exfoliated thick (~100 nm) nanoflake device that we can measure the energy
dependent optical absorption through the photothermoelectric effect. Spectral
signatures are seen for a number of optical transitions between the valence and
conduction bands, including a broad peak at 1.5 eV which is likely dominated by
bulk band-to-band optical transitions but is at the same energy as the well
known optical transition between the two topologically protected conducting
surface states. We also observe a surprising linear polarization dependence in
the response of the device that reflects the influence of the metal contacts.
