  Topological insulators in the Bi2Se3 family have an energy gap in the bulk
and a gapless surface state consisting of a single Dirac cone. Low frequency
optical absorption due to the surface state is universally determined by the
fine structure constant. When the thickness of these three dimensional
topological insulators is reduced, they become quasi-two dimensional insulators
with enhanced absorbance. The two dimensional insulators can be topologically
trivial or non-trivial depending on the thickness, and we predict that the
optical absorption is larger for topological non-trivial case compared with the
trivial case. Since the three dimensional topological insulator surface state
is intrinsically gapless, we propose its potential application in wide
bandwidth, high performance photo-detection covering a broad spectrum ranging
from terahertz to infrared. The performance of photodetection can be
dramatically enhanced when the thickness is reduced to several quintuple
layers, with a widely tunable band gap depending on the thickness.
