  Transient reflectivity (TR) from thin films (6 - 40 nm thick) of the
topological insulator Bi2Se3 reveal ultrafast carrier dynamics, which suggest
the existence of both radiative and non-radiative recombination between
electrons residing in the upper cone of initially unoccupied high energy Dirac
surface states (SS) and holes residing in the lower cone of occupied low energy
Dirac SS. The modeling of measured TR traces allowed us to conclude that
recombination is induced by the depletion of bulk electrons in films below ~20
nm thick due to the charge captured on the surface defects. We predict that
such recombination processes can be observed using time-resolved
photoluminescence techniques.
