  We performed broadband optical transmission measurements of Bi2Se3 and
In-doped Bi(1-x)In(x)2Se3 thin films, where in the latter the spin-orbit
coupling (SOC) strength can be tuned by introducing In. Drude and interband
transitions exhibit In-dependent changes that are consistent with evolution
from metallic (x=0) to insulating (x=1) nature of the end compounds. Most
notably, an optical absorption peak located at hw=1eV in Bi2Se3 is completely
quenched at x=0.06, the critical concentration where the phase transition from
TI into non-TI takes place. For this x, the surface state (SS) is vanished from
the band structure as well. The correlation between the 1eV optical peak and
the SS in the x-dependences suggests that the peak is associated with the SS.
We further show that when Bi2Se3 is electrically gated, the 1eV-peak becomes
stronger(weaker) when electron is depleted from (accumulated into) the SS.
These observations combined together demonstrate that under the hw=1eV
illumination electron is excited from a bulk band into the topological surface
band of Bi2Se3. The optical population of surface band is of significant
importance not only for fundamental study but also for TI-based optoelectronic
device application.
