Observation of optical absorption correlated with surface state of topological insulator

We performed broadband optical transmission measurements of Bi2Se3 and In-doped (Bi1-xInx)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 the metallic (x=0) to insulating (x=1) nature of the end compounds. Most notably, an optical absorption peak located at ω=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) has vanished from the band structure as well. The correlation between the 1 eV optical peak and the SS in the x dependencies suggests that the peak is associated with the SS. We further show that when Bi2Se3 is electrically gated, the 1 eV peak becomes stronger (weaker) when electron is depleted from (accumulated into) the SS. These observations combined together demonstrate that under the ω=1eV illumination electron is excited from a bulk band into the topological surface band of Bi2Se3. The optical population of the surface band is of significant importance not only for fundamental study but also for TI-based optoelectronic device application.