Observation of optical absorption correlated with surface state of topological insulator

TL;DR

This study links a 1 eV optical absorption peak in Bi2Se3 to its topological surface state, showing the peak's disappearance at the phase transition and its modulation by electrical gating, highlighting its significance for optoelectronic applications.

Contribution

It demonstrates that the 1 eV optical absorption peak is directly associated with the topological surface state and varies with phase transition and gating, providing insight into surface state dynamics.

Findings

The 1 eV peak is quenched at the topological phase transition.

Gating modulates the 1 eV peak strength, indicating surface state involvement.

Optical excitation at 1 eV populates the topological surface band.

Abstract

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.