Study of the surface and bulk states of Bi${}_2$Te${}_3$ topological insulators using terahertz time-domain spectroscopy

Topological insulators (TIs) are widely recognized as a new state of quantum matter with semiconductor-like or insulator-like bulk states (BSs) and Dirac-like surface states (SSs). Herein, the terahertz (THz) properties of the SSs and BSs in Bi${}_2$Te${}_3$-based TIs, as well as the effects of temperature on them, are examined. By using THz time-domain spectroscopy, the optical conductance of Bi${}_2$Te${}_3$ is obtained for TI thickness of about 10 and 20 nm. It is shown that the THz responses of the SSs and BSs can be distinguished by purely optical means in a wide temperature range (80-280 K). A Drude formula, which includes two terms corresponding to the SSs and BSs, is applied to describe the behavior of the optical conductance. Moreover, the evidence of bulk-to-surface scattering and additional electron injection from the BSs to the SSs is also observed without the help of electric measurement. These results are helpful to gain an in-depth understanding of the THz properties of TIs and to explore new applications in topologically protected plasmonic and optoelectronic devices. (© 2023 Wiley‐VCH GmbH)