Study of the Surface and Bulk States of Bi₂Te₃ 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 Bi2Te3-based TIs, as well as the effects of temperature on them, are examined. By using THz time-domain spectroscopy, the optical conductance of Bi2Te3 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.