Surface State-Dominated Photoconduction and THz Generation in Topological Bi₂Te₂Se Nanowires

Topological insulators constitute a fascinating class of quantum materials with nontrivial, gapless states on the surface and insulating bulk states. By revealing the optoelectronic dynamics in the whole range from femto- to microseconds, we demonstrate that the long surface lifetime of Bi₂Te₂Se nanowires allows us to access the surface states by a pulsed photoconduction scheme and that there is a prevailing bolometric response of the surface states. The interplay of the surface and bulk states dynamics on the different time scales gives rise to a surprising physical property of Bi₂Te₂Se nanowires: their pulsed photoconductance changes polarity as a function of laser power. Moreover, we show that single Bi₂Te₂Se nanowires can be used as THz generators for on-chip high-frequency circuits at room temperature. Our results open the avenue for single Bi₂Te₂Se nanowires as active modules in optoelectronic high-frequency and THz circuits.