Surface state-dominated photoconduction and THz-generation in topological Bi$_{2}$Te$_{2}$Se-nanowires

Topological insulators constitute a fascinating class of quantum materials with non-trivial, gapless states on the surface and trivial, insulating bulk states. In revealing the optoelectronic dynamics in the whole range from femto- to microseconds, we demonstrate that the long surface lifetime of Bi$_{2}$Te$_{2}$Se-nanowires allows 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 state dynamics on the different timescales gives rise to a surprising physical property of Bi$_{2}$Te$_{2}$Se-nanowires: their pulsed photoconductance changes polarity as a function of laser power. Moreover, we show that single Bi$_{2}$Te$_{2}$Se-nanowires can be used as THz-generators for on-chip high-frequency circuits at room temperature. Our results open the avenue for single Bi$_{2}$Te$_{2}$Se-nanowires as active modules in optoelectronic high-frequency and THz-circuits.