Picosecond Competing Dynamics of Apparent Semiconducting-Metallic Phase Transition in the Topological Insulator Bi₂Se₃

Resolving the complex interplay between surface and bulk response is a long-standing issue in the topological insulators (TIs). Some studies have reported surface-dominated metallic responses, yet others show semiconducting-like bulk photoconductance. Using ultrafast terahertz spectroscopy with the advent of Fermi-level engineered TIs, we discovered that such difference arises from the time-dependent competing process of two parameters, namely, the Dirac-carrier surface scattering rate and the bulk Drude weight. After infrared femtosecond pulse excitation, we observed a metal-like photoconductance reduction for the prototypical n-type Bi₂Se₃ and a semiconductor-like increased photoconductance for the p-type Bi₂Se₃. Surprisingly, the bulk-insulating Bi₂Se₃, which is presumably similar to graphene, exhibits a semiconducting-to-metallic phase apparent transition at 10 ps. The sign-reversed, yet long-lasting (≥500 ps) metallic photoconductance was observed only in the bulk-insulating Bi₂Se₃, indicating that such dynamic phase transition is governed by the time-dependent competing interplay between the surface scattering rate and the bulk Drude weight. Our observations illustrate new photophysical phenomena of the photoexcited-phase transition in TIs and demonstrate entirely distinct dynamics compared to graphene and conventional gapped semiconductors.