Abstract
Two-dimensional surface-scattering dynamics are central in the physics of topological insulators. Numerous electrical and optical measurements have evidenced that the origins of novel optoelectronic response can be traced back to Dirac surface-electron dynamics. Intrinsic surface dynamics, however, remain elusive because these experiments cannot access the frequencies of the surface-scattering rate. Time-resolved terahertz spectroscopy is the only apparatus for directly probing the collective response of low-energy electronic transitions. Here, by utilizing ultrafast optical-pump terahertz-probe spectroscopy, we discovered anomalous characteristics of the surface-scattering dynamics. Upon photoexcitation, the surface-scattering rate is increased and results in negative dynamic conductance at low temperature. Surprisingly, the differential changes of the surface-scattering rate are strongly reduced by photoexcited electrons at elevated temperature. We find that this nontrivial surface-electron dynamics is due to opening a carrier-relaxation channel from bulk to the surface state---one distinct characteristic of topological insulators. Our observations reveal unexpected surface dynamics of hot Dirac electrons, providing experimental a priori knowledge toward ultrafast optoelectronic operations.
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