Ultrafast Nonequilibrium Carrier Dynamics in Topological Insulator Bi2Se3 Probed by Terahertz Spectroscopy at Room Temperature

Topological insulators (TIs) feature unique Dirac fermion-hosting surface states with exceptional electronic properties, rendering them promising candidates for optoelectronic and spintronic applications. Herein, we investigate the relaxation dynamics of photoexcited carriers in Bi2Se3 films via optical pump–terahertz (THz) probe spectroscopy (OPTP) at room temperature. Under 800 nm pump pulse excitation, the time-dependent real part of the pump excitation conductivity Δσ exhibits a positive-to-negative sign reversal as carriers relax toward equilibrium, which is further validated by frequency-dependent conductivity spectra at varied pump-probe delays. The initial positive Δσ originates dominantly from bulk carrier contributions, while the negative component at prolonged delays is ascribed to Dirac surface states, driven by enhanced scattering of photoexcited carriers. Using the Drude–Smith model to fit the differential conductivity spectra, we quantitatively extracted time-dependent transport parameters of bulk and surface states. These results unravel the comprehensive carrier relaxation mechanism in Bi2Se3, clarify the distinct roles of surface and bulk contributions, and lay the groundwork for designing TI-based THz devices.