Spin-dependent bandgap renormalization and state-filling effect in Bi2Se3 observed by ultrafast Kerr rotation

We investigate the ultrafast spin dynamics of the prototypical topological insulator Bi2Se3 using time-resolved Kerr rotation (polarization-change) measurements across near-infrared wavelengths. The Kerr rotation angle ΔθK of Bi2Se3 was found to significantly depend on the photon energy around a resonance transition (∼1.0 eV) of bulk states, as well as the ellipticity of the pump light, in the presence of spin excitation. The observed photon-energy dependence of ΔθK can be well simulated by assuming spin-dependent refractive-index changes in the presence of bandgap renormalization and state-filling effect upon photoexcitation. Our study delivers comprehensive insights into the opto-spintronic properties of bulk Bi2Se3 and the fundamental physical processes underlying polarization changes. These findings are expected to be crucial in developing ultrafast magneto-optical memory devices, which can perform read-and-write operations in the terahertz regime.