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 Δ θ K 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 Δ θ K 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.