Direct observation of spin-dependent optical excitation in topological surface states

To investigate optical excitation processes in spin-polarized topological surface states, we performed time- and angle-resolved photoemission spectroscopy on a carrier-tuned topological insulator (Sb,Bi)₂Te₃, which possesses a Dirac point near the Fermi energy (E_F). In this experiment, two types of pump pulses were used: a mid-infrared (MIR, 0.30 eV) pump corresponding to resonant excitation between the lower and upper Dirac cones, and a non-resonant visible (VIS, 2.58 eV) pump. Under VIS excitation, direct excitation from deeper bulk states to the entire upper Dirac cone was observed at t ~ 0 ps. In contrast, under MIR excitation, direct excitation to the upper Dirac cone was observed around 0.20 eV above E_F, while almost no photoelectron intensity was detected around 0.08 eV. This result implies that the direct transition from the lower to the upper Dirac cones is prohibited (allowed) around 0.08 eV (0.20 eV). To elucidate these differences, k_x-k_y mapping was performed. It revealed that while the constant energy contours around 0.08 eV are nearly isotropic, those around 0.20 eV are significantly distorted into a snowflake-like shape due to the warping effect. In the distorted region, the topological surface states host not only in-plane but also out-of-plane spin components, whereas in the isotropic region, in-plane spin components dominate. These differences in spin texture may govern the presence or absence of photoexcitation in the topological surface states.