Ultrafast Spin-to-Charge Conversion in Topological-insulator/Ferromagnet Heterostructures

Strong spin-orbit coupling, resulting in the formation of spin-momentum-locked surface states in topological insulators (TIs), is predicted to possess superior spin-to-charge conversion (SCC) efficiency via the inverse Rashba-Edelstein effect and/or inverse spin Hall effect. The knowledge of the timescale of such SCC processes is crucial for the optimization of TI-based spintronic devices. Here, we investigate the ultrafast SCC in topological-insulator/ferromagnet heterostructures via terahertz emission spectroscopy. We demonstrate highly-efficient THz emission from Bi₂Se₃/Co heterostructures, where the surface states play a dominant role. Compared to the instantaneous shift current terahertz emission mechanism in Bi₂Se₃, we directly observe a temporal delay in the spin-current-related component — 0.12 ps, which characterizes the SCC timescale in TIs. In addition, we show that the SCC efficiency is temperature independent in Bi₂Se₃ as expected from the nature of the surface states. Our study provides fundamental insights into the ultrafast SCC processes, and demonstrate the practical applications of TIs, especially the surface states, in terahertz spintronic devices at room temperature.