Spin-Charge Conversion in NiFe/Bi₂Se₃ Heterostructures Observed by Terahertz Emission Spectroscopy

When a femtosecond laser pulse hits a magnetic material, the resulting ultrafast demagnetization can generate a spin current that lasts on a picosecond timescale. This ultrafast spin current can be converted to a charge current that subsequently radiates terahertz (THz). In typical spintronic THz emitters, the spin-charge conversion (SCC) is achieved by interfacing the magnetic material (usually a ferromagnetic (FM) layer) with a heavy metal (HM). Because of the strong spin-orbit coupling in HM films, the injected spin current gives rise to a transverse charge current through the inverse spin hall effect. Recently, topological insulators (TI) have been shown as an alternative to HMs for converting spin current to charge current. In a TI, the spin of the topological surface states is locked at right angles to the carrier momentum leading to enhanced SCC. In this work, we demonstrate SCC in NiFe/Bi₂Se₃ (a FM/TI bilayer) using time-resolved THz emission spectroscopy. We characterize the NiFe/Bi₂Se₃ emitter as a function of magnetic orientation, laser polarization, and laser fluence and find that SCC is the dominant mechanism for the THz emission. Furthermore, we present results comparing bulk-insulating Bi₂Se₃ and bulk-conducting Bi₂Se₃ to identify the SCC mechanisms at the interface.