Spin-charge conversion in topological materials via THz emission spectroscopy

In this talk I will show THz emission data from spin-to-charge conversion processes occuring in topological material (TM)/ferromagnet (FM) heterostructures, where the TM is the Bi₂Se₃ topological insulator or the MoS₂ 2D transition metal dichalcogenide, both of which possess large spin-orbit coupling. By measuring the emitted THz pulses in the time and frequency domain, as a function of pump polarization angle, pump fluence, magnetic field direction, sample direction, TM thickness, FM thickness and temperature, both in the pure TM film and the TM/FM heterostructure, we demonstrate highly-efficient THz emission from both Bi₂Se₃/Co and MoS₂/Co. In Bi₂Se₃/Co, we conclude that the Bi₂Se₃ surface states play a dominant role in THz emission via the inverse Rashba-Edelstein effect. Compared to the instantaneous shift current THz emission mechanism in pure Bi₂Se₃, we observe a temporal delay in the spin-current-related component ~0.12 ps, which characterizes the spin-to-charge conversion timescale in topological insulators. In MoS₂/Co, we observe and disentangle the contributions of inverse spin Hall effect and inverse Rashba-Edelstein effect to the spin-to-charge conversion. Our analysis gave estimates for the spin Hall angle, which is a measure of the spin-charge conversion efficiency, and the spin diffusion length, that enables us to resolve the spin diffusion dynamics from the Co layer into the TM surface.