Ultrafast Spin‐to‐Charge Conversion at the Surface of Topological Insulator Thin Films

Strong spin–orbit coupling, resulting in the formation of spin‐momentum‐locked surface states, endows topological insulators with superior spin‐to‐charge conversion characteristics, though the dynamics that govern it have remained elusive. Here, an all‐optical method is presented, which enables unprecedented tracking of the ultrafast dynamics of spin‐to‐charge conversion in a prototypical topological insulator Bi 2 Se 3 /ferromagnetic Co heterostructure, down to the sub‐picosecond timescale. Compared to pure Bi 2 Se 3 or Co, a giant terahertz emission is observed in the heterostructure that originates from spin‐to‐charge conversion, in which the topological surface states play a crucial role. A 0.12 ps timescale is identified that sets a technological speed limit of spin‐to‐charge conversion processes in topological insulators. In addition, it is shown that the spin‐to‐charge conversion efficiency is temperature independent in Bi 2 Se 3 as expected from the nature of the surface states, paving the way for designing next‐generation high‐speed optospintronic devices based on topological insulators at room temperature.