Ultrafast Spin-To-Charge Conversion at the Surface of Topological Insulator Thin Films

TL;DR

This study uses an all-optical method to track ultrafast spin-to-charge conversion in topological insulator heterostructures, revealing a sub-picosecond timescale and temperature-independent efficiency, advancing high-speed spintronic device development.

Contribution

It introduces a novel all-optical technique to observe ultrafast spin-to-charge conversion dynamics in topological insulators at sub-picosecond timescales.

Findings

Giant terahertz emission from heterostructures indicates efficient spin-to-charge conversion.

Identified a 0.12-picosecond speed limit for the process.

Conversion efficiency remains temperature independent, suitable for room-temperature applications.

Abstract

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, we present an all-optical method that 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, we observe a giant terahertz emission in the heterostructure than originates from spin-to-charge conversion, in which the topological surface states play a crucial role. We identify a 0.12-picosecond timescale that sets a technological speed limit of spin-to-charge conversion processes in topological insulators. In addition, we show 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 opto-spintronic devices based on topological insulators at room temperature.