Spatiotemporal Terahertz Emission Nanoscopy of Spintronic Photocurrents

Abstract

Capturing ultrafast spin and charge photocurrents on nanoscopic scales is essential for fundamental research in physics and engineering, as well as for future applications, such as novel spinorbitronic devices. Accessing the fundamental dynamics driven by changes in electronic energy, linear momentum, and angular momentum requires probing that simultaneously resolves their native spatiotemporal scales: femtoseconds and nanometers. However, experimental approaches achieving this simultaneous resolution remain scarce and instrumentally demanding. While near-field probing offers promising platforms to combine ultrafast and nanometer resolution, a major challenge is the observation of in-plane propagation dynamics, required for studying many application-relevant, high-speed phenomena from carrier transport in 2D materials to spin-to-charge conversion in spintronic terahertz emitters (STEs). However, near-field scanning probes are primarily sensitive to out-of-plane electric fields. Here we present a solution to this challenge by performing terahertz (THz) emission nanoscopy (TEN) of a photoexcited fiber-coupled STE using a scanning-probe microscope. We uncover a counterintuitive, dipolar evolution of the near-field THz signal, which we show originates from the scanning-probe's inherent sensitivity to local out-of-plane electric fields that emerge in our case from spatially confined in-plane charge currents. Our findings resolve ambiguities regarding the sensitivity of near-field probes to ultrafast in-plane charge transport and establish TEN as a fully vectorial probe for the spatiotemporal imaging of coupled nanoscale THz charge and spin dynamics.