Spintronic THz emission: driving force and applications

Ultrabroadband terahertz (THz) radiation is a useful and versatile tool in the fields of spintronics and ultrafast magnetism. THz emission spectroscopy exploits this radiation as a direct probe: a metallic heterostructure is excited with a femtosecond optical pulse, and the resulting sub-picosecond charge-current dynamics are read out through the emitted THz field, giving access to spin transport on femtosecond timescales. The insights obtained are relevant not only for a better understanding of fundamental processes involving electron spins but also for applications in THz photonics. We discuss this technique along its three constituent steps: driving force, transport, and detection. To launch THz spin transport (driving force), we excite a ferromagnetic-metal thin film (F), such as Fe or Co, magnetized in the plane, with a femtosecond optical pulse. As a result of its increased electron temperature, F exhibits a transient spin voltage, i.e., an excess of magnetization, which is released by spin-angular-momentum transfer to the crystal lattice or by a spin current jS into an adjacent metal layer (N) [1,2] . To detect jS, we use heavy metals such as Pt (platinum) for N because they exhibit a large inverse spin Hall effect (ISHE) and thus convert jS into an in-plane charge current jC. The charge current jC, in turn, acts as a source of an electromagnetic pulse with frequencies reaching the THz range [3]. Finally, we extend this scheme to THz scanning near-field optical microscopy (THz-SNOM), which brings the spatial resolution of spintronic THz emission nanoscopy (STEN) down to the 10s-of-nanometer range while keeping the femtosecond time resolution intact [4].   [1] Rouzegar et al., Physical Review B 106, 144427 (2022) [2] Rouzegar et al., ArXiv (2026) [3] Seifert et al., Nature Photonics 10, 483 (2016) [4] Paries et al., ArXiv (2026)