Ultrafast spin-orbitronics with terahertz electromagnetic pulses

Tobias Kampfrath

Freie Universität Berlin and Fritz Haber Institute of the Max Planck Society, Berlin

To take advantage of the electron spin in future electronics, spin angular momentum needs to be transported and detected. Electric fields and temperature gradients have been shown to efficiently drive spin transport at megahertz and gigahertz frequencies. However, to probe the initial elementary steps that lead to the formation of spin currents, we need to launch and measure transport on much faster, i.e., femtosecond time scales.

To achieve this goal, we apply optical femtosecond laser pulses to induce a spin voltage in a metallic ferromagnetic layer F [1]. The resulting spin current into an adjacent layer is measured by conversion into a charge current in a layer N and detection of the concomitantly emitted terahertz electromagnetic pulse [2]. Interesting applications such as terahertz spin-conductance spectroscopy [3] and the generation of ultrashort terahertz electromagnetic pulses emerge [4]. In all cases, the emitted terahertz signal arises from regions close to the F/N interface, thereby making spintronic terahertz emission very interface-sensitive [5]

This methodology can be transferred from the spin to the so far highly unexplored orbital angular momentum of electrons. We obtain new insights into orbitronic phenomena on their natural time scales, for example, time-domain signatures of giant propagation lengths of orbital currents in tungsten [6].

Finally, one can implement a reciprocal experimental scheme and use intense terahertz electromagnetic pulses to drive electric currents and control magnetic order on ultrafast time scales through, for example, Néel spin-orbit torques [7].

References
[1] Rouzegar et al., Phys. Rev. B 106, 144427 (2022)
[2] Kampfrath et al., Nature Nanotech. 8, 256 (2013)
[3] Rouzegar et al., arXiv:2305.09074 (2023)
[4] Seifert et al., Nature Photon. 10, 483 (2016); Rouzegar et al., Phys. Rev. Appl. 19, 034018 (2023)
[5] Gueckstock et al., Advanced Materials 33, 2006281 (2021)
[6] Seifert et al., Nature Nanotech. 18, 1132-1138 (2023)
[7] Behovits et al., Nature Communications 14, 6038 (2023)

Figure: Schematic of optically driven spin-orbitronic emission of ultrabroadband terahertz electromagnetic pulses from a generic F|N thin-film stack. 𝑴 is the magnetization of layer F.