Description
The orbital angular momentum of electrons offers a promising yet largely unexplored degree of freedom for ultrafast, energy-efficient information processing. As the foundation of orbitronics, understanding how orbital currents propagate and convert into charge currents is essential – but remains elusive due to the challenge in disentangling orbital and spin dynamics in ultrathin films. Although orbital currents have been predicted to propagate over long distances in materials, recent theoretical studies argue that lattice symmetry may constrain their mean free paths (MFPs) to the scale of a single atomic layer. In this work, we provide direct experimental evidence for ultrashort orbital MFPs in heavy metals (HMs) – W, Ta, Pt – revealed by terahertz emission spectroscopy on precisely engineered wedge-shaped HM|Ni heterostructures. Using a multi-component terahertz-emission model, we quantitatively extract the orbital MFPs for each HM, consistently finding them shorter than their spin counterparts. Furthermore, control experiments rule out interfacial conversion mechanisms, confirming that orbital-to-charge conversion is dominated by the bulk inverse orbital Hall effect. Our results resolve a longstanding debate on the transport and conversion of light-induced orbital currents, providing critical insights for the development of orbitronic terahertz devices.
