Topological light meets topological semimetal

A Weyl semimetal carries topological charges at the Weyl nodes; a light beam can also carry a topological charge, when it has an orbital angular momentum (OAM) from spatial phase and/or polarization gradients. Recently there has been a lot of interest in understanding how the spin angular momentum (SAM) of light interacts with materials to induce photocurrents (circular photogalvanic effect, CPGE), but not many studies have focused on photocurrents generated by the OAM of light. Here we report a unique orbital photogalvanic effect (OPGE) in a type-II Weyl semimetal WTe₂, featured by a photocurrent winding around the axis of OAM-carrying beams, whose intensity is directly proportional to the topological winding number of the light field, and can be attributed to a discretized dynamical Hall effect. In addition to obtaining new microscopic insights into light-matter interactions with topological light, these measurements show promise for fabricating on-chip photodetectors on Weyl semimetals capable of detecting OAM modes, which can be useful for building the next-generation, high-capacity optoelectronic circuits.