Clarification of the transverse orbital angular momentum of spatiotemporal optical vortices

Abstract

Advances in the generation and the application of spatiotemporal optical vortices (STOV) are proceeding fast, but fundamental aspects of their nature remain obscure. Phys. Rev. A 107, L031501 (2023) (PRA) and Prog. Electromagn. Res. 177, 95 (2023) (PIER) provide contradictory results on the transverse orbital angular momentum (OAM) carried by STOVs. We show that the results by Porras in PIER and by Bliokh in PRA refer to different STOVs and are all correct. In PIER, STOVs are elliptical at given cross section and time, or in space-time, but not in three-dimensional space. In PRA, STOVs are elliptical in space but not in space-time. This is evidenced from two dual, equivalent theories on the transverse OAM where a wave packet is seen in space-time evolving with propagation distance or in space evolving in time, that accounts for all values of the total, intrinsic and extrinsic OAM in PIERS and PRA. However, the intrinsic OAM with respect to the photon wave function center in PRA is not generally conserved, which advocates for the energy center in PIER as the STOV center. We argue that STOVs are generated in experiments to purportedly have elliptical symmetry in space-time. The values provided in PIER should then be taken as the reference for elliptical STOVs, and the theory therein to evaluate the transverse OAM of other wave packets. Hancock et al. in Phys. Rev. Lett. 127, 193901 (2021) and Phys. Rev. X. 14, 011031 (2024) erroneously attribute the transverse OAM of elliptical STOVs in space to the elliptical STOVs in space-time they consider theoretically and can generate in their experiments.