Transverse spin and orbital angular momenta in rotating-wave structured light

TL;DR

This paper introduces a new form of structured light with transverse orbital angular momentum and spin-momentum locking, revealing conserved angular momentum densities and local spin-orbital conversion in free space.

Contribution

It demonstrates the existence of rotating-wave structured light with transverse angular momentum and uncovers its unique spin-momentum locking and conservation properties.

Findings

Transverse orbital angular momentum is carried by wave vortices.

Total angular momentum density is conserved in free space.

Transverse spin orientation is linked to Poynting momentum direction.

Abstract

Within monochromatic optical fields, we demonstrate the rotating-wave structured light with wave vortex carrying an intrinsic transverse orbital angular momentum orthogonal to the propagation direction of light. Remarkably, we find that such a rotating-wave structured light reveals highly nontrivial features of transverse spin and orbital angular momentum densities. The normalized total angular momentum density is conserved universally, suggesting a mutual conversion of the intrinsic transverse spin and orbital angular momentum in free space. Despite such mutual conversion at local level, the integral intrinsic orbital angular momentum can be well defined with the topological charge of the vortex per photon. Moreover, the orientation of transverse spin density is governed by the direction of the Poynting momentum density, manifesting a spin-momentum-locking effect in free space.