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

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.