  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.
