Photonic spin rings formed by toroidal spin momentum

The transverse spin angular momentum (TSAM) of light, also called photonic wheels, has brought important applications, but its direction can only be along a straight line. Here we employ a tightly focused circular polarization to show that the generated TSAM can be oriented along a 3D ring. Firstly, we show that due to the π/2 phase difference between the circularly polarized light and the induced longitudinal polarization component, the resulting spin momentum away from the focus circulates about a ring, leading to an azimuthal TSAM. Secondly, we reveal that the TSAM is closely related to the spatial curl of polarization orientation. Finally, it is shown that the TSAM manifests itself as an array of photonic rings since the spin directions are reversed every one half wavelength. The 3D spin ring adds a new degree of freedom to photonic spin and may be valuable in developing novel energy and information transfer methods.