Emergence of Transverse Optical Spin in a Subwavelength Grating Ring Resonator

The local polarization of the electromagnetic field plays a crucial role in the interaction of light with spin- and valley-polarized quantum sources. Unlike free-space electromagnetic waves, whose polarization degeneracy enables flexible polarization manipulation, planar integrated optical structures lack such a degree of freedom because of intrinsic structural anisotropy. Here, we propose a planar optical ring resonator based on a subwavelength grating (SWG) waveguide that supports two quasi-degenerate modes. We demonstrate that coupling of these modes in the ring resonator leads to the formation of the resonances with a predominant direction of electric-field rotation in the vicinity of the resonator, resulting in the nonzero transverse optical spin. The average degree of circular polarization (DoCP) in the proposed structures reaches values of up to 70%. Simulations reveal that emission from dipole sources positioned near the resonator can be directionally routed into a side-coupled bus waveguide while simultaneously exhibiting Purcell enhancement of the decay rate. The theoretical predictions are corroborated by experimental validation in the microwave spectral range. Our findings suggest a viable route toward the realization of on-chip optical spintronic and valleytronic interfaces.