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 degree of freedom owing to intrinsic structural anisotropy. Here, we propose a planar optical ring resonator based on a subwavelength grating 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 non-zero transverse optical spin. The average degree of circular polarization in the proposed structures reaches values of up to 70%. The theoretical predictions are corroborated by experimental validation in the microwave spectral range. Our findings suggest a viable route toward realization of on-chip optical spintronic and valleytronic interfaces.