Tiny Ring Structure Twists Light to Create New Spin-Based Technologies

Researchers have demonstrated the emergence of transverse optical spin within a compact, planar ring resonator, offering a potential pathway towards novel on-chip spintronic and valleytronic devices. Nikita Iukhtanov, Chao Sun, and Georgiy Kurganov, from ITMO University and Harbin Engineering University, alongside Dmitry Zhirihin, Andrey Bogdanov, and Roman Savelev et al., report a design utilising a subwavelength grating waveguide that supports quasi-degenerate modes. Their work overcomes limitations inherent in planar integrated structures, which typically lack the polarization flexibility found in free space, by achieving a significant degree of circular polarization, up to 70% , through resonant coupling. This innovative approach generates a predominant direction of electric-field rotation near the resonator, effectively creating transverse spin and validating the concept experimentally in the microwave spectrum.

Unlike conventional optical structures where polarization manipulation is limited by intrinsic anisotropy, this new design overcomes these restrictions by harnessing the unique properties of subwavelength grating waveguides.

The work centres on a ring resonator supporting two quasi-degenerate modes, and demonstrates that coupling these modes creates resonances with a predominant direction of electric-field rotation near the resonator surface. This directed rotation directly results in a non-zero transverse spin, a crucial element for controlling spin- and valley-polarized quantum sources.

Specifically, the team engineered a periodic ring resonator based on a subwavelength grating waveguide, carefully tuning its parameters to support the two quasi-degenerate modes. Theoretical investigations revealed that bending the waveguide into a ring induces hybridization of these modes, breaking mirror symmetry and enabling control over the polarization of the local electric field.

This hybridization leads to the formation of ring resonances exhibiting a distinct direction of electric field rotation within the plane of the resonator, a phenomenon not observed in conventional single-mode ring resonators. Simulations predict that the average degree of circular polarization within these structures can reach up to 70%.

The theoretical predictions have been experimentally validated using microwave spectral range measurements with a resonator fabricated from ceramic particles. This corroboration confirms the feasibility of the design and its ability to generate significant transverse optical spin. The implications of this breakthrough extend to the creation of more efficient and compact on-chip interfaces for spintronics and valleytronics, potentially enabling advanced functionalities in data processing, sensing, and quantum technologies by addressing limitations inherent in current integrated photonic structures. The ability to achieve high degrees of circular polarization without requiring precise positioning of quantum emitters represents a significant step forward in the field.

Hybridization of quasi-degenerate modes in curved subwavelength grating ring resonators

A subwavelength grating (SWG) waveguide forms the basis of the planar ring resonator investigated in this study. This waveguide supports two quasi-degenerate modes, intentionally designed to provide an additional degree of freedom for controlling the polarization of the local electric field. Researchers bent this SWG waveguide into a ring resonator to examine the interaction between these modes and reveal the formation of resonances exhibiting a predominant direction of electric-field rotation.

The design leverages the hybridization of the two quasi-degenerate modes when the waveguide is curved, breaking the mirror symmetry inherent in a straight waveguide. This hybridization induces a finite unit-cell-averaged spin density, a crucial factor in achieving non-zero transverse optical spin. Numerical analysis demonstrated that, unlike conventional ring resonators where averaged circular polarization vanishes due to symmetry, the proposed periodic ring resonator achieves average degrees of circular polarization reaching up to 70%.

Experimental validation corroborated these theoretical predictions using a resonator fabricated from ceramic particles within the microwave spectral range. The team characterised the resonances and confirmed the predominant direction of electric field rotation in the vicinity of the resonator. This methodology, combining theoretical modelling with microwave experimentation, provides a viable route towards realising on-chip spintronic and valleytronic interfaces by manipulating the polarization handedness of the local electric field. The work addresses limitations of previous designs requiring precise emitter positioning by engineering asymmetry directly into the resonator structure.

High degrees of circular polarisation achieved via transverse spin coupling in planar ring resonators

Average degrees of circular polarization in the proposed planar ring resonators reach values of up to 70 percent. These structures, based on a subwavelength grating waveguide, support two quasi-degenerate modes whose coupling forms resonances with a predominant direction of electric-field rotation. This rotation results in a non-zero transverse spin in the vicinity of the resonator, a key characteristic of the design.

Theoretical predictions regarding these polarization levels have been experimentally validated within the microwave spectral range, confirming the feasibility of the approach. The work demonstrates that average DoCP values can reach tens of percent, contrasting with conventional ring resonators where averaged DoCP is approximately zero due to symmetry.

Analysis focused on the electric contribution to the z-component of the spin density, denoted as s(E)z, revealing its spatial distribution and relationship to field polarization. Averaging the DoCP over the volume containing emitters provides a convenient metric for ensembles, assuming they reside within the same material.

Ring resonators were formed using silicon blocks with a refractive index of 3.5 partially embedded in silicon dioxide, possessing a refractive index of 1.45. Waveguide dispersion was engineered to achieve accidental degeneracy between two modes at a frequency of approximately 202 terahertz. Geometric parameters included a waveguide width of 820 nanometers, a height of 440 nanometers, and a period of 295 nanometers, with a 30 nanometer silicon dioxide cladding layer.

Distributions of the electric field and the z-component of the electric contribution to the optical spin density were analysed for both straight-waveguide modes and ring resonances. For the straight waveguide modes, the distributions of s(E)z were antisymmetric, resulting in zero average values for both s(E)z and DoCP when averaged over the unit cell.

However, linear superposition of these modes with a relative phase of ±π/2 maximizes total spin, demonstrating a pathway to control polarization. Resonances with an azimuthal number of 26 exhibited distinct field distributions and non-zero integrated s(E)z values, further validating the design’s ability to generate controlled polarization.

Enhanced Circular Polarisation and Non-reciprocal Effects via Planar Resonator Coupling

Researchers have demonstrated a planar ring resonator capable of generating resonances with a predominant direction of electric-field rotation. This device, based on a subwavelength grating waveguide, supports two quasi-degenerate modes which, when coupled, create a non-zero transverse spin in the vicinity of the resonator.

The average degree of circular polarization achieved in these structures reaches up to 70%. Theoretical predictions regarding the resonator’s performance have been experimentally validated in the microwave spectral range. This design offers a potentially viable route towards the creation of on-chip spintronic and valleytronic interfaces, circumventing the need for asymmetric placement or manipulation of materials.

The inherent optical spin density within the proposed structure enables strong coupling asymmetry and non-reciprocal phase shifts, even with uniform material placement, simplifying device fabrication and improving reproducibility. The authors acknowledge that discrepancies between simulated and measured field components may arise from the finite size of the near-field probe used in their experiments.

Future research could focus on scaling the current design to operate across different spectral ranges, adapting it for use with various material platforms. Further investigation into the potential for magneto-optical isolation and strong exciton, photon coupling regimes is also warranted, building upon the enhanced effects observed in this resonant structure.