Non-local metasurface generates highly efficient transmission vortex by intrinsic singularity and generalized Kerker effect

Optical vortex beams, characterized by their helical phase fronts and orbital angular momentum (OAM), are pivotal in applications such as high-capacity communication, optical manipulation, and quantum information processing. Their ability to encode information in OAM states enhances data transmission capabilities, making them crucial for next-generation technologies like 5G/6G communication systems and other photonic technologies.

Metasurfaces, composed of precisely engineered subwavelength meta-atoms, have emerged as a versatile platform for generating optical vortex beams through tailored vortex geometries. These meta-atoms typically behave as electric and magnetic dipoles or higher-order multipoles, enabling precise control over electromagnetic wavefronts even at the nanoscale. Traditionally, vortex beams are constructed by arranging these dipoles into specific patterns, leveraging their space-dependent responses to shape the desired phase and amplitude profiles of the output beams. However, individual dipole scatterers inherently possess singularities -- such as their poles -- that can intrinsically generate vortex-like electromagnetic fields. Leveraging these intrinsic vortex behaviors offers the advantage of eliminating the need for complex vortex geometries with an optical center, potentially simplifying design and enhancing robustness.

Despite this intrinsic capability, a challenge persists: the directivity and forward scattering efficiency of single dipole scatterers around these singularities are typically low, resulting in inefficient and poorly directed beams. To address these limitations and fully leverage the advantages of the intrinsic capability, we propose a non-local metasurface that combines the generalized Kerker effect with non-local collective interactions. Non-local metasurface can generate narrowband spatially customized wavefronts at multiple selected wavelengths, it has also been applied in the field of wireless communication capabilities in 2023. In this paper, by tuning the interference between electric dipole (ED) and magnetic dipole (MD) resonances, the Kerker effect enhances forward scattering and suppresses backscattering. Furthermore, the non-local coupling between unit cells sharpens the beam’s directivity through wavevector redistribution via Bragg scattering. This approach leverages the intrinsic singularities of dipoles while significantly improving the efficiency and directivity of vortex generation. Our design offers a scalable, alignment-free solution with practical applications in millimeter-wave communication and advanced photonic systems.

Specifically, this work demonstrated that the intrinsic singularities of dipoles, coupled with the generalized Kerker effect and non-local collective interactions, provide a robust framework for generating highly efficient and directional vortex beams using a DRNM. The combination of ED and MD resonances enabled enhanced forward scattering and suppressed backscattering, leading to a cross-polarization transmission efficiency of 41% at 39.99 GHz, significantly outperforming the SRNM.

The synergy between these three mechanisms -- intrinsic singularities, the generalized Kerker effect, and non-local collective coupling -- was critical to the performance of the DRNM. The intrinsic singularities of the dipoles facilitated the generation of phase vortices, while the generalized Kerker effect ensured efficient forward scattering by balancing the ED and MD resonances. At the same time, non-local collective interactions between unit cells enabled wavevector redistribution through Bragg scattering, further enhancing the beam’s directivity and efficiency. These three effects worked in concert to achieve the high-performance results observed.

We also explored a simplified metasurface structure, which maintained these essential mechanisms while reducing fabrication complexity. Despite the simplifications, the structure achieved a vortex conversion efficiency of 14.1% at 31.5 GHz, demonstrating that the core principles of the design can be preserved while optimizing for practical scalability.

In summary, this work highlights a novel, efficient, and alignment-free approach to vortex beam generation. By leveraging intrinsic dipole singularities, non-local collective coupling, and the generalized Kerker effect, the DRNM offers a scalable solution with robust performance in millimeter-wave communication and photonic systems. The simplified structure further enhances the practicality of the design for real-world applications, where ease of fabrication and alignment-free operation are crucial advantages.