Optical vortices matter because they carry orbital angular momentum, enabling parallel information channels for high-capacity communications, richer quantum photonic states, and improved tools for imaging and optical manipulation. But conventional vortex generators often demand near-perfect alignment with the optical beam’s geometric center, and they can be sensitive to wavelength, fabrication imperfections, beam size, and incidence angle.

The study tackles a known bottleneck in nonlocal metasurfaces: earlier designs worked, but their useful resonances were narrow and overly dispersive, so only a limited slice of the incoming light could be converted efficiently into a high-quality vortex.

Here, the authors introduce a cavity-assisted reflective nonlocal metasurface that forms a Fabry–Pérot cavity using a silicon photonic crystal slab, a silicon dioxide spacer, and a gold mirror. This cavity environment reshapes the optical response so that bound states in the continuum and degeneracy points—both carrying topological signatures—are engineered to hybridize.

That hybridization produces quasi-flat optical bands with strong scattering, effectively creating a broader “working region” across angles and wavelengths. In practical terms, the device can convert a much wider range of incident light into vortex beams without requiring the same stringent alignment that plagues many real-space phase shapers.

Numerical simulations predict near-unity conversion at resonance, more than 90% overall efficiency, and high orbital angular momentum purity. Experiments then confirm broadband operation from 1480 to 1600 nm, with a peak overall efficiency approaching 80% and an OAM purity of 91.7%.

Beyond standard vortex modes, the researchers also demonstrate conversion of a zero-order Bessel beam into a second-order Bessel vortex—often described as a “perfect vortex beam”—showing the approach can maintain mode quality under structured illumination.

Just as importantly, the platform shows robustness against edge effects, beam position, beam profile, and variations in numerical aperture—exactly the kind of resilience needed for real-world photonic systems.

If momentum-space band engineering becomes a reliable design principle, cavity-assisted nonlocal metasurfaces could accelerate compact, efficient, and broadband vortex sources for next-generation meta-optics and integrated photonic platforms, with future directions including visible-wavelength extensions and dynamically tunable or multiplexed vortex generation.

Keywords

nonlocal metasurfaces; orbital angular momentum; bound states in the continuum; band engineering; broadband operation; vortex beam generation
Subject of Research: Cavity-assisted nonlocal metasurfaces for momentum-space broadband optical vortex generation
Article Title: Cavity-assisted nonlocal metasurfaces for momentum-space broadband-operational optical vortice generation with maximum efficiency approaching 80%
Web References: http://dx.doi.org/10.29026/oea.2026.250296
References: Wang KR, Sun KL, Du J et al. Opto-Electron Adv 9, 250296 (2026). DOI: 10.29026/oea.2026.250296
Image Credits: OEA