Cavity-assisted nonlocal metasurfaces for momentum-space broadband-operational optical vortice generation with maximum efficiency approaching 80%

Nonlocal metasurfaces generate optical vortices via momentum-space topological singularities, eliminating the need for complex meta-atom phasing or precise beam alignment. Their utility has been constrained by narrow bandwidth and low efficiency stemming from the steep dispersion of high-Q resonances. Here, we introduce a cavity-assisted reflective nonlocal metasurface that overcomes these limits by hybridizing bound states in the continuum (BICs) with degeneracy points (DPs). A Fabry-Pérot cavity supplies a single, addressable control for BIC-DP coupling, enabling deterministic tuning of dispersion, radiative Q, and polarization to realize quasi-flat bands with strong scattering. Simulations predict near-unity on-resonance conversion and >90% overall efficiency—3–4× higher and >15× broader than conventional designs. Experiments confirm operation from 1480 to 1600 nm with ~80% peak efficiency and 91.7% OAM purity, while suppressing edge effects and markedly reducing sensitivity to alignment, beam profile, and numerical aperture. Crucially, the device enables broadband, efficient conversion of zero-order Bessel beams into high-quality, OAM-carrying perfect vortex beams—performance not achieved by prior metasurfaces. These results establish a practical, scalable route to broadband, high-efficiency vortex generation for high-dimensional optical communications, advanced imaging, and quantum photonics.