Generation of dual-mode OAM vortex beam via BICs in a photonic crystal slab

Abstract We propose a metal-dielectric hybrid photonic crystal (PhC) slab for generating orbital angular momentum (OAM) vortex beams. The approach exploits multiple bound states in the continuum (BICs) supported by the PhC slab, which serve as polarization vortex center in momentum space. These vortices carry distinct topological charges, leading to the generation of diverse OAM modes. Unlike conventional spiral phase plates, our design scheme is composed of double-layer periodic units and without a distinct optical geometric center. This PhC slab exhibits insensitivity to the position of the incident wave and is conducive to the flexible configuration of the OAM generator. We further investigate a compact OAM antenna system based on the proposed PhC slab lens. Simulation and experimental results demonstrate that OAM modes with topological charges l = − 2 (6.5–7.0 GHz) and l = + 2 (5.7–6.0 GHz) are generated as a right-handed circularly polarized wave is incident on the PhC slab, the OAM mode purity exceeds 95%. Due to the symmetry of the system, OAM modes with opposite topological charges can be achieved for a left-handed circularly polarized incident wave. The proposed approach expands the application of OAM vortex beam via BICs.