High-efficiency nonlocal reflection-type vortex beam generation based on bound states in the continuum

Momentum-space polarization vortices centered at symmetry-protected bound states in the continuum of a periodic structure, e.g. photonic crystal slab, provide a novel nonlocal approach to generate vortex beams. This approach enjoys a great convenience of no precise alignment requirements, although the generation efficiency of the nonlocal generators requires further optimization before the practical application. In this work, we propose a temporal-coupled-mode-theory-based guideline for high-efficiency nonlocal reflection-type vortex generator design. The conversion efficiency of the vortex beam is found to be limited by the ratio of the radiative loss to the intrinsic absorption in practical systems. To increase this ratio through mode selection and structure design, the photonic crystal slabs are theoretically designed and experimentally characterized, showing a maximum on-resonance conversion efficiency of up to 86%. Combining high efficiency with simple fabrication and no requirement for precise alignment, reflection-type photonic crystal slabs could offer a new and competitive way to generate vortex beams flexibly.