Low-profile and low-divergence vortex beams generation based on spoof surface plasmon polaritons

Vortex beams carrying orbital angular momentum (OAM) have garnered considerable interest for their unique helical phase fronts and mode orthogonality, which offer promising prospects in modern wireless communications and detection. However, their inherent divergence poses a fundamental challenge for long-range applications. To overcome this limitation, we propose a low-profile, low-divergence vortex beam emitter utilizing spoof surface plasmon polariton (SSPP) waveguides, capitalizing on their tunable dispersion and structural adaptability. The emitter incorporates 16 sub-emitters configured into a uniform concentric circular array (UCCA) to effectively reduce beam divergence angle, suppress sidelobes, and simplify the feeding network. Each sub-emitter is implemented using a double-sided comb-shaped widened (DSCSW) SSPP waveguide, which not only enables efficient circularly polarized radiation via leaky-wave mechanisms but also concurrently serves as a reflective surface, thereby obviating the need for additional reflectors and achieving a minimal profile with low metal coverage. Simulations and measurements confirm that the proposed design features a simple feeding network, an ultra-thin profile of 0.067λ (λ is free-space wavelength at 10.1 GHz), and generates vortex beams with a notably low divergence angle of 3° and a gain of 16 dBic.