Demonstration of microwave plasmonic-like vortices with tunable topological charges by a single metaparticle

Light beams with helical wave fronts, also called optical vortices, have attracted great interest in the community of optics and photonics. They provide an additional degree of freedom for light manipulation, leading to wide-ranging potential applications in micro-particle trapping, optical microscopy, and even quantum information processing. Recently, metallic microstructures are introduced to confine the plasmonic vortices into deep subwavelength dimension, which benefits photonic integration on chip. In this Letter, exploiting the excitation of spoof surface plasmon, we experimentally demonstrate the near-field optical vortices with tunable topological charges supported by a single metaparticle in the microwave regime. These microwave plasmonic-like vortices are excited by surface waves with a spatial asymmetric distribution of electromagnetic field, which are launched by a metallic comb-shaped waveguide. Experimental characterization of highly localized and controllable near-field vortices with the nature of deep subwavelength confirms the numerical simulation. In addition, an equivalent physical model based on the coupled mode theory is proposed to understand the generation mechanism of these spoof plasmonic vortices. Our approach offers an efficient way to generate deterministic subwavelength optical vortices, which provides the potential for critical vortex elements on photonic integrated chip.