ABSTRACT
The vortex beam provides a promising alternative for next-generation wireless communication, but it is a long-standing challenge to generate a multi-mode and robust vortex beam. In this Letter, a multi-mode vortex beam emitter is introduced and experimentally verified based on spoof surface plasmon polaritons (SSPP). The SSPP on a helical grating carries multi-mode orbital angular momentum and can be converted into a high-purity vortex beam via the diffraction of a ring array. The operation frequency and topological charge are determined by that of the SSPP. This emitter can achieve the function of beam-scanning in each radiation band. The beam-scanning and vortex characteristics are experimentally verified. The designed emitter is compact and robust, and we are confident that this work will have great application prospects in communication systems.
ABSTRACT
Recently, ultrathin localized spoof surface plasmon (LSSP) resonators are found to have intrinsic defects of relatively low quality factors (Q-factors) because of unavoidable material and radiation losses. In this paper, multilayer structures of planar-circular-grating resonators and their magnetic-coupling schemes are proposed to achieve effective excitation of high-Q LSSPs modes. By adopting the multilayer structures with air between the layers, the power dissipation effected by both material and radiation losses is significantly suppressed. Experimental results show that the Q-factors could reach more than 200 and the excitation efficiencies could reach more than 90%. Numerical simulations show the distribution of the electromagnetic field and illustrate the principle of magnetic coupling. Besides, the Q-factors of resonators with different structural parameters were measured and analyzed. This study aims to provide some inspirations on planar gyro-devices and to improve the performance of existing applications, such as sensors and filters.
