Fractional-Order Localized Spoof Surface Plasmons for In-Phase or Out-of-Phase Power Division

Localized spoof surface plasmons (LSSPs) are extensively used to develop various functional components due to their excellent characteristics of subwavelength confinement and concomitant near-field enhancement. In this paper, a rat-race coupler based on fractional-order LSSPs is proposed to achieve in-phase or out-of-phase power division. It is a four-port passive component with the advantage of miniaturization. In simulation and experiment, the feasibility of the rat-race coupler is demonstrated and the existence of the rare fractional-order LSSPs modes is conformed. More interestingly, a general design method is suggested, based on which arbitrary fractional-order LSSPs modes can be constructed by skillfully tailoring the electrical lengths between ports. This paper expands the applications of the LSSPs and enriches the topological properties of the LSSPs.