Concept, Theory, Design, and Applications of Spoof Surface Plasmon Polaritons at Microwave Frequencies

Surface plasmon polaritons (SPPs) are highly localized surface waves that exist on the interface of two media with opposite permittivities at optical frequencies. To produce spoof SPPs at microwave or terahertz frequencies, plasmonic metamaterials have been proposed by making subwavelength structures on a metal surface. Spoof SPPs inherit the properties of natural SPPs, including dispersion characteristics, field confinement, and subwavelength resolution, and therefore are highly expected to offer new solutions for advanced circuits and systems with high integration, compact size, and excellent performance. In this article, the development of spoof SPPs in recent years is reviewed, focusing primarily on the basic concept, theory, design method, and applications in microwave engineering. First, the concept and theory of SPPs and spoof SPPs are introduced, along with development from bulky waveguides to ultrathin transmission lines (TLs) and the unique merits of this new type of TL. Then, the design method is studied, including the feeding strategy of spoof SPP TLs and the realization of reconfigurable spoof SPPs. Recent progress on the engineering realization of plasmonic circuits is also demonstrated, including passive SPP circuits, active SPP devices, and SPP antennas. Finally, the future directions and potential applications of microwave spoof SPPs are discussed.