Surface plasmon polaritons (SPPs) are interactions between incident electromagnetic (EM) waves and free electrons on the metal-dielectric interface in the optical regime. To mimic SPPs in the microwave frequency, spoof SPPs (SSPPs) on ultrathin and flexible corrugated metallic strips were proposed and designed, which also inherit the advantages of lightweight, conformal, low profile, and easy integration with the traditional microwave circuits. In this paper, we review the recent development of SSPPs, including the basic concept, design principle, and applications along with the development from unwieldy waveguides to ultrathin transmission lines. The design schemes from passive and active devices to SSPP systems are presented respectively. For the passive SSPP devices, the related applications including filters, splitters, combiners, couplers, topological SSPPs, and radiations introduced. For the active SSPP devices, from the perspectives of transmission and radiation, we present a series of active SSPP devices with diversity and flexibility, including filtering, amplification, attenuation, nonlinearity, and leaky-wave radiations. Finally, several microwave systems based on SSPPs are reported, showing their unique advantages. The future directions and potential applications of the ultra-thin SSPP structures in the microwave and millimeter-wave regions are discussed.
Polarization plays an important role in practical applications, and hence it is an essential task to generate the desired polarization of a spatial propagating wave (SPW). Here, the authors propose a method to design radiation metasurface, or metasurface emitter, which can produce arbitrarily polarized SPW. Different from the conventional transmission and reflection metasurfaces that can only manipulate the incoming spatial wave, the proposed radiation metasurface can be regarded as an electromagnetic (EM) wave emitter that can generate SPW by itself. More importantly, the polarization of the generated SPW can be flexibly customized by designing the phase distribution on metasurface. The metasurface is fed by a monopole antenna, and the energy is first coupled into the metasurface to form a surface wave. Modulated by the metasurface, the surface wave is then converted into SPW. As a proof of concept, they design and fabricate the radiation metasurfaces that are capable of generating radially, azimuthally, linearly, and circularly polarized SPWs, as well as dual‐beam radiations with different polarizations, respectively. The measured results have a good match to the theoretical predictions and full‐wave simulations. The proposed method provides an efficient way to generate SPWs with any desired polarizations for easy integration.
An ultra‐compact spoof surface plasmon polariton (UCSSPP) transmission line (TL) based on a meander line is proposed, which can achieve almost the same transmission performance as the conventional rectangular‐groove SSPP TL, but the linewidth is greatly reduced by more than 40%. The coupling effect between two UCSSPP TLs in different line arrangements has been well studied, revealing that excellent crosstalk suppression can be achieved when two TLs are arranged back‐to‐back, while strong coupling occurs when they are arranged face‐to‐back or face‐to‐face. The signal integrity of back‐to‐back UCSSPP TLs is analyzed and demonstrated, which shows that the signal integrity can be greatly improved compared with microstrip (MS) TLs. In addition, based on the principle of mode mismatch, MS and UCSSPP TLs are arranged alternately to solve the mutual coupling problem in multi‐line systems. In contrast, benefiting from the strong coupling of face‐to‐face UCSSPP TLs, an ultra‐miniaturized SSPP coupler working at 9 GHz is designed, whose size can be reduced by over 75% compared with conventional MS coupler. The measurement results show good agreement with full‐wave simulations, indicating that the proposed UCSSPP TL has significant advantages in signal integrity and device miniaturization over previously reported SSPP TLs.
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