This work proposes an approach to enable efficient coupling of evanescent waves below the cutoff frequency of a standard rectangular waveguide (RWG) system based on ultrathin planar capacitive metasurfaces (CMSs). It is demonstrated that the CMS at the RWG’s cross-section can couple the evanescent wave from one port to the other, and thus enable an efficient frequency-selective transmission below the cutoff frequency. Theoretical formulas, together with the equivalent circuit, are presented to quantitatively illustrate the operational mechanism of the proposed approach. The performance characteristics, including out-of-band suppression, Q-factor, transmission loss, and coupling bandwidth, are studied in a comprehensive manner. Three CMS samples with different surface impedance values are designed, fabricated, and measured. The measurement results show good agreement with the numerical values, validating their highly efficient evanescent wave coupling performance characteristic at selected frequencies below the cutoff frequency, with approximately 100% transmission efficiency. Benefiting from the admirable low-frequency coupling performance, this approach has potential applications in many miniaturized and high-performance transmission components, such as compact evanescent wave resonators, filters, and antennas.

这项工作提出了一种方法，可以在基于超薄平面电容超表面 (CMS) 的标准矩形波导 (RWG) 系统的截止频率以下实现消逝波的有效耦合。结果表明，RWG 横截面处的 CMS 可以将衰减波从一个端口耦合到另一个端口，从而实现低于截止频率的有效频率选择性传输。提出了理论公式和等效电路，以定量说明所提出方法的运行机制。性能特性，包括带外抑制，Q因子、传输损耗和耦合带宽，进行了全面的研究。设计、制造和测量了三个具有不同表面阻抗值的 CMS 样品。测量结果显示出与数值的良好一致性，验证了它们在低于截止频率的选定频率下的高效倏逝波耦合性能特性，传输效率约为 100%。受益于令人赞叹的低频耦合性能，这种方法在许多小型化和高性能传输组件中具有潜在应用，例如紧凑型渐逝波谐振器、滤波器和天线。