Efficient coupling of evanescent waves in rectangular waveguides based on ultrathin planar capacitive metasurfaces

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.