Compact backward effective spoof surface plasmon polaritons (BESSPPs) for high-performance millimeter-wave application

The control of electromagnetic waves has garnered significant attention across various scientific and engineering disciplines. While spoof surface plasmon polaritons (SSPPs) have enabled field confinement and effective wave manipulation at microwave and terahertz frequencies, they suffer from limitations such as high insertion loss, reduced cutoff frequency, and bulky transition structures. To address these challenges, we propose a backward effective spoof surface plasmon polariton (BESSPP) mode, supported by an array of dielectric posts within a parallel-plate waveguide (PPW). The BESSPP mode exhibits characteristics similar to natural SPPs in the optical regime, achieving strong field confinement with an ultra-narrow line depth while maintaining a broad operational bandwidth. Unlike conventional SSPPs, the BESSPP mode allows independent control of lower and upper cutoff frequencies, facilitating the design of flexible bandpass filters. Furthermore, it presents backward wave behavior and eliminates radiation losses even in non-enclosed structures, significantly reducing insertion loss in millimeter-wave applications. The BESSPP mode also removes the need for large transition structures, enabling compact circuit designs. Experimental results confirm the BESSPP spectrum and validate its excitation method, demonstrating its potential for high-speed and high-density integrated circuits.