Multi-order ESSPP modes with independently controllable cutoff frequencies for tri-band filters.

Spoof surface plasmon polaritons (SSPPs) exhibit strong field confinement and slow-wave characteristics for microwave and terahertz applications. However, existing multiband SSPP structures generally lack independent control of cutoff frequencies and often suffer from radiation losses. Here, a multilayer substrate-integrated waveguide (SIW) supporting multi-order effective SSPP (ESSPP) modes is proposed for independently controllable multiband transmission. By exploiting structural dispersion in rectangular waveguides, multiple equivalent epsilon-negative/epsilon-positive interfaces are created to excite different-order ESSPP modes in distinct frequency regions. Analytical dispersion relations and closed-form expressions for the cutoff frequencies are derived, showing that each passband is controlled by a single layer, thereby enabling independent frequency tuning and analytical filter design. A tri-band ESSPPs filter is designed, fabricated, and experimentally validated. The measured results agree well with theory and simulations, demonstrating three passbands at 5-5.7 GHz, 6.3-6.7 GHz, and 8.3-8.8 GHz with low insertion losses. The proposed structure provides a compact and low-radiation platform for multiband microwave and millimeter-wave applications.