Transition-free ultrawideband waveguide filter based on effective surface plasmon polaritons

Traditionally, the excitation of effective surface plasmon polaritons (ESPPs) in rectangular waveguides (RWGs) relies on gradient-transition structures to facilitate momentum and impedance matching between the transverse electric ${\mathrm{TE}}_{10}$ and ESPP modes. However, the inherently large and bulky nature of these transitions imposes significant limitations on the practical implementation of ESPP-based functional devices. In this work, we propose a novel transition-free ultrawideband ESPP waveguide filter that enhances the ESPP mode excitation through improved wideband momentum matching. By eliminating the bulky transition structures, the proposed design achieves significant miniaturization while maintaining superior in-band performance. A simple and effective method for extracting the characteristic impedance of ESPPs is introduced, enabling further optimization of the passband response via impedance matching within the transition-free framework. Compared with conventional ESPP- or spoof surface plasmon polariton (SSPP)-based filters that rely on gradient transitions, the proposed design offers a substantial reduction in the overall length without compromising performance. As a proof of concept, a prototype filter centered at 9.3 GHz was fabricated and tested, achieving a compact length of only $0.981{\ensuremath{\lambda}}_{0}$, a fractional bandwidth of 58%, a shape factor of 1.064, and an insertion loss of just 0.25 dB. These results establish a robust foundation for the practical application of compact and efficient ESPP devices.