Labyrinthine Spoof SPP Multi-band Bandpass Filters

The surface plasmon polariton (SPP) is an electromagnetic wave mode that occurs at the interface of a metal and a dielectric material. It possesses unique properties such as enhancing the strength of the electromagnetic field at the metal surface, achieving sub-wavelength focusing of light waves, and exhibiting low loss. Due to these characteristics, SPP holds great promise in various applications including super-resolution imaging, terahertz technology, biosensing, and optical communication. This paper proposes two Spoof SPP-based tri-band bandpass filters that replace the conventional sawtooth cell structure with a miniaturized labyrinth resonator structure. Upon investigating the dispersion characteristics of the resonators, we found that both resonator unit 1 and resonator unit 2 exhibit three modes, resulting in three notch points for each filter. Unlike most SPP-based structures, our design features a compact structure fed by a co-planar waveguide (CPW) without an added ground at the bottom, thereby reducing losses and improving efficiency. To achieve a smoother transition from CPW to the transmission structure, we utilize a segment of microstrip line structure synthesized by a logarithmic function. Both filters are of the same size, with dimensions totaling 192 mm × 42 mm. Based on our study, we have designed two filters with multiple notch points and have obtained good agreement between the simulation results and the actual test results.