  We propose wideband bandpass filters based on multipole resonances of spoof
localized surface plasmons (SLSPs). The resonance characteristics and geometric
tunability of SLSPs are investigated under microstrip excitations. Strong
coupling with interlayer microstrip lines is proposed to join discrete
multipole resonances into a continuous and flat passband. The SLSP filters
exhibit wide passbands in compact sizes and well-balanced shapes, while holding
satisfactory spurious rejection bands, group delays, and geometric tunability.
This work exposes the SLSPs' application potential in filters as novel
resonators.
