In this letter, a novel ultra-compact spoof surface plasmon polaritons (SSPPs) low-pass filter which shows a high-efficiency transmission and an excellent out-of-band suppression is investigated. It consists of a pair of double-layer SSPPs, the top layer using interdigital strip and meander-line, and the bottom layer is metal gratings of fish-bone shape. The vast advantages of the proposed SSPPs structure in lower asymptotic frequency mode compared with traditional SSPPs structure with the same size enables the proposed lowpass filter to be more miniaturized. The dimensions of the miniaturized structure are 0.75λ 0 × 0.1λ 0 (λ 0 is the wavelength corresponding to the central operating frequency). The high-efficient mode conversion is achieved by gradient meander-line lengths. Moreover, we also investigate the transmission performance of the proposed structure when it is conformal. To validate the design concept, the prototype of a straight and bend low-pass filter has been manufactured and measured. The experimental results of the fabricated sample agree with the simulation results well, in which the insertion loss is less than 0.5 dB, the reflection coefficient less than −15 dB, and excellent out-of-band rejection of the attenuation is greater than 30 -dB, which proves the design validity. The proposed ultra-compact SSPPs structure may have great potential applications in miniaturized integrated circuits in the microwave and terahertz (THz) frequencies.

A novel bandpass filter (BPF) based on spoof surface plasmon polaritons (SSPPs) using a compact folded slotline structure is proposed and experimentally demonstrated. The proposed novel SSPPs structure compared with a conventional plasmonic waveguide with slot line SSPPs unit structure at the same size, the considerable advantages in much lower asymptotic frequency with tight field confinement, which enable the proposed filter to be more miniaturization. A high-efficient mode conversion structure is designed to transition from TE-mode to SSPPs-mode by gradient slotline lengths. The low-frequency stop-band can be committed with microstrip to slotline evolution on both sides of the dielectric, while the high-frequency cutoff band is realized by the proposed SSPPs structure. The influence of dispersion relation, electric field distribution, surface current, and structural parameters on the transmission characteristics of the proposed BPF are analyzed by finite difference time domain (FDTD). To validate the design concept, the prototype of the miniaturized SSPPs BPF has been manufactured and measured. The experimental results show high performance of the fabricated sample, in which the working in a range of 0.9 GHz–5.2 GHz with the relative bandwidth is 142%, the insertion loss less than 0.5 dB, the reflection coefficient less than −10 dB, and the group delay is less than one ns. This works provides a mirror for realizing the miniaturization of waveguides, and the application and development of high-confinement SSPPs functional devices in the microwave and THz regimes.