Ultra-compact spoof surface plasmon polariton waveguides and notch filters based on double-sided parallel-strip lines

In this paper, we propose a new variety of strong field confinement, ultra-compact size, easy-to-integrate spoof surface plasmon polariton (SSPP) waveguides based on double-sided parallel-strip lines (DSPSL), which are formed by etching opposite spiral grooves on the upper and lower metal layers of the DSPSL. By analyzing the dispersion characteristics, we found that the opposite spiral grooves can significantly reduce the asymptotic frequency of the dispersion curve. Compared with the conventional comb-shaped SSPP structures at the same asymptotic frequency, the size of the structure is reduced to 19.85%, which facilitates miniaturization and integration of microwave devices. A broadband transition is also designed to achieve smooth impedance and momentum matching for the proposed SSPP waveguide. Owing to the dual-conductor structure, the proposed waveguides can be easily integrated with active devices and can also support low-frequency or time-domain signals without causing distortions. The proposed SSPP waveguides have strong field confinement and can concentrate the field around the spiral groove, implying great potential applications in sensing and detection. In parallel, we also designed two notch filters based on the proposed SSPP waveguide, which can achieve adjustable narrowband or broadband filtering by loading different capacitors. The proposed design can be widely used in various plasmonic devices, which opens up a new door to build up large-scale plasmonic integrated circuits in the microwave and terahertz regimes.