Compact Spoof Surface Plasmon Polariton Waveguides and Notch Filters Based on Meander-Strip Units

Abstract

We demonstrate a new variety of spoof surface plasmon polariton (SSPP) waveguides and notch filters with periodic arrays of ultra-thin metallic meander-strip units for high-efficiency and strong confinement microwave SSPPs transmission and notch rejection. Firstly, the dispersion characteristics of the proposed SSPPs waveguide based on meander-strip units are investigated. The dispersion curve of the proposed waveguide presents a unique negative group velocity characteristic, and the asymptotic frequency greatly reduces to near half as that of SSPP waveguide based on traditional rectangular-grooved units with the same transverse size. To validate the proposed design, an SSPP waveguide prototype is fabricated and tested, showing good lowpass filtering performances with a high transmission coefficient ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text {S}21 &gt; {-}1$ </tex-math></inline-formula> dB) and a low reflection coefficient ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text {S}11 &lt; {-}15$ </tex-math></inline-formula> dB) below the asymptotic frequency of 5.2 GHz. Based on this waveguide structure, a notch filter is designed by loading a split ring resonator (SRR). The transmission coefficient (S21) of this filter is −18 dB at the notch frequency of 2.52 GHz. Furthermore, a PIN diode is integrated into the SRR of the notch filter to make the notch frequency switchable. As the PIN diode turns from the ON state to the OFF state, the notch frequency switches from 2.5 GHz to 3.5 GHz. The proposed waveguides and notch filters with electronically adjustable functionalities may have great potential applications in microwave integrated plasmon circuits and programmable systems.