Spoof surface plasmon polariton waveguide with spiral structure units

By introducing periodic structure such as grooves into metal surface, spoof surface plasmon polaritons (SSPPs) at microwave and terahertz bands with similar characteristics to optical SPPs can be transmitted along the metal/dielectric interface. In order to reduce the cutoff frequency of a planar SSPP waveguide, periodic elements of rectangular spiral structure are applied in this paper. This waveguide is excited by a microstrip feedline whose ground plane has an opening angle. The experimental results show that only a small periodic conductor width is required to achieve a low cutoff frequency suitable for sub-6 GHz applications. Meanwhile, the passband range can be adjusted by controlling the length of the spiral arm, while the waveguide width remains unchanged. However, a negative group velocity of the fundamental mode can be introduced by using longer spiral arms to enhance the field confinement.