Spoof Surface Plasmon Polaritons Based on Balanced Coplanar Stripline Waveguides

Ultra-wideband spoof surface plasmon polaritons (SSPPs) based on compact balanced coplanar stripline (CPS) waveguides are proposed. Compared with the conventional CPS-based terahertz (THz) SSPP unit cell, the proposed one has achieved a size reduction of 55.2% under the condition of the same asymptotic frequency. The propagation and attenuation characteristics of the proposed SSPP waveguide can be easily manipulated by adjusting the geometry dimensions of the SSPP unit cell. It indicates that the cut-off frequency of the SSPP waveguide has its tunable flexibility thereby facilitating the filter design. To further validate the proposed idea, a similar topology in microwave regime is designed and measured, where the microwave feeding can be easily realized by embedding a balun structure from unbalanced microstrip line to balanced CPS. The measurement of the microwave filter prototype illustrates ultra-wideband bandpass filtering characteristics with return losses of over 10 dB and average insertion losses of 3.2 dB in the passband of 1.9-14 GHz. The presented work may have significant potentials to develop the miniaturization of various planar plasmonic devices and integrated circuits in microwave and THz regimes.