Terahertz broadband spoof surface plasmon polaritons using high-order mode developed from ultra-compact split-ring grooves.

Terahertz (THz) broadband spoof surface plasmon polaritons (SSPPs) using new structure of ultra-compact split-ring grooves are proposed. The high-order mode propagation is highly concentrated around the proposed structure with lower radiation loss implying improved operating bandwidth. More importantly, a size reduction of 83.5% can be realized as compared to the traditional grounded SSPP structure with the same high-order asymptotic frequency. To further verify the proposed idea, a similar structure in microwave regime is designed and measured, where the excitation is easily achieved by directly connecting the microstrip line to the proposed SSPP waveguide. The gradient transition section, such as flaring ground, can be avoided, which decreases the waveguide's longitudinal and transversal lengths and simplifies the design procedure. The measured results of the microwave prototype illustrate that it has good lowpass filtering performance, in which the reflection coefficient is better than -10 dB up to 13 GHz, with the smallest and worst insertion losses of 0.5 dB and 4.5 dB, respectively. To the best of the authors' knowledge, this work presents THz high-order broadband SSPP propagation for the first time, having significant potential for plasmonic integrated circuits application at microwave/THz frequencies.

Spoof plasmonic waveguide developed from coplanar stripline for strongly confined terahertz propagation and its application in microwave filters.

A novel planar terahertz (THz) plasmonic waveguide developed from coplanar stripline (CPS) is proposed for the first time to achieve strongly confined THz propagation performance based on the concept of spoof surface plasmon polaritons (SSPP). Guided-wave characteristics of the proposed plasmonic waveguide are theoretically investigated by eigen-mode simulation technique and finite-difference time-domain solutions. It is found that the waveguide propagation characteristics can be directly manipulated by designing the SSPP unit cells, which exhibit flexible tuning ability of the asymptotic frequency and strong THz field confinement. The idea has been validated through fabricated filter experiments in microwave frequency regime by scaling up the geometry size of the proposed structure. The measured results illustrate high performance of the ultra-wideband filter, in which the reflection coefficient is better than -10 dB from 3 to 13.1 GHz with the smallest and worst insertion losses of 2.2 dB and 5.6 dB, respectively. This work presents a new SSPP waveguide developed from CPS to realize the THz-wave propagation with strong field confinement, which may have promising potential applications in various integrated THz plasmonic devices.