Investigation of the Transitions for Coplanar Waveguide to Terahertz Spoof Surface Plasmon Polariton Waveguides

In this article, we present the investigation and design of high-performance transitions from coplanar waveguide (CPW) to single-conductor, corrugated waveguides (WGs), namely, terahertz spoof surface plasmon polariton (SSPP) WGs. In contrast to the previous studies in the literature on CPW-to-SSPP WG transitions, we propose a novel methodology to examine the relationship between the guided wavenumber and momentum matching, which is based on analytical calculations, together with the effects of flaring grounds on transition performance. This novel methodology allows us to design a low-loss transition circuit (TC) for any given CPW and SSPP dimensions, which is demonstrated for the first time in the literature. We verify the proposed methodology by the design, fabrication, and measurement of several TCs. The measurement results show that the insertion losses of the proposed TCs can be as low as -1.2, -1, and -2.4 dB at 0.25, 0.275, and 0.3 THz, respectively, which brings a significant improvement of 0.6-2.3 dB in the 0.25-0.3 THz band, compared with the previously reported studies in the literature for the terahertz band.