Capacitively-Coupled Resonators for Terahertz Planar-Goubau-Line Filters

Low-loss planar Goubau lines show promising potential for terahertz (THz) applications. However, a single-wire waveguide exhibits less design freedom than standard multiconductor lines, which is a significant constraint for realizing standard components. Existing filters for planar-Goubau-line lack clear design procedures preventing the synthesis of an arbitrary filter response. In this work, we present a design for a bandpass/bandstop filter for planar Goubau line by periodically loading the line with capacitively-coupled$\lambda /2$resonators, which can be easily tuned by changing their electrical length. The filter's working principle is explained by a proposed transmission-line model. We designed and fabricated a passband filter centered at 0.9 THz on a 10-$\mu$m silicon-membrane substrate and compared measurement results between 0.5 and 1.1 THz to electromagnetic simulations, showing an excellent agreement in both$S_{11}$and$S_{21}$. The measured passband has an insertion loss of 7 dB and a 3-dB bandwidth of 31%. Overall, the proposed filter design has a good performance while having a simple design procedure.