Development of a model for evaluating propagation loss of metal-coated dielectric terahertz waveguides

A simple physical model for evaluating propagation loss of a metal-coated dielectric terahertz (THz) waveguide with different metal film thicknesses was developed for those fabricated by three-dimensional printing and film coating techniques. Our model enables a comprehensive understanding of the propagation loss mechanism and two key values: the critical film thickness to behave like the bulk material and loss in a sufficiently thick film. To develop the model, in addition to reflection at the metal–dielectric interface, the thickness-dependent electrical conductivity of the metal film was considered. The model was validated by an in-house multi-channel Au-coated THz parallel-plate waveguide in the lowest transverse-electric mode. The estimated critical thickness of our Au film was 171–207 nm at 0.72–1.4 THz. Our model clarified the contribution of three loss components to the overall loss: penetration loss, ohmic loss by bulk conductivity, and ohmic loss by a decrease in conductivity due to thin-film effects. Evaluation of loss over a broader frequency range (0.03–3.0 THz), which corresponds to fifth- to sixth-generation mobile network, revealed that the critical thickness decreased by up to 1.0 THz but increased above this range due to the transition of the dominant loss component from penetration loss to ohmic loss by a decrease in conductivity. As all three loss components and the critical thickness depend on film quality, a deposition process to yield high-quality films is necessary for high-performance waveguides. Our model is applicable to various waveguides, including rectangular waveguides, at any frequency and with any metal film.