Suitability of metallic materials for constructing metal-coated dielectric terahertz waveguides

Abstract

We aimed to identify metallic materials that could be used to construct metal-coated dielectric terahertz (THz) waveguides. We examined seven different metals: gold (Au), copper (Cu), silver (Ag), aluminum (Al), nickel (Ni), chromium (Cr), and titanium (Ti). The propagation losses of our in-house metal-coated dielectric parallel-plate waveguide (PPWG) were experimentally determined. We developed a physical model to estimate the two key parameters determining the performance of metal-coated waveguides: the critical film thickness required for bulk material-like behavior and the propagation loss in a film with a thickness greater than critical film thickness. Film quality, as revealed by the thickness-dependent electrical conductivity of the metal film, was measured prior to experiments and used for model calculations because propagation loss is influenced by film conductivity, which differs from bulk conductivity and depends on film thickness. After experimentally validating the applicability of the model to different metals, suitable metals were identified based on the two key parameters calculated by the model, assuming the same high film quality. Cu was identified as the optimal metal. The effect of film quality on the two key parameters is discussed in this paper. The impact of the surface oxide (CuOx) layer on THz wave propagation was experimentally evaluated using CuOx/Cu-coated PPWG; no detectable transmittance decrease was observed regardless of the CuOx thickness (1.5–176 nm), when the underlying Cu film was of sufficient thickness. Our model also indicated that a CuOx layer <1  μm-thick had a negligible impact on THz wave propagation. Thus, native oxidation is not an issue when using Cu.