Terahertz (THz) waveguide is an essential component for propagating THz waves. However, it is challenging for fabricating THz waveguides owing to their sub-millimeter sized aperture and high-aspect-ratio feature. Thus, three-dimensional (3D) printing technology combined with the metal coating is developed to fabricate metal-coated dielectric THz waveguides. For coating metal film on 3D printed substrate, electroless plating followed by electroplating has commonly been adopted, yet its low film quality and poor step coverage confine application on THz waveguides. Therefore, we proposed to apply supercritical fluid deposition (SCFD), which enables high-quality film onto high-aspect-ratio structures for fabricating THz devices. In the development of metal-coated dielectric waveguides, the required film thickness and suitable material are essential. Thus, we conducted material selection and derived a model for evaluating the required film thickness. However, the previous study concentrated on transverse-magnetic (TM) mode only. Considering high loss in TM_1 mode and incomplete confinement in TM_0 mode, the lowest transverse electric (TE_1) mode is attracted to low-loss propagation. This study investigated the required film thickness by evaluating the propagation loss in the silicon-based parallel-plate waveguides (PPWG) with different length and different coated Au film thicknesses. The model to estimate propagation loss was expressed in TE mode.
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