Hollow-core antiresonant THz waveguides based on polymer- and metal-coated sapphire tube

While the existing terahertz (THz) fiber optics components suffer from high loss and dispersion, low technological reliability, poor environmental resistance and radiation strength, as well as large cross-section, THz applications in different fields still require hardware for the sensing and exposure of hard-to-access objects. To mitigate this difficulty, we develop the two variants of hollow-core THz waveguides, those exploit the antiresonant reflecting optical waveguiding (ARROW) mechanism and use (as a key element) a few-millimeter-diameter sapphire tube produced by the edge-defined film-fed growth (EFG) technique. In the all-dielectric arrangement, the outer surface of this tube is coated by a sub-millimeter-thick polytetrafluoroethylene (PTFE) film, while in the metal-coated one-by a sub-micrometer-thick reflecting copper layer. These coatings increase the guiding efficiency and underlie different performance of the two geometries. Both waveguides are studied numerically and experimentally in the 0.56-0.7 THz frequency range. The observed discrepancies between the theoretical and measured propagation loss are attributed to fluctuation of the cross-section geometry over the waveguide length. In narrow frequency bands, the metal-coated waveguide offers the propagation loss as small as 5.0 dB/m, which is significantly lower than that of the all-dielectric one. Furthermore, the outer metal coating completely prevents mode leakage, whereas in an all-dielectric waveguide, some of the evanescent field extends into the surrounding space and still can be de-coupled. Our findings highlight that the ARROW sapphire THz waveguides provide a reasonable compromise between the guiding efficiency and the cross-section dimensions, thus, forming a favorable platform for the THz sensing and exposure.