Bend losses in flexible polyurethane antiresonant terahertz waveguides

One important shortcoming of terahertz technology is the relative absence of convenient, flexible, and reconfigurable waveguides with low attenuation and small bend losses. While recent years have been marked by remarkable progress in lowering the impact of material losses using hollow-core guidance, such waveguides often have centimeter-scale diameter and are therefore not flexible. Here we experimentally and numerically investigate antiresonant dielectric waveguides made of thermoplastic polyurethane, a commonly used dielectric with a low Young's modulus. The hollow-core nature of antiresonant fibers leads to low transmission losses using simple structures, whereas the low Young's modulus of polyurethane makes them extremely flexible. The structures presented enable millimeter-wave manipulation in the same spirit as conventional (visible- and near-IR-) optical fibers, i.e. conveniently and reconfigurably, despite their centimeter-thick diameter. We investigate two canonical antiresonant geometries formed by one- and six-tubes, experimentally comparing their transmission, bend losses and mode profiles. The waveguides under investigation have loss below 1 dB/cm in their sub-THz transmission bands, increasing by 1 dB/cm for a bend radius of about 10 cm. We find that the six-tube waveguide outperforms its one-tube counterpart for smaller bend radii (here: 10cm); for larger bend radii, coupling to cladding tube modes can lead to a drop in transmission at specific frequencies in the six-tube waveguide that does not occur in the one-tube waveguide.