Bend losses in flexible polyurethane antiresonant terahertz waveguides

Abstract

The quest for practical waveguides operating in the terahertz range faces two major hurdles: large losses and high rigidity. While recent years have been marked by remarkable progress in lowering the impact of material losses using hollow-core guidance, such waveguides are typically not flexible.Here we experimentally and numerically investigate antiresonant dielectric waveguides made of 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 centimeter-thick waveguides in the same spirit as conventional (visible-and near-IR-) optical fibers, i.e. conveniently and reconfigurably. 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, which is analogous to bending standard 125 µm diameter fiber to a 1.2 mm radius. I. INTRODUCTIONTerahertz frequencies (0.1-10 THz), located between the microwave and infrared, can be harnessed across a wide range of application areas, including molecular [1], gas [2] and DNA [3] sensing, imaging and security [4-6], pharmaceutical research [7], short-haul communication (6G) [8], and other industrial applications [9]. Owing to the relative infancy of accessible THz sources and detectors [10], devices in this range are relatively underdeveloped, with most systems still relying on free space propagation, in contrast to near-infrared and visible frequencies, where optical fibers and photonic circuits are decades ahead [11,12].To address this limitation, the past few years have shown rapid development of novel terahertz waveguides, e.g., polymer-based [13], sub-wavelength [14], porous [15], and hollowcore tubes/fibers [16,17] We refer the reader to some excellent recent reviews on terahertz fibers [18][19][20], which discuss common structures and the physics underlying their guidance mechanisms in great detail.