Terahertz Hollow Core Antiresonant Fibre

Abstract

Research on fibres operating in the terahertz frequency range is rapidly growing with numerous potential applications such as in spectroscopy, imaging, security, and transmission. However, designing a terahertz fibre with controllable and desirable transmission characteristics is challenging due to the complex cladding structure. In this thesis, we study hollow core antiresonant photonic crystal fibre (HC-ARPCF) for electromagnetic transmission and refractometric sensing in the terahertz regime. The HC-ARPCF consists of an air-core surrounded by a structured polymer cladding, which confines most of the power within the air-core region. The idea behind hollow-core antiresonant fibres is that light is guided in the hollow air core, thus drastically reducing the transmission loss. Guidance of light is achieved via reflection provided by thin membranes of the antiresonant tubes that surround the core, behaving effectively as a Fabry-P´erot cavity. At antiresonant frequencies, the thin membranes reflect the light towards the core because of the higher refractive index of the membranes. The guidance mechanism of the HC-ARPCF can also be explained due to the inhibited coupling mechanism (coupling between core and cladding mode is forbidden in guidance), where the cladding mode maintains a lower density of states (ηeff) than the fundamental core mode. Inhibited coupling guidance in HC-ARPCF offers broad bandwidth. At resonance frequencies, the light couples to the thin membranes and the core mode becomes more lossy, which can assist in gas sensing. The idea for the terahertz HC-ARPCF is inspired by those in the well-developed infrared and mid-infrared range. The effect of cladding pattern, cladding material, and cladding sector angle are analysed to investigate and tune the transmission loss, bending loss, and modal properties. The detailed simulations of several designs give a new understanding of the effect of the cladding elements on the leakage loss. The HCARPCFs are considered as a suitable candidate for low loss and broadband terahertz transmission. In addition, we model and simulate a simple hollow-core antiresonant terahertz waveguide, show the linear properties and explore the mechanism of achieving nonlinearity. First, the linear properties of HC-ARPCF are discussed, and then the nonlinear properties of the same structure are demonstrated, considering a gas-filled core in the terahertz regime. Furthermore, this thesis describes two different fabrication techniques for terahertz HC-ARPCF, using Zeonex and UV-resin as the bulk materials via a 3D printing process. The Zeonex filaments are made by using a Filabot EX2 Filament Extruder designed for filament production. To measure the effective material loss of the Zeonex, a circular disc with an uneven thickness of 0.65±0.05mmand a diameter of 24mmis printed.We demonstrate the first successful fabrication of Zeonex and UV resin fibre using Fused Decomposition modelling (FDM) and Steriolithography Apparatus (SLA) methods, respectively, to investigate the surface quality and thickness variations of the printed structure. These printing approaches have potential to replace conventional costly terahertz fibre drawing process. The fabricated fibres are then experimentally investigated for terahertz transmission. Fibres fabricated using the FDM and SLA methods are also investigated numerically and the results are compared against the experimental results. The detailed simulations suggest their attenuation can be improved by orders of magnitude with improvements in the quality of the fabrication process. We also discuss the possible post-processing techniques that can be useful for improving fibre quality and consistency in future work.