Photonic Crystal Millimetre Wave and Terahertz Waveguides and Functional Components

This work discusses both the theoretical and experimental guidance of low-loss single-mode millimetre-wave (mmW) and terahertz (THz) waves within microstructured photonic crystal fibre or waveguides, as well as functional components which can be built upon them. The aim of this work is to provide good interconnects for mmW and THz system. The interconnects are desired to be low loss, single mode, low dispersion, as well as easy to fabricate and integrate. In this work, photonic crystal structures, which can easily manipulate the wave-behaving photons by artificially changing its geometrical and material properties, are used in the proposed mmW and THz waveguides. The proposed photonic crystal waveguides includes cylindrical Bragg fibres and flat hollow photonic crystal integrated waveguides. The geometrical differences between Bragg fibres and photonic crystal integrated waveguides make them work better for different challenges. The former are promising for long distance guidance of signals due to its ultra-low loss, while the latter are strong candidates for compact and multilayer packaging applications since its flatness and other exceptional properties. The thesis has three primary themes. The first them is about the design principles, analysis, and fabrication and measurement of low-loss asymptotically single-mode THz Bragg fibres. A design principle for manipulating the photonic bandgap of Bragg fibres, which is called as the generalized half-wavelength condition, is proposed. Based on the design principle, an ultra-low loss THz Bragg fibre with single mode and low dispersion is proposed, verified by the simulation. Considering practical fabrication challenges, a sub-THz Bragg fibre is fabricated using 3D printing technology and characterized to be one of the lowest loss waveguide at around 300 GHz. The mode transition and filtering in the fabricated sub-THz Bragg fibre is investigated, disclosing the mechanisms of asymptotically single-mode operation pattern of Bragg fibres. The second theme is about the design, fabrication and measurement of single-mode mmW flat and hollow photonic crystal integrated waveguides with low loss and zero group velocity dispersion. The hollow photonic crystal integrated waveguides comprise of air-core line-defect photonic crystal structures sandwiched by a pair of metallic parallel plates. Two different types of photonic crystals are used in the designs, namely hexagonal lattice array of air holes in dielectric slab and Bragg reflectors that consist of periodic arrangement of dielectric layers and air layers. Therefore, two types of hollow photonic crystal integrated waveguides are designed. The designs are fabricated and verified at Ka-band by measurements. The hollow photonic crystal integrated waveguides possess the merits of both substrate integrated waveguide and photonic crystal waveguide, but eliminates their drawbacks, making them strong candidates for compact and multilayer mmW and THz system-in-package applications. The third theme is about the design and simulation of mmW and THz functional components built upon the previously designed microstructured photonic crystal fibres and flat waveguides. The functional components that have been designed include waveguide bends, power splitters or combiners, cavity, h-plane horn antenna, and circular Bragg fibre horn antenna. This theme aims to demonstrate the expansibility and flexibility of the proposed microstructured photonic crystal fibres and flat waveguides as promising platforms for designing mmW and THz functional components. Though each theme discusses the theoretical analysis and/or experimental measurements of distinct phenomena, they are deeply related within the overall theme of engineering low-loss single-mode fibres or waveguides and their integration into mmW or THz systems.