Terahertz waveguides: a study of microwires and porous fibres.

Abstract

This Thesis reports the development of fibres to guide terahertz (THz) or T-ray radiation. It demonstrates the theoretical studies of THz microwires (air-clad solid core fibres) and a new form of waveguide: the porous fibre. Porous fibre has an arrangement of sub-wavelength featured air-holes in the cross-section, resulting in improved confinementof the propagating mode while retaining the low loss characteristic compared to air-clad sub-wavelength waveguide or microwires. Porous fibres also offer lower frequency dependent loss and dispersion compared to microwires. Furthermore, introducing asymmetrical discontinuity leads to high birefringence, which is comparable to recently achieved high birefringence in photonic crystal fibres. Furthermore, this thesis involves the first successful fabrication of highly porous polymer fibres, with both symmetrical and asymmetrical discontinuities, via an extrusion process. In order to achieve rapid and reproducible waveguide cross-sections three different cleaving techniques—based on the use of a semiconductor dicing saw, focused ion beam milling, and a 193 nm ultraviolet laser—have been investigated for cleaving of polymer porous fibres. Finally, two different techniques have been utilised for characterisation of porous fibres. The first approach leads to the first experimental verification of frequency dependence of effective refractive indices of polymer porous fibres and microwires. The second approach exploits a micromachined photoconductive probe-tip for sampling of the THz pulse along the waveguide, from which the frequency dependent absorption coefficient and refractive index are determined. Moreover, the evanescent field distribution of porous fibres as a function of frequency is measured for the first time.