Novel hardware for terahertz time-domain spectroscopy (THz-TDS).

Abstract

Terahertz time-domain spectroscopy (THz-TDS) systems have been generally limited to a single mode of operation, either in transmission or reflection geometries. The possibility of systems able to operate simultaneously in both geometries opens new possibilities for material characterisation. This Thesis designs and characterises a novel system able to simultaneously capture spectra from samples at normal incidence transmission and reflection. This enables materials that are opaque and/or partially reflective, as well as materials that exhibit non-unity values of permittivity and/or permeability to be thoroughly investigated. In addition to a dual geometry system, this Thesis presents two novel beam-splitters useable in the terahertz (THz) range of frequencies from 0.1 to 10 THz. Optical components in the THz frequency range have been limited, with ongoing developments being made to fabricate and characterise lenses, polarizers and waveguides, with beam-splitters that are polarization dependent. The presented original contributions include a low-cost beam-splitter fabricated from an ultra-thin polymer substrate and silver paint, and a novel beam-splitter fabricated from conductive polymers. These beam-splitters provide a near frequency and polarization independent response. An introductory background into THz-TDS along with generation and detection methods are also offered as part of this Thesis. Four auxiliary investigations are also described in the appendices: (i) a dual scanning THz-TDS system, to improve acquisition times, (ii) a mini investigation into food quality control using THz-TDS, (iii) an investigation into security applications for THz-TDS and (iv) second harmonic generation (SHG) using a β radiation damaged barium borate (BBO) crystal and a Ti:Sapphire laser.