Engineering aspects of terahertz time-domain spectroscopy.

Terahertz time-domain spectroscopy (THz-TDS) is a technique capable of measuring optical constants ofmaterialswith T-ray frequencies, bounded between 0.1 and 10 THz. Owing to the infancy of the technology, much work has to be carried out to improve its utility and reliability. Engineering aspects become vital to support its operation that relies on physical phenomena. This thesis, in the arena of engineering, encompasses a variety of original THz-TDS projects, which aim for (Part I) signal enhancement and classification, (Part II) system evaluation and optimisation, and (Part III) T-ray optics: \nPart I is relevant to enhancement and classification of T-ray signals via digital signal processing. In one project, information underlying T-ray signals is enhanced through numerical removal of unwanted artefacts that are introduced by the response of water vapour during the measurement. In another project, machine learning is recruited in classification of visually indistinguishable T-ray signals probing materials of the same general class. \nPart II focuses on THz-TDS systems with a particular interest in the measurement precision. An ISO standard for the evaluation of measurement uncertainty is adopted for assessing the uncertainty in THz-TDS measurements. The result is an analytical uncertainty model, which allows an improvement in the measurement precision through optimisation of a model parameter in the subsequent \nwork. \nPart III involves design, fabrication, and characterisation of THz-TDS hardware components, i.e., antireflection windows and multilayer interference filters. The designs are based upon conventional optical interference theory. Despite that, required materials and fabrication processes are completely different from those used in optics due to the distinctive operating wavelengths, which dictate material responses and structural dimensions. \nIn addition to these parts of the original contributions, the thesis offers an introductory background to THz-TDS, in the areas of hardware, applications, and data processing.