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Parameter Extraction and Uncertainty in Terahertz Time-Domain Spectroscopic Measurements

Greenall, Nicholas Robert (2017) Parameter Extraction and Uncertainty in Terahertz Time-Domain Spectroscopic Measurements. PhD thesis, University of Leeds.

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Abstract

Terahertz (THz) time domain spectroscopy is emerging as a powerful tool to characterise samples both chemically and physically. In this work different methods of estimating spectroscopic parameters of a sample, its thickness and the uncertainty of these estimates is presented. A number of case studies are also examined including paracetamol polymorphs and a method of creating a spectroscopic simulant of Semtex-H is presented. Approximation of the sample spectroscopic parameters, real refractive index and absorption coeficient were formed by building up a simple model of the samples interaction with THz radiation. Methods of correcting unwrapping error in the real refractive index were developed, including a method to correct in the presence of discontinuities in the refractive index itself. These approximations were then applied to extract parameters of both lactose and paracetamol samples. An algorithm to generate spectroscopic simulants was developed and applied to Semtex-H. These simulants consisted of simple mixtures of inert compounds, which were measured and found to have similar spectrum to the target sample. Methods of fitting resonant models to the sample response were developed to extract both the spectroscopic parameters and sample thickness. These were refined by calibrating for the Gaussian beam profile of the THz radiation, which was shown to increase the accuracy of the extracted thickness. The thickness and spectroscopic parameters of a lactose sample were measured with temperature, and it was found that the spectroscopic parameter change was underestimated when thickness was assumed constant. A resonant model for multilayered samples was then developed and used to characterise IPA in a flowcell measurement. This was then combined with a method of time segmentation of the sample response, to extract spectroscopic parameters and sample thickness simultaneously. This was then applied to a two layer sample, to extract the spectroscopic parameters of a silicon and a quartz layer from a single measurement. Finally, methods of propagating the uncertainty from the time domain to the spectroscopic parameters were developed. These were based on a multivariate normal statistical model of the measurements andwere compared to numerical bootstrap and Monte–Carlo estimates. These were used to develop confidence intervals for the extracted refractive index, absorption coefficient and thickness. These methods were applied to both a lactose and quartz sample.

Item Type: Thesis (PhD)
Keywords: Terahertz spectroscopy uncertainty multivariate normal explosives gouy gaussian refractive index absorption coefficient multilayer time-frequency time-domain genetic algorithms
Academic Units: The University of Leeds > Faculty of Engineering (Leeds)
The University of Leeds > Faculty of Engineering (Leeds) > School of Electronic & Electrical Engineering (Leeds)
The University of Leeds > Faculty of Engineering (Leeds) > School of Electronic & Electrical Engineering (Leeds) > Institute of Microwaves and Photonics (Leeds)
Depositing User: Mr Nicholas Robert Greenall
Date Deposited: 24 Jan 2018 16:01
Last Modified: 24 Jan 2018 16:01
URI: http://etheses.whiterose.ac.uk/id/eprint/19045

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