Lying between the microwave and far infrared (IR) regions, the "terahertz gap" is a relatively unexplored frequency band in the electromagnetic spectrum that exhibits a unique combination of properties from its neighbors. Like in IR, many materials have characteristic absorption spectra in the terahertz (THz) band, facilitating the spectroscopic "fingerprinting" of compounds such as drugs and explosives. In addition, non-polar dielectric materials such as clothing, paper, and plastic are transparent to THz, just as they are to microwaves and millimeter waves. These factors, combined with sub-millimeter wavelengths and non-ionizing energy levels, makes sensing in the THz band uniquely suited for many NDE applications. In a typical nondestructive test, the objective is to detect a feature of interest within the object and provide an accurate estimate of some geometrical property of the feature. Notable examples include the thickness of a pharmaceutical tablet coating layer or the 3D location, size, and shape of a flaw or defect in an integrated circuit. While the material properties of the object under test are often tightly controlled and are generally known a priori, many objects of interest exhibit irregular surface topographies such as varying degrees of curvature over the extent of their surfaces. Common THz pulsed imaging (TPI) methods originally developed for objects with planar surfaces have been adapted for objects with curved surfaces through use of mechanical scanning procedures in which measurements are taken at normal incidence over the extent of the surface. While effective, these methods often require expensive robotic arm assemblies, the cost and complexity of which would likely be prohibitive should a large volume of tests be needed to be carried out on a production line. This work presents a robust and efficient physics-based image processing approach based on the mature field of parabolic equation methods, common to undersea acoustics, seismology, and other areas of science and engineering. The method allows the generation of accurate 3D THz tomographic images of objects with irregular, non-planar surfaces using a simple planar scan geometry, thereby facilitating the integration of 3D THz imaging into mainstream NDE use.

In this thesis, two-dimensional materials such as graphene are tested for their suitability for opto-electronic applications using terahertz time domain spectroscopy (THz-TDS). This ultrafast all-optical technique can probe the response of novel materials to photoexcitation, and yield information about the dynamics of the material systems. Graphene grown by chemical vapour deposition (CVD) is studied using optical-pump THz-probe time domain spectroscopy in a variety of gaseous environments in Chapter 4. The photoconductivity response of graphene grown by CVD is found to vary dramatically depending on which atmospheric gases are present. Adsorption of these gases can open a local bandgap in the material, allowing stimulated emission of THz radiation across the gap. Semiconducting equivalents to graphene, molybdenum disulphide (MoS₂) and tungsten diselenide (WSe₂), grown by CVD, are investigated in Chapter 5. These members of the transition metal dichalcogenide family show sub-picosecond responses to photoexcitation, suggesting promise for use in high-speed THz devices. In Chapter 6, an alternative production route to CVD is studied. Liquid-phase exfoliation offers fast, easy production of few-layer materials. THz spectroscopy reveals that the dynamics of these materials after photoexcitation are remarkably similar to those in CVD-grown materials, offering the potential of cheaper materials for future devices. Finally in Chapter 7, it is shown that carbon nanotubes can be used to make ultrafast THz devices. Unaligned, semiconducting single walled carbon nanotubes can be photoexcited to produce an ultrafast, dynamic THz polariser. The work in this thesis demonstrates the potential for these novel materials in future opto-electronic applications. THz spectroscopy is shown to be an important tool for the characterisation of new materials, providing information that can be used to understand the dynamics of materials, and improve production methods.

Ce mémoire de thèse est consacré à l'étude, la fabrication et la caractérisation de métamatériaux en vue d'applications dans le domaine millimétrique et submillimétrique. Dans un premier temps, nous avons tenu à rappeler les propriétés remarquables ainsi que les processus physiques mis en jeux par cette nouvelle génération de matériaux. Le manuscrit regroupe essentiellement des résultats issus d’études réalisées sur différentes structures en microondes et en terahertz : métamatériaux composites, métamatériaux entièrement diélectriques à base de résonances de Mie, ouvertures sublongueur d’ondes basés sur la transmission extraordinaire assistée par plasmons de surface. Nos investigations ont permis d’ouvrir la voie à de multiples applications dans les domaines des capteurs et des télécommunications. / This PhD dissertation is dedicated to the study, fabrication and characterization of metamaterials for millimeter and submillimeter applications. First of all we proposed to remind the extraordinary properties and physical processes involving within this new generation of materials. The manuscript contains results obtained from studies performed on different categories of metamaterials at microwave and terahertz frequencies: composite metamaterials, all dielectric metamaterials based on Mie resonances, subwavelength apertures based on the extraordinary transmission assisted by surface Plasmon polaritons. Our investigations have contributed to open the path to multiple potential applications in the field of sensors and telecommunications.

Sy, Ming Yiu. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (p. 95-100). / Abstracts in English and Chinese. / Abstract --- p.1 / Acknowledgments --- p.5 / List of figures --- p.9 / List of tables --- p.13 / List of abbreviations --- p.14 / List of publications and awards --- p.15 / Awards --- p.16 / Chapter Chapter 1 --- Introduction --- p.17 / Chapter 1.1 --- Terahertz radiation --- p.18 / Chapter 1.1.1 --- Terahertz sources --- p.19 / Chapter 1.1.2 --- Terahertz systems --- p.20 / Chapter 1.1.3 --- Reflection and Transmission geometries --- p.21 / Chapter 1.2 --- Terahertz biomedical applications --- p.24 / Chapter 1.2.1 --- Biomolecules --- p.24 / Chapter 1.2.2 --- THz biomedical imaging --- p.25 / Chapter 1.2.3 --- THz Spectroscopy --- p.26 / Chapter 1.3 --- Objectives --- p.26 / Chapter 1.4 --- Structure of this thesis --- p.26 / Chapter Chapter 2 --- Theory and experimental system --- p.28 / Chapter 2.1 --- Theory --- p.28 / Chapter 2.1.1 --- Propagation of electromagnetic field through dielectric media --- p.29 / Chapter 2.1.2 --- A de-noising method --- p.32 / Chapter 2.1.3 --- Baseline calculation --- p.34 / Chapter 2.1.4 --- Frequency-dependent Refractive Index and Absorption Coefficient.. --- p.37 / Chapter 2.2 --- Experimental Configuration --- p.41 / Chapter 2.2.1 --- Terahertz generation and detection --- p.41 / Chapter 2.2.2 --- Configuration of our system --- p.44 / Chapter 2.2.3 --- Hand-held TPI Setup --- p.46 / Chapter 2.3 --- Data Structure --- p.48 / Chapter 2.4 --- Pre-processing and the user interface --- p.49 / Chapter 2.4.1 --- Data pre-processing 1 (Chopping) --- p.49 / Chapter 2.4.2 --- Data pre-processing 2 (Angle selection) --- p.50 / Chapter 2.4.3 --- The user interface for the data processing --- p.52 / Chapter Chapter 3 --- Ex-v/vo experiment: investigating the origin of contrast --- p.54 / Chapter 3.1 --- Liver Cirrhosis --- p.54 / Chapter 3.1.1 --- Liver --- p.54 / Chapter 3.1.2 --- Liver cirrhosis --- p.54 / Chapter 3.1.3 --- The trend of liver cirrhosis --- p.56 / Chapter 3.1.4 --- Technique for diagnosing liver cirrhosis --- p.57 / Chapter 3.2 --- Experiment protocol --- p.58 / Chapter 3.2.1 --- Formalin fixing --- p.58 / Chapter 3.2.2 --- Sample preparation --- p.58 / Chapter 3.2.3 --- Formalin fixing protocol --- p.60 / Chapter 3.2.4 --- Histopathology --- p.61 / Chapter 3.2.5 --- Protocol for measuring sample water content --- p.61 / Chapter 3.3 --- Results and discussion --- p.62 / Chapter 3.3.1 --- Optical parameters of the fresh and fixed samples --- p.62 / Chapter 3.3.2 --- Consistency with previous results --- p.63 / Chapter 3.3.3 --- The relationship between water content and optical parameters --- p.64 / Chapter 3.3.4 --- Conclusion --- p.68 / Chapter Chapter 4 --- In-vivo experiment: imaging of human skin --- p.69 / Chapter 4.1 --- Human skin --- p.69 / Chapter 4.1.1 --- The structure of human skin --- p.69 / Chapter 4.1.2 --- Skin thickness --- p.70 / Chapter 4.1.3 --- The structure and regeneration of stratum corneum --- p.70 / Chapter 4.1.4 --- Stratum corneum related Skin disease --- p.72 / Chapter 4.2 --- Combing reflections of electromagnetic wave --- p.73 / Chapter 4.3 --- Experiment protocol --- p.74 / Chapter 4.3.1 --- THz response of human skin --- p.74 / Chapter 4.3.2 --- Protocol for measuring human skin --- p.75 / Chapter 4.4 --- Results and discussion --- p.76 / Chapter 4.4.1 --- The variation due to the position --- p.76 / Chapter 4.4.2 --- The variation due to the temperature and humidity --- p.78 / Chapter 4.4.3 --- Discussion --- p.79 / Chapter Chapter 5 --- Noise evaluation --- p.80 / Chapter 5.1 --- The main noise source --- p.80 / Chapter 5.2 --- SNR and DR --- p.80 / Chapter 5.2.1 --- Signal to noise ratio (SNR) --- p.80 / Chapter 5.2.2 --- Dynamic range (DR) --- p.81 / Chapter 5.2.3 --- SNRVS DR --- p.82 / Chapter 5.3 --- Simulation of noise impact on the complex refractive index --- p.83 / Chapter 5.3.1 --- Methodology --- p.83 / Chapter 5.3.2 --- SNR: Simulation results and discussions --- p.85 / Chapter 5.3.3 --- DR: Simulation results and discussions --- p.87 / Chapter 5.3.4 --- Conclusion --- p.89 / Chapter Chapter 6 --- Conclusion and future work --- p.90 / Chapter 6.1 --- Conclusion --- p.90 / Chapter 6.1.1 --- A summary of Terahertz pulsed imaging (TPI) techniques --- p.90 / Chapter 6.1.2 --- Our system and calculations --- p.90 / Chapter 6.1.3 --- Terahertz spectroscopy of liver cirrhosis: investigating the origin of contrast --- p.91 / Chapter 6.1.4 --- In-v/vo study: skin measurement --- p.91 / Chapter 6.1.5 --- SNR sensitivity --- p.92 / Chapter 6.2 --- Suggestions for future work --- p.92 / Chapter 6.2.1 --- Algorithms --- p.92 / Chapter 6.2.2 --- Understanding the origin of contrast --- p.93 / Chapter 6.2.3 --- Application in cardiovascular diagnosing imaging --- p.93 / Chapter 6.3 --- Concluding remarks --- p.94 / References --- p.95

Kan, Wai Chi. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references (p. 111-116). / Abstract --- p.i / Acknowledgement --- p.iii / List of Publications --- p.vi / List of Figures --- p.xi / List of Tables --- p.xii / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Terahertz Radiation --- p.1 / Chapter 1.2 --- Biomedical Applications of Terahertz Imaging --- p.3 / Chapter 1.3 --- THz Spectroscopy --- p.4 / Chapter 1.4 --- Osteoarthritis --- p.4 / Chapter 1.5 --- Aim and motivation --- p.5 / Chapter 1.6 --- Overview of thesis --- p.5 / Chapter 2 --- Theory --- p.7 / Chapter 2.1 --- Propagation of electromagnetic field through dielectric media --- p.7 / Chapter 2.2 --- Deconvolution --- p.10 / Chapter 2.3 --- Baseline offset --- p.12 / Chapter 2.4 --- Frequency-dependent Refractive Index and Absorption Coefficient --- p.15 / Chapter 2.4.1 --- Reflection Geometry --- p.15 / Chapter 2.4.2 --- Transmission Geometry --- p.17 / Chapter 2.5 --- Conversion of Optical Delay into Depth --- p.22 / Chapter 2.6 --- Finite Difference Time Domain Method --- p.23 / Chapter 2.7 --- Summary --- p.25 / Chapter 3 --- Terahertz Systems --- p.26 / Chapter 3.1 --- Terahertz Pulsed Generation --- p.26 / Chapter 3.2 --- Terahertz Pulsed Detection --- p.28 / Chapter 3.3 --- Terahertz Pulsed Imaging (TPI) System --- p.29 / Chapter 3.4 --- Reflection System --- p.29 / Chapter 3.4.1 --- Flatbed System --- p.29 / Chapter 3.4.2 --- Probe --- p.32 / Chapter 3.5 --- Transmission System --- p.36 / Chapter 3.5.1 --- Antenna --- p.39 / Chapter 3.6 --- Data Acquisition --- p.40 / Chapter 3.6.1 --- Flatbed System --- p.40 / Chapter 3.6.2 --- Probe --- p.42 / Chapter 3.7 --- Baseline Validation --- p.46 / Chapter 4 --- Osteoarthritis --- p.48 / Chapter 4.1 --- Introduction --- p.48 / Chapter 4.2 --- Cartilage Composition and Structure --- p.49 / Chapter 4.3 --- 〇A symptoms --- p.51 / Chapter 4.4 --- Other Imaging Techniques --- p.52 / Chapter 4.5 --- Sample Preparation and Histology --- p.54 / Chapter 5 --- THz Pulsed Imaging of OA --- p.58 / Chapter 5.1 --- Results --- p.58 / Chapter 5.1.1 --- Optical Delays --- p.59 / Chapter 5.1.2 --- Estimation of surface refractive index --- p.69 / Chapter 5.1.3 --- Conversion of Optical Delay into Cartilage Thickness --- p.72 / Chapter 5.1.4 --- Correlation with Histology --- p.74 / Chapter 5.1.5 --- Errors and Problems --- p.80 / Chapter 5.2 --- FDTD of cartilage layers --- p.85 / Chapter 5.3 --- Conclusion --- p.87 / Chapter 6 --- Sliced Cartilage Sample and Bone Measurement --- p.88 / Chapter 6.1 --- Sliced Cartilage Samples --- p.88 / Chapter 6.1.1 --- Multi-reflections of sliced cartilage samples --- p.89 / Chapter 6.1.2 --- The influence of pressure on cartilage thickness --- p.91 / Chapter 6.1.3 --- Estimation of surface refractive index of sliced cartilage samples --- p.93 / Chapter 6.1.4 --- Comparison between sliced cartilage and knee joint measurements --- p.95 / Chapter 6.2 --- Bone --- p.97 / Chapter 7 --- Transmission System Result --- p.99 / Chapter 7.1 --- Data Validation --- p.99 / Chapter 7.1.1 --- Water spectrum --- p.99 / Chapter 7.1.2 --- Quartz measurement --- p.100 / Chapter 7.2 --- Liquid cell --- p.100 / Chapter 7.3 --- Cartilage Transmission Result --- p.103 / Chapter 7.4 --- Difficulties and problems --- p.105 / Chapter 7.5 --- Conclusions --- p.106 / Chapter 8 --- Conclusions and future work --- p.107 / Chapter 8.1 --- Summary --- p.107 / Chapter 8.2 --- Discussion --- p.107 / Chapter 8.3 --- Suggestions for further study --- p.109 / Bibliography --- p.111

Terahertz (THz) spectroscopy can potentially be used to probe and characterize inhomogeneous materials, however spectroscopic identification of such materials from spectral features of diffuse returns is a relatively underdeveloped area of study. In this thesis, diffuse THz scattering from granular media is modeled by applying radiative transfer (RT) theory for the first time in THz sensing. Both classical RT theory and dense media radiative transfer (DMRT) theory based on the quasi-crystalline approximation (QCA) are used to calculate diffuse scattered intensity. The numerical solutions of the vector radiative transfer equations (VRTE) were coded and calculated in MATLAB. The diffuse scattered field from compressed Polyethylene (PE) pellets containing steel spheres was measured in both transmission and reflection using a THz time domain spectroscopy (THz-TDS) system. Measurement results showed energy redistribution by granular media due to volume scattering as well as angle dependent spectral features due to Mie scattering. The RT model was validated by successfully reproducing qualitative features observed in experimental results. Diffuse intensity from granular media containing Teflon, lactose sugar, and C4 explosive was then calculated using the RT models. Simulation results showed the amplitude of diffuse intensity is affected by factors such as grain size, fractional volume of grains, thickness of scattering layer, and scattering angles. Spectral features were also observed in the diffuse intensity spectra from media containing grains with THz spectral signatures. The simulation results suggest the possibility of identifying materials from diffuse intensity spectra.

Terahertz (THz) wavelengths have attracted recent interest in multiple disciplines within engineering and science. Situated between the infrared and the microwave region of the electromagnetic spectrum, THz energy can propagate through non-polar materials such as clothing or packaging layers. Moreover, many chemical compounds, including explosives and many drugs, reveal strong absorption signatures in the THz range. For these reasons, THz wavelengths have great potential for non-destructive evaluation and explosive detection. Three-dimensional (3-D) reflection imaging with considerable depth resolution is also possible using pulsed THz systems. While THz imaging (especially 3-D) systems typically operate in transmission mode, reflection offers the most practical configuration for standoff detection, especially for objects with high water content (like human tissue) which are opaque at THz frequencies. In this research, reflection-based THz synthetic-aperture (SA) imaging is investigated as a potential imaging solution. THz SA imaging results presented in this dissertation are unique in that a 2-D planar synthetic array was used to generate a 3-D image without relying on a narrow time-window for depth isolation [1]. Novel THz chemical detection techniques are developed and combined with broadband THz SA capabilities to provide concurrent 3-D spectral imaging. All algorithms are tested with various objects and pressed pellets using a pulsed THz time-domain system in the Northwest Electromagnetics and Acoustics Research Laboratory (NEAR-Lab).

The purpose of this study is to investigate the dielectric properties of fuel oils and their ethanol mixtures in the THz spectral region. We presented frequency dependent absorption coefficients, refractive indices, and dielectric constants calculated from the measurements of pure and mixtures of fuel oils. As the mixing ratio changes, meaningful shifts were observed in refractive index and absorption coefficient of the mixtures. For pure liquids, we used Debye model which provides a good estimate for the dielectric parameters of pure liquids in microwave region and also in the THz region. Bruggeman model, which is used for describing the interaction between liquids in binary mixtures, did not work for ethanol mixtures of gasoline within our assumptions. However, these mixtures were modeled successfully with a modified Debye model in which the mixture behavior was described with a basic contribution approach. The results suggest that there is no strong interaction between the ethanol and the molecules in the gasoline. We concluded that this new approach offers a simple and useful method to determine the concentration of ethanol in gasoline with 3% (by volume) maximum error.

Thesis (Ph. D.)--University of California, Riverside, 2009. / Vita. Includes abstract. Includes bibliographical references (leaves 148-155). Issued in print and online. Available via ProQuest Digital Dissertations.

Thesis (Ph. D.)--Syracuse University, 2007. / "Publication number: AAT 3295517."