Les dernières années montrent des nombreuses applications de la spectroscopie Teraheretz (THz) dans le domaine de sécurité postale, contrôle de la qualité, médecine et biologie. Après les premières expériences de l'imagerie avec un seul élément / détecteur, l'étape suivante est l'utilisation de matrices de détecteurs. Par conséquent, la nécessité de détecteurs THz sensibles, très rapides, opérant à température ambiante et intégrable facilement en matrice est devenue crucial. Les transistors à effet de champ semblaient être les candidats les plus appropriés pour la construction du première matrice pour l'imagerie THz. Ce travail présente les études des transistors à effet de champ à base de GaAs et GaN en vue de leur application comme imageurs THz. Dans la première partie, nous présentons les études de FET à base de GaAs sur une plage de fréquence très large (0,25 _2.54 THz). Nous étudions également les moyens d'accroître leur sensibilité et d'optimiser leurs puissance équivalente de bruit. Dans la deuxième partie nous étudions les transistors à base de GaN. Ce matériau , avec un grand bande interdite, pourrait être un candidat potentiel comme imageur THz travaillant à des températures élevées et / ou dans des environnements difficiles. Leur sensibilité a été étudiée en fonction de différents paramètres physiques, tels que la tension de grille, la longueur de grille, le courant de fuite de grille, la température et la direction de polarisation du rayonnement THz incidente. Nous avons étudié également l'influence de l'application de courant de drain. Les comportements observés ont été interprété / compris en utilisant des simulations numériques basées sur les modèles théoriques existants. Enfin, nous avons étudié des transistors à base de GaAs avec une structure de couplage spécifique réseaux de grille périodiques. La présence de la structure périodique permet d'améliorer le couplage entre l'onde THz incidente et le transistor. Une estimation théorique de la longueur caractéristique de détection, combinée à des calculs de l'intensité des champs THz locaux ont été utilisés pour interpréter nos résultats expérimentaux. Un bon accord avec le modèle théorique a été obtenue montrant que la détection a lieu principalement dans les zones appauvri du canal. / Last years clearly show many emerging applications of Teraheretz spectroscopy in postal and airport security, quality control, medicine and biology. After first demonstrations of imaging with a single element/detector the evident next step is use of detector matrixes. Therefore , the need for sensitive, rapid, room temperature operating, and easily integrable THz detectors became critical. Field effect transistors appeared to be the most suitable candidates for building the first matrixes focal plane arrays. This work presents the studies of different GaAs and GaN based field effect transistors in view of they application in Terahertz imagers. In the first part we present the studies of GaAs based FETs over a very wide frequency range (0.25 _2.54 THz). We study also the ways to increase their sensitivity and optimize their Noise Equivalent Power. In the second part we study the transistors based on GaN technology. This wide gap material can be a potential candidate for Terahertz imagers working at elevated temperatures or/and harsh environments. Their sensitivity has been studied as a function of various physical parameters, such as the gate voltage, gate length, the gate leakage current, temperature and direction of polarization of the incident THz radiation. We studied also the influence of a drain current. The observed behaviour was interpreted/understood using numerical simulations based on existing theoretical models. Finally, we studied GaAs based transistors with a specific coupling structure - periodic double-granting gate. The presence of periodic structure allows to improve the coupling between incident THz wave and a transistor. A theoretical estimate of the characteristic length of detection, combined with calculations of the intensity of THz local fields were used to interpret our experimental results. Good agreement with theoretical model was obtained showing that the detection takes place mainly in depleted portions of the channel.

L’objectif de ce travail est de positionner l’imagerie terahertz comme un nouvel outil dédié à la métrologie et au contrôle non destructif. Ses propriétés étonnantes à la fois de forte pénétration dans les matériaux diélectriques et de longueur d’onde de dimension millimétrique voire submillimétrique en font un outil très enthousiasmant pour des applications comme le contrôle non destructif (CND).En premier lieu, nous avons présenté deux techniques d’imagerie : une consistant à déplacer l’objet, point par point, dans un faisceau terahertz focalisé pour reconstruire, pixel par pixel, une image en deux dimensions. L’autre technique repose sur l’utilisation d’un objectif et d’un capteur matriciel à l’état de l’art fonctionnant à température ambiante. Ainsi nous avons imagé des objets d’intérêts, sans déplacement mécanique. Bien que cette dernière technique soit beaucoup plus rapide que l’imagerie point par point, la qualité des images obtenues n’est pas comparable et ceci à cause de deux verrous. Ainsi, une partie du travail a consisté en l’étude des stratégies d’éclairage pour limiter les effets optiques interférentiels induits par la cohérence de la source. De plus, en déployant des simulations de toute la chaine optique avec le logiciel Zemax, de nombreuses images de qualité permettent d’envisager des applications en imagerie 2D (métrologie, CND) et en imagerie 3D. Ainsi, deux techniques complémentaires de reconstruction tomographique sont expérimentées sur des images obtenues en temps réel ; une technique inspirée de l’imagerie par rayon X et une technique utilisée en microscopie optique 3D qui exploite la profondeur de champ réduite de l’objectif. Enfin, plusieurs études spectroscopiques et métrologiques sont menées afin d’évaluer le comportement des matériaux et la dimension des objets à partir de leurs images THz en 2D ou avec leur reconstruction tomographique 3D. Les travaux effectués constituent les briques de base au déploiement de l’imagerie terahertz vers le domaine applicatif grâce à une qualité d’image incomparable et des acquisitions en temps réels. / The aim of this work is to position terahertz imaging as a new tool dedicated to metrology and non-destructive testing. Its astonishing properties of both high penetration in dielectric materials and wavelength millimeter or even submillimetric make it a very exciting tool for applications such as non-destructive testing (NDT).First, we presented two imaging techniques: one to move the object, point by point, in a focused terahertz beam to reconstruct, pixel by pixel, a two-dimensional image. The other technique is based on the use of a lens and a matrix sensor in the state of the art operating at room temperature. Thus we have imaged objects of interest, without mechanical displacement. Although this last technique is much faster than point-by-point imaging, the quality of the images obtained is not comparable and this because of two drawbacks. Thus, part of the work consists in the study of lighting strategies to limit the interferential optical effects induced by the coherence of the source. In addition, by deploying simulations of the entire optical chain with Zemax software, numerous quality images make it possible to consider applications in 2D (metrology, NDT) and 3D imaging. Thus, two complementary tomographic reconstruction techniques are tested on images obtained in real time; a technique inspired by X-ray imaging and a technique used in 3D optical microscopy that exploits the reduced depth of field of the lens. Finally, several spectroscopic and metrological studies are carried out in order to evaluate the behavior of the materials and the dimension of the objects starting from their THz images in 2D or with their 3D tomographic reconstruction. The work done is the foundation for deploying terahertz imagery to the application domain with unmatched image quality and real-time acquisition.

Após a excitação de uma amostra semicondutora por um pulso ultracurto, os fotoporadores interagem com a rede excitando modos longitudinais ópticos. Essa interação provoca variações no índice de refração do material, produzindo modulações na resposta óptica do meio (efeito eletro-óptico). Por outro lado, esta dinâmica origina polarizações dependentes do tempo o que gera emissão de radiação terahertz. Experimentos recentes (pump-probe) observaram modulações do campo através de medidas da refletividade resolvidas no tempo. A refletividade e o campo estão relacionados segundo o efeito eletro-óptico. Também se resolve temporalmente o campo irradiado pela amostra, através de antenas que operam na faixa de terahertz. Tanto as medidas eletro-ópticas quanto de emissão terahertz fornecem informações sobre a interação dinâmica do plasma com a rede após a excitação óptica. Nesse trabalho simulamos a interação dinâmica de plasma e fônons em n-GaAs e Telúrio (\"bulk\") após estes serem excitados por um pulso ultracurto. Utilizamos equações hidrodinâmicas para descrever transporte de cargas e uma equação fenomenológica de oscilador harmônico forçado, para descrever oscilações longitudinais ópticas da rede. Complementando nossa descrição temos a equação de Poisson, com a qual calculamos o campo gerado pelo plasma e pela polarização da rede semicondutora. Essas equações constituem um sistema de seis equações diferencias (quatro parciais) acopladas. Para resolvê-las utilizamos o método das diferenças finitas. Do cálculo numérico obtemos a evolução temporal do campo elétrico no interior do material. Com esse campo determinamos as freqüências de oscilação do sistema e calculamos o campo irradiado. Nossos resultados apresentam acordo qualitativo com os experimentos / Above-band-gap optical excitation of semiconductors generates highly non-equilibrium photocarriers which interact with phonons thus exciting vibrational modes in the system. This interaction induces refractive-index changes via the electro-optic effect. Moreover it gives rise to electromagnetic radiation at characteristic frequencies (terahertz). Both effects have been measured by time-resolved ultra fast spectroscopy. Recent pump-probe experiments have found strong modulations of the internal electric field through electro-optic measurements. The emitted electromagnetic radiation has also been detected by a terahertz dipole antenna. Both electro-optic and terahertz emission measurements provide information about the coupled dynamics of photocarriers and phonons. In this work we simulate the dynamics of plasmon-phonon coupled modes in n-GaAs and Tellurium (bulk) following ultrafast laser excitation. The time evolution of the photocarrier densities and currents is described semi classically in terms of the moments of the Boltzmann equation. Phonon effects are accounted for by considering a phenomenological driven-harmonic-oscillator equation, which is coupled to the electron-hole plasma via Poisson\'s equation. These equations constitute a coupled set of differential equations. We use finite differencing to solve these equations. From the numerical results for the evolution of internal fields we can calculate both the characteristic frequencies of system and its terahertz radiation spectrum. Our results are consistent with recent experimental data

Low-loss waveguides with minimal dispersion are desired throughout the electromagnetic spectrum. These properties are difficult to achieve in the Terahertz (THz) region due to material and geometric constraints. This thesis focuses on the design, fabrication, and testing of waveguide-based devices using two promising technologies: the free-space metallic-slit waveguide (MSWG) and the coplanar strip (CPS) waveguide on a thin (1 um) commercial silicon nitride membrane. The work presented here differs from standard THz waveguide research which commonly uses the field radiated by a photoconductive antenna (THz optics) for excitation and detection. To improve upon system integration, a focus is placed on planar waveguide devices without refractive THz elements. Three main waveguide devices are investigated. First, an edge-coupled MSWG-based linear tapered slot antenna (LTSA) was used for THz-Time Domain Spectroscopy (TDS). This device functions as an alternative to a standard photoconductive switch coupled to a silicon lens and maintains comparable performance. Next an edge-coupled tapered MSWG was investigated. The MSWG conductor separation was increased to a low-loss configuration where the field propagated for 24 mm, after which the conductors were tapered to focus the field onto the receiving active region where a THz-bandwidth pulse was detected. Finally a CPS waveguide was fabricated on a thin silicon nitride membrane where a THz-bandwidth pulse was detected after propagating for 10 mm. The active regions for this device were fabricated using a unique method. This method results in the creation of thousands of small (40 um x 20 um) active regions (from a 4 mm x 4 mm host substrate) which can be placed anywhere for THz excitation and detection. The small active regions in conjunction with the CPS waveguide on the silicon nitride membrane provide an excellent platform for THz system testing. A single membrane can host many THz circuits which can be made ``active" by the placement of a few thin-film photoconductive devices. Main potential future applications include waveguide-based spectroscopy and coherent THz-bandwidth circuit analysis. / Graduate

Finally, based on theoretical calculations and experiments, we present a development model (DDRA model) to describe the interaction between the protein and its solvent molecule. The parameters derived from this model provide good fits to the experimentally determined complex dielectric constant, making it of the model valuable benchmarks for other theoretical treatments of bio-molecular system. / Secondly, we focus our aims on investigating protein molecules due to the possibility of being able to explain the mechanism of molecular interactions more clearly. Two lands of labeled immunoglobulin G were investigated using a reflective THz-IDS system. The dielectric properties were sensitive to the conjugation of the antibody. Additionally, terahertz spectroscopy is able to evaluate the depth of the hydrogen shell and shows that the hydrogen-bonded networks of charged protein solutions play an important role in determining the dielectric. / The bio-molecular interaction has been one of the most challenging subjects to probe due to its complexity. In the thesis, we have been attempting to answer fundamental questions about bio-molecular interactions in the terahertz (THz) region from the macroscopic to microscopic level. Terahertz radiation (defined as 0.1--10 THz) can excite intermolecular interactions such as the librational and vibrational modes. These attributes make it feasible to probe the dynamic characteristics of the bio-molecular system. Furthermore, it is worth investigating whether terahertz technology could potentially be used as a novel tool in the biomedical diagnosis field in the near future. / Thirdly, using a transmission THz-TDS system we investigated a biomarker protein and observed distinct spectral differences at various temperatures. This work demonstrates that terahertz spectroscopy can be used to evaluate the anharmonicity of the vibrational potential. By comparing the absorption spectra of the THz-TDS and Synchrotron results it is possible to deduce the approximate localization of the vibrational modes within the molecular chain. / We develop a controlled study to investigate the effects of formalin fixing on the THz properties of two different tissue types. The optical properties are measured using THz reflection spectroscopy. The results present how the fixing process can affect image contrast in THz images of biological samples. / Sun, Yiwen. / Advisers: Emma MacPherson; Yuan-ting Zhang. / Source: Dissertation Abstracts International, Volume: 73-03, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references (leaves 120-140). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

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

Cette thèse est consacrée à l'étude de puits quantiques de HgCdTe / CdHgTe dans le plan [013]. Les données expérimentales sont obtenues par photoconductivité et résonance cyclotron et concordent avec les résultats de simulations numériques. Les calculs sont effectués par approximation des fonctions enveloppe avec un Hamiltonien effectif 8x8. Afin de décrire les hétérostructures dont la croissance est faite sur un plan atomique arbitraire, une approche générale basée sur l'expansion d'invariants et les transformations par rotation, a été développé. La rotation généralisée peut être appliquée à un modèle avec l'ensemble des bandes de façon arbitraire. Le spectre d'énergie du Hamiltonien effectif a été calculé en utilisant l'expansion en onde plane des fonctions enveloppe dans la direction de croissance de l'hétérostructure. Un champ magnétique quantifiant a été pris en compte avec la substitution et l'expansion de Pierls sur les fonctions d'onde des électrons libres en présence du champ magnétique. Les mesures de photoconductivité ont montré une photoréponse correspondant à des transitions interbandes pour des énergies de photons allant jusqu'à 30 meV. Dans l'intervalle de 7 à 30 meV nous avons observé une photoconductivité vraisemblablement due à la photo-ionisation de certains centres de diffusion dans les barrières de CdHgTe. L'analyse théorique de la possibilité d'amplification du rayonnement Terahertz sur les transitions intrabandes dans les puits quantiques HgCdTe / CdHgTe a été effectuée. Des mesures de résonance ont montré la dépendance de la masse cyclotron sur la concentration des porteurs de charges due à la forte non-parabolicité de la bande de conduction. En champs magnétiques quantifiants (jusqu'à 45 T) des résonances interbandes et intrabandes ont été observées. Les mesures de magnétoabsorption ont été également effectuées avec un spectromètre à transformée de Fourier en champs magnétiques quantifiants allant jusqu'à 16 T. Les transitions interbandes et intrabandes ont été étudiés. Dans les puits quantiques de HgTe ayant une structure de bande inversée, une ligne de résonance cyclotron (CR) liée aux trous a été observée. Dans les échantillons ayant des bandes normales, en plus des transitions CR liées aux électrons, une ligne très marquée et probablement liée à des transitions d'impuretés a été découverte. Un anti-croisement des niveaux de Landau d'électrons et de trous dans les échantillons de bandes inversées a été confirmée par l'observation du splitting d'une résonance. Des simulations numériques ont montré que l'anticroisement des niveaux de Landau est causé par l'asymétrie par inversion du cristal massif (Bulk Inversion Asymmetry - BIA) et devrait disparaître pour certaines directions de croissance des hétérostructures. La comparaison des résultats expérimentaux et théoriques a montré de manière générale un bon accord qualitatif, mais un désaccord quantitative systématique allant dans le même sens pour toutes les expériences en champs magnétique. L'accord a été obtenu par l'ajustement de l'offset de la bande de valence du CdTe et du HgTe, et du paramètre de Kane, Ep. / The thesis is devoted to study of narrow-gap HgCdTe/CdHgTe quantum wells grown on [013] plane. The experimental data are obtained by means of photoconductivity and cyclotron resonance measurements and fit with results of numerical simulations.The calculations are made within envelope functions approximation with 8x8 effective Hamiltonian. In order to describe heterostructures grown on arbitrary atomic plane a general approach based on expansion over invariants and rotation transformation was developed. The generalized rotation can be applied to a model with arbitrary basis band set.The energy spectrum of the effective Hamiltonian was calculated using plain wave expansion of the envelope functions in heterostructure growth direction. Quantizing magnetic field have been taken into account with Pierls substitution and expansion over wave functions of free electron in magnetic field.Photoconductivity measurements have demonstrated interband photoresponse for photon energies down to 30~meV. In the range from 7 to 30 meV we have observed photoconductivity presumably concerned with photoionization of some centers in CdHgTe barriers. Theoretical analysis of possibility of FIR radiation amplification on intraband transitions in HgCdTe/CdHgTe quantum wells have been performed.Cyclotron resonance measurements in quasiclassical magnetic fields have shown dependence of cyclotron mass on carrier concentration caused by strong non-parabolicity of conduction band. In quantizing magnetic fields (up to 45 T) both interband and intraband magnetoabsorption have been observed.Magnetoabsorption have been also measured with Fourier transform spectrometer in quantizing magnetic fields up to 16 T. Both interband and intraband magnetoabsorption have been studied. In semimetallic HgTe quantum well with inverted band structure, a hole CR line was observed. In normal-band sample in addition to electron CR transitions a strong absorption line presumably related to impurity transitions was discovered.Avoided crossing of electron and hole Landau levels in inverted-band sample was confirmed by splitting of two spectral lines. Numerical simulations showed that Landau level anticrossing is caused by bulk inversion asymmetry and should disappear for some heterostructure growth directions.Comparison of experimental and theoretical results have shown good qualitative agreement with systematic quantitative disagreement in the same direction in all experiments involving magnetic fields. The agreement was achieved by adjustment of valence band offset of CdTe and HgTe, and the Kane parameter Ep.

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

In this study, suspensions of metallic nanoparticles in base fluids, nanofluids, are investigated by using terahertz time domain spectroscopy (THz-TDS). Nanofluids are used as the working fluid in a variety of applications especially for the purpose of heat transfer enhancement. Polar fluids are being used as the base in nanofluids for their tendency to stop aggregation and sedimentation. Polar fluids highly absorb THz signal. In order to select the best possible host, various polar liquids have been investigated, and isopropanol (99.5%) is selected to be the best candidate for its low THz absorptivity when compared to ethanol (99.5%), ethylene glycol (99%), methanol (95%) and distilled water. Ag, Pd and Cu nanoparticles have been custom-made in isopropanol by laser ablation method, and the size distributions have been characterized by Zeta Potential Analyzer. The nanoparticle diameters are measured to be on average 10 nm, 12 nm and 75 nm for Ag, Cu and Pd, respectively. Nanofluids of 1X, 2X and 3X concentrations of Ag, Cu and Pd nanoparticles have been prepared by diluting with pure (99.5%) isopropanol. Measurements have been repeated after 7 days up to 12 days in order to check for aggregations and sedimentations. THz-TDS is a strong tool to analyze the refractive index and absorption coefficient, but no distinct difference was observed in the frequency domain analysis for the nanofluid samples. On the other hand, in the time domain data analysis, a shift on the time data with a change in transmission was observed. For Ag nanoparticles a positive time shift with a decrease in transmission with increasing concentration was observed. For Cu nanoparticles an interesting negative time shift and an increase in the intensity was observed with increasing concentration. The Pd nanoparticle solution scans showed almost no shift initially, but a negative time shift after a wait period on the order of days. A model of the transmission of the THz pulse through the nanofluid was developed based on transmission/reflection coefficients due to both dielectric and conducting layered media. The model well explains the positive time shift seen with Ag nanoparticle suspensions but fails to explain the shift seen with the Cu nanoparticle suspensions due to the long path length inside the nanofluid. Negative time-shifts can only be explained by decreasing the path length which suggests additional layering inside the nanofluid medium, or assuming that the chemical composition of the isopropanol host has changed with the addition of Cu and/or Pd nanoparticles. The positive time shifts observed with the Ag nanoparticle suspensions allowed for estimating the change in refractive index of the base fluid. From this change, using effective medium theory based on Maxwell-Garnett model, the concentrations of the nanoparticles were estimated. The results agree within an order of magnitude to commercially available nanofluids which are also non-aggregate.

Disertacijos darbo tikslas buvo sukurti ir ištirti puslaidininkinius terahercinių (THz) impulsų emiterius ir detektorius, skirtus sistemoms, naudojančioms 1 μm bangos ilgio femtosekundinę lazerinę spinduliuotę. THz impulsų generavimo ir detektavimo sistema, kurios optoelektroninius puslaidininkinius komponentus aktyvuoja femtosekundiniai lazerio impulsai, yra plačiai taikoma terahercinėje laikinės srities spektroskopijoje. Tradiciškai tokiose sistemose naudojami Ti:safyre femtosekundiniai lazeriai, kurių spinduliuotės bangos ilgis yra ~800 nm. Šios sistemos nėra patogios dėl jų matmenų, nes lazeriai turi sudėtingą kelių pakopų kaupinimo sistemą. Pastaruoju metu THz impulsų generavimui vis dažniau naudojami femtosekundiniai kietakūniai ir šviesolaidiniai lazeriai, kurių spinduliuotės bangos ilgis patenka į artimosios IR spinduliuotės sritį. Tačiau šios sistemos vis dar neturi tinkamos medžiagos fotolaidiems elementams gaminti, kurie būtų žadinami 1 – 1,55 µm bangos ilgio lazeriais. Tokios medžiagos, visų pirmą, turi būti jautrios optinei spinduliuotei, o jų draustinės energijos tarpas turi atitikti žadinamos spinduliuotės fotonų energiją, be to sluoksniai turi pasižymėti didele tamsine varža bei labai trumpomis krūvininkų gyvavimo trukmėmis (~ 1 ps). Šioje disertacijoje yra pateikiami THz impulsų generavimo panaudojus puslaidininkių paviršius ir fotolaidžias antenas rezultatai, žadinant 1 µm bangos ilgio femtosekundiniais lazerio impulsais. / The aim of dissertation was to develop and explore the semiconductor material terahertz (THz) pulse emitters, for Terahertz time–domain spectroscopy (THz–TDS) systems using a 1 μm wavelength femtosecond laser radiation. THz pulse generation and detection using optoelectronic semiconductor components in THz–TDS excited by femtosecond laser pulses become these days a powerful experimental technique. Traditionally, mode-locked Ti:sapphire lasers emitting at the wavelengths ~800 nm are used. However Ti:sapphire lasers require many-stage optical pumping arrangement, the system is quite bulky and complicated. The solution could be the lasers emitting in 1 – 1.55 µm, which can be directly pumped by diode laser bars. Recently, several compact, efficient and cost-effective solid-state and fiber laser systems that generate femtosecond pulses at near-infrared wavelengths have been developed and employed for activating THz–TDS systems. The main obstacle of these systems is the lack of material with appropriate bandgap, high dark resistivity and short (~ ps) carrier lifetimes.