Thesis (M.E.E.)--University of Delaware, 2006. / Principal faculty advisor: James Kolodzey, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

Our group purchased a THz photomixer from the Millimeter Wave Technology Group at the Rutherford Appleton Laboratory in London, UK. A photomixer is based on optical heterodyne conversion where a photoconductor or photodiode is illuminated by two laser beams with a difference frequency in the THz region. The beat frequency of the two lasers will modulate the conductivity of the photomixer material. The variation of the conductivity is converted to electrical current and finally radiation by applying a bias voltage across the active region of the device. My thesis concerns the determination of the characteristics of the photomixer, which is an expensive and extremely sensitive piece of equipment, to optimize its performance in support of a number of research activities within the group. / xvii, 120 leaves : ill. (chiefly col.) ; 29 cm

Thesis (Ph. D.)--University of California, Riverside, 2010. / Includes abstract. Title from first page of PDF file (viewed May 18, 2010). Includes bibliographical references. Issued in print and online. Available via ProQuest Digital Dissertations.

In this thesis, we deal with the application of transmission terahertz spectroscopy as an analysis tool for the study of molecular solids, in particular organic crystals of pharmaceutical interest. Most of the work has been performed using two computational packages aimed at the interpretation of the spectra, one based on molecular forcefields (DMACRYS), the other on solid state density functional theory (CASTEP). We compare low temperature determinations of several molecular organic crystals to calculated spectra, and attempt to assign calculated modes of vibrations to absorption peaks, based on the similarity in frequency between the measured and calculated peaks. One of the main aims of this work is to establish the limits of our forcefield approach, which is based on the approximation that the intramolecular degrees of freedom can be neglected. We analyse the normal modes of vibration calculated with CASTEP, evaluating the amount of rigid molecule rotational and translational contribution to each eigenvector as a function of frequency, in order to validate our forcefield approach. We also compare the two sets of eigenvectors from the DMACRYS and CASTEP calculations to assess the similarity between the two approaches. We perform the same eigenvectors analysis on several hydrate systems in order to understand the role of water in the lattice dynamics of crystalline hydrates. We attempt a classification of the eigenvectors based on the strength of the forces involved in the molecular vibrations and based on the amount of the water contribution to each normal mode. A set of isostructural crystals is analysed in order to understand the effect that small variations (in the molecular formula and in the unit cell arrangement) have on the measured and calculated absorption spectra of a crystal. Finally, we discuss the use and development of computational methods that allow us to have a more realistic description of the molecular electrostatic in DMACRYS.

This doctoral dissertation focuses on the nonlinear optical response of water vapour as well as some solids at terahertz (THz) frequencies. In this study, the propagation of broadband single-cycle THz pulses through a medium with the third-order nonlinear optical response is theoretically investigated. Also, a technique to measure the nonlinear response of transparent materials based on the time-domain THz spectroscopy is developed, which provides frequency dispersion curves of the nonlinear Kerr coefficient (n₂). A numerical model is used to simulate the THz pulse propagation. This model takes into account non-paraxial effects, self-focusing, and diffraction, as well as dispersion, in both the linear and nonlinear optical regimes. The contribution of non-instantaneous Kerr-type nonlinearity to the overall instantaneous and delayed Kerr effect at the THz frequencies is investigated. It is shown how increasing the nonlinear relaxation time and its dispersion modifies the THz pulse after the propagation through a transparent medium. The effect of linear dispersion on self-action during pulse propagation is also discussed. Moreover, the nonlinear spectroscopy of water vapour at THz frequencies is reported. Atmospheric water vapour has a rich spectrum with several strong resonances at frequencies below 3 THz, falling within the range of operation of most existing THz sources. An extremely large nonlinear response to THz radiation is observed at the positions of these resonances. Using the optical Kerr model for the nonlinear response, a minimum nonlinear refractive index of the order of 10² m²/W is estimated. The results provide insight into the energy levels of the water molecule and give a more accurate picture of its response to electromagnetic radiation, paving the way to more accurate THz spectroscopy, imaging, and sensing systems, and thereby facilitating future emerging THz technologies. Finally, the nonlinear response of solids at THz frequencies is studied. It has been shown that a phonon-induced THz Kerr effect can result in a larger nonlinear refractive index than the nonlinear refractive index at the visible or near-infrared range (optical Kerr effect). This pronounced nonlinear optical behavior is verified using a time-domain characterization approach. The results indicate a large delay occurred to the THz fields as they transmit through some of the material samples. In the frequency domain, the induced nonlinear phase shift of the intense THz field is shown to be relatively large of the order of 0.1 rad. From the phase information, the nonlinear phase is extracted by which the dispersion profile of n₂ is obtained.

No description available.

Studying dynamics of proteins in their biological milieu such as water is interesting because of their strong absorption in the terahertz range that contain information on their global and sub-global collective vibrational modes (conformational dynamics) and global dynamical correlations among solvent water molecules and proteins. In addition, water molecules dynamics within protein solvation layers play a major role in enzyme activity. However, due to the strong absorption of water in the gigahertz-to-terahertz frequencies, it is challenging to study the properties of the solvent dynamics as well as the conformational changes of protein in water. In response, we have developed a highly sensitive megahertz-to-terahertz dielectric spectroscopy system to probe the hydration shells as well as large-scale dynamics of these biomolecules. Thereby, we have deduced the conformation flexibility of proteins and compare the hydration dynamics around proteins to understand the effects of surface-mediated solvent dynamics, relationships among different measures of interfacial solvent dynamics, and protein-mediated solvent dynamics based on the complex dielectric response from 50 MHz up to 2 THz by using the system we developed. Comparing these assets of various proteins in different classes helps us shed light on the macromolecular dynamics in a biologically relevant water environment. / Doctor of Philosophy / Proteins are complicated biomolecules that exist in all living creatures and they are, mostly, involved in building up structures and cell functions in various biological systems. Not only their existence but also their complex movements and dynamics are vital to cell functions in living beings. Until recently, their chemical functions and dynamics have been extremely challenging to investigate and track in their native environments. Thanks to various efforts by researchers all over the world to learn more about their convoluted behavior, new techniques have arisen to study these properties. We, as a part of this community, have been able to develop highly sensitive megahertz-to-terahertz dielectric spectroscopy system to probe proteins and other biomolecules dynamics in picosecond to microsecond range. Using our benchmark system, we have been able to map the detailed dynamical properties of biomolecules as well as their exclusive hydration shell characterizations. In this work, we gathered details about three well-known proteins and biomolecules by studying their dielectric responses. Thus, we have been able to discuss the movements, relaxation processes and hydration shell properties of these molecules in liquid water as their basic native environment.

Ce travail de cette thèse porte sur la dynamique des protéines, accompagnées de leur eau d'hydratation, une couche de solvant autour des protéines vitale pour leur fonction biologique. Chacune de ces deux composantes s'accompagne d'une dynamique qui lui est propre et dont la réunion reforme le paysage énergétique du système biologiquement actif. La mise en application conjointe de la deutération sélective, de la diffusion incohérente de neutrons ainsi que la spectroscopie terahertz a permis d'explorer de manière indépendante la dynamique des protéines et de celles des couches d'hydratation. L'influence de l'état de repliement de la protéine sur sa dynamique a été étudié par diffusion élastique de neutrons. Les protéines globulaires se sont montrées moins dynamiques que ses analogues intrinsèquement désordonnées. Eux même semblent être plus rigides que les protéines dépliées non physiologiques. L'état d'oligomérisation et les conséquences sur la dynamique de ces protéines ont été développés. Les agrégats d'une protéine globulaire se sont avérés être plus flexibles que la forme soluble. A l'inverse, les agrégats d'une protéine désordonnée voient leur dynamique moyenne baisser par rapport à la forme soluble. Ces observations témoignent de la grande diversité de dynamiques à travers le protéome. Les expériences de diffusion incohérente de neutrons sur les couches d'hydratation des protéines globulaires et désordonnées ont permis d'obtenir des informations sur la nature des mouvements de l'eau autour des protéines. Les mesures ont mis en évidence la présence de mouvements translationnels concomitants à l'apparition de la transition dynamique dans les couches d'hydratations, vers 220 K. Les mesures ont de même montré un couplage plus fort entre une protéine désordonnée et son eau d'hydratation que celui entre une protéine globulaire et son eau d'hydratation. La nature de la couche d'hydratation et son influence sur sa dynamique ont été explorés, avec l'utilisation de polymères qui miment le comportement de l'eau et agissent comme source de flexibilité pour la protéine. Pour terminer, la dynamique des groupements méthyles, impliqués dans les modifications dynamiques observées à 150 et 220 K, a été étudiée. / This thesis work focused on the dynamics of proteins, surrounded by their hydration layer, a water shell around the protein vital for its biological function. Each of these components is accompanied by a specific dynamics which union reforms the complex energy landscape of the system. The joint implementation of selective deuteration, incoherent neutron scattering and terahertz spectroscopy allowed to explore the dynamics of proteins and that of the hydration shell. The influence of the folding state of protein on its dynamics has been studied by elastic neutron scattering. Globular proteins were less dynamic than its intrinsically disordered analogues. Themselves appear to be stiffer than non physiological unfolded proteins. The oligomerization state and the consequences on the dynamics were investigated. Aggregates of a globular protein proved to be more flexible than the soluble form. In contrast, aggregates of a disordered protein showed lower average dynamics compared to the soluble form. These observations demonstrate the wide range of dynamics among the proteome. Incoherent neutron scattering experiences on the hydration layer of globular and disordered proteins have yielded information on the nature of water motion around these proteins. The measurements revealed the presence of translational motions concomitant with the onset of the transition dynamics of hydration layers, at 220 K. Measurements have also shown a stronger coupling between a disordered protein and its hydration water, compared to a globular protein and its hydration shell. The nature of the hydration layer and its influence on its dynamics has been explored with the use of polymers that mimic the water behavior and that act as a source of flexibility for the protein. Eventually, the dynamics of methyl groups involved in the dynamical changes observed at 150 and 220 K, was investigated.

Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Terahertz radiation is currently used in security, information and communication technology (ICT), and biomedical sciences among others. The usability of terahertz (THz) radiation, in many of its applications depends on characteristics of the materials being investigated in the THz range. At the heart of THz usage is a THz spectroscopy system necessary for the generation and detection of the THz radiation. In this thesis, we characterise such a THz spectroscopy system. In our typical THz spectrometric system, we make use of femtosecond (fs) laser technology and pump-probe principles for emission and detection of THz radiation. Background about the principles of generation THz radiation using fs triggered antennas and the principles of the spectroscopy technique and appropriate literature references are presented. Using an assembled commercially available kit, we reproduce known spectra in order to confirm correct functionality (for calibration) of the assembled spectroscopy system and to gain experience in interpreting these spectra. By introducing a suitable x - y scanning device we construct a crude THz imaging device to illustrate the principle. / AFRIKAANSE OPSOMMING: Terahertsstraling word deesdae wyd in die sekuriteits, inligting-en-kommunikasie en biomediese sektore aangewend. Die gepastheid van terahertsstraling (THz) vir ’n spesifieke toepassings hang af van die eienskappe van die materiale wat ondersoek word. Vir die uitvoer van sulke eksperimente word ’n THz-spektroskopie sisteem benodig vir die opwekking en meting van THz-straling. In hierdie tesis word so ’n THz-spektroskopie sisteem beskou en gekarakteriseer. In die sisteem word van ’n femtosekondelaser (fs) gebruik gemaak in ’nn pomp-en-proef opstelling vir die uitstraling en meting van THz-straling. Die beginsels rakende die opwekking van THz-straling, deur gebruik te maak van ’n antenna wat deur ’n fs-laser geskakel word, asook die beginsels van die spektroskopiese tegniek, met toepaslike verwysings, word in die tesis aangebied. Deur gebruik te maak van’n kommersiële THz opstelling is bekende spektra gemeet om die korrekte funksionering (vir kalibrasie doeleindes) na te gaan en om ondervinding op te doen in die interpretasie van hierdie spektra. ’n X-Y-translasie toestel is tot die opstelling bygevoeg om THz-afbeelding moontlik te maak en sodoende hierdie beginsel te illustreer.

Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Terahertz (THz) radiation is often used in many promising applications such as information and communication technology and airport security. Optimized and adapted terahertz fields hold huge promise for leading this technology further. This study is focused on terahertz time domain spectroscopy (THz-TDS). In THz-TDS the electric field is measured, therefore both amplitude and phase information of the THz pulse can be obtained. We used the pump-probe technique in order to measure a THz pulse from the photoconductive antenna. A pulsed fiber laser with FWHM of ' 100 fs was used for this. The frequency spectrum of the measured THz pulse was obtained via a fast Fourier transform. We studied the principles of the THz pulse generation as well as detection, with a photoconductive antenna as emitter and detector. In this study terahertz spectroscopy has been used to investigate the refractive index and absorption coefficient of different types of materials in the terahertz region. The last part of this study deals with a simple process for material parameter extraction of a polymer sample using commercial software called Teramat1.0. It uses the sample thickness, the reference THz pulse and the transmitted THz pulse to retrieve the complex refractive index of the sample. / AFRIKAANSE OPSOMMING: Terahertz (THz) straling word gereeld gebruik vir belowende toepassings soos inligting en kommunikasie tegnologie en lughawe sekuriteit. Geoptimeerde en aangepaste terahertz velde dra by tot die bevordering van die tegnologie. Hierdie studie fokus op terahertz tyd domein spektroskopie (THz-TDS). In THz-TDS word die elektriese veld gemeet en dus word beide amplitude en fase inligting van die THz puls verkry. Ons gebruik ’n pomp en toets tegniek om die THz puls deur ’n fotogeleidende antenna te bepaal. ’n Gepulseerde vesel laser met FWHM van 100 fs word hiervoor benut. Die frekwensie spektrum van die gemete THz puls word bereken deur ’n vinnige Fourier transvorm te bereken. Die beginsels van die generering en deteksie van THz pulse is bestudeer met ’n fotogeleidende antenna as sender en ontvanger. In die studie is terahertz spektroskopie gebruik om die brekingsindekse en die absorpsie koeffisiënte van verskillende materiale in die terahertz gebied te bepaal. Die laaste gedeelte van die studie handel oor ’n eenvoudige proses om die materiaal parameters van ’n polimeer te bepaal deur gebruik te maak van kommersiële sagteware Teramat 1.0. Die monster dikte, die THz verwysingspuls en die deurgelate puls word gebruik om die komplekse brekingsindeks van die materiaal te bereken.