This literature analysis examines terahertz (THz) imaging as a non-invasive tool for the imaging of friction ridge detail from the first painted layer (imprimatura) in multilayered painted works of art. The paintings of interest are those created utilizing techniques developed during the Renaissance and still in use today. The goal of analysis serves to answer two questions. First, can THz radiation penetrate paint layers covering the imprimatura to reveal friction ridge information? Secondly, can the this technology recover friction ridge detail such that the fine details are sufficiently resolved to provide images suitable for comparison and identification purposes. If a comparison standard exists, recovered friction ridge detail from this layer can be used to establish linkages to an artist or between works of art. Further, it can be added to other scientific methods currently employed to assist with the authentication efforts of unattributed paintings. Flanked by the microwave and far-infrared edges, THz straddles the electronic and optic perspectives of the electromagnetic spectrum. As a consequence, this range is imparted with unique and useful properties. Able to penetrate and image through many opaque materials, its non-ionizing radiation is an ideal non-destructive technique that provides visual information from a painting’s sub-strata. Imaging is possible where refractive index differences exist between different paint layers. Though it is impossible, at present, to determine when a fingerprint was deposited, one can infer approximately when a print was created if it is recovered from the imprimatura layer of a painting, and can be subsequently attributed to a known source. Fingerprints are unique, a person is only able to deposit prints while their physical body is intact and thus, in some cases, the multiple layer process some artists use in their work may be used to the examiner’s advantage. Impressions of friction ridge detail have been recorded on receiving surfaces from human hands throughout time (and have also been discovered in works of art). Yet, the potential to associate those recorded impressions to a specific individual was only realized just over one hundred years ago. Much like the use of friction ridge skin, the relatively recently discovered THz range is now better understood; its tremendous potential unlocked by growing research and technology designed to exploit its unique properties.

In this thesis, I present my research into the development and use of a magneto-spectrometer operating at terahertz frequencies. Molecular samples with unpaired electrons were studied using electron paramagnetic resonance (EPR) spectroscopy and the properties of semiconducting samples were measured by cyclotron resonance. A terahertz time-domain spectrometer was constructed, fine-tuned and benchmarked. The use of a large area photoconductive antenna (PCA), with a bias voltage modulated at 1 MHz, allowed for a signal-to-noise ratio of 13800:1 to be achieved, which was equal or superior to comparable systems in use. The thermal behaviour of the PCA was studied and modelled to find that a 28% increase in emission occurred when the PCA was cooled to 100 K and a 22% increase in the maximum usable bias voltage was possible when the emitter substrate was cooled in a liquid nitrogen cryostat. The spectrometer was used to study a mixed type I/type II GaAs quantum well heterostructure as a test sample, in the process extending the existing research performed on this controllable terahertz filter. Together with a Gunn diode, bolometer and a Schottky diode, a pulsed magnet was developed into a 135 GHz, continuous-wave EPR spectrometer. EPR was detected in DPPH allowing for the spectrometer magnetic field accuracy to be determined and in ruby, which provided the crystal orientation of the sample and the base temperature of the spectrometer. Following this, three novel mixed-metal fluoride-centred triangles were studied on a multiple frequency EPR spectrometer allowing for the exchange interactions to be calculated along with the g-factors for two energy states. Two dimetallic cobalt complexes were studied to find that small changes to the molecules resulted in significant changes in the measured EPR spectra and so these molecules could be a useful part of the study of exchange coupled systems. The terahertz time-domain spectrometer was then combined with the pulsed magnet and an asynchronous optical sampling (ASOPS) system. Use of ASOPS allowed the number of waveforms measurable during a magnetic field pulse to be increased by at least a factor of 20 compared to contemporary alternatives. The spectrometer was used study cyclotron resonance in a two-dimensional electron gas (2DEG) formed at a GaAs/AlGaAs interface. Cyclotron resonance was successfully measured in the 2DEG at frequencies up to 1.6 THz, finding an electron effective mass of 0.071 m e , a sheet electron density of 3.3 × 10 11 cm -2 and a mobility of 1.5 × 10 5 cm 2 V -1 s -1 . Furthermore, using this system, the detection of EPR in a ruby sample was achieved, providing the first demonstration of ASOPS being used to study EPR at terahertz frequencies.

In this thesis, I report on the investigation of the low frequency vibrational modes in a number of different systems using ultrafast spectroscopic techniques. These consist of biological systems, including the enzyme, morphinone reductase (MR) and the related biomolecules, riboflavin (Rb) and flavin mononucleotide (FMN), as well as non-biological systems, including the semiconductor gallium nitride (GaN) and gold nanoparticles (Au NPs). The term low frequency refers to terahertz (THz) frequencies, where vibrational modes exist at the molecular level, with molecular rotations, lattice vibrations and inter- and intra-molecular vibrations occurring in the THz spectral range. These vibrational modes occur on sub-picosecond timescales and therefore ultrafast techniques utilising femtosecond laser pulses provide a means of studying these modes, and are employed throughout this thesis. The two ultrafast techniques of transient absorption (TA) spectroscopy and terahertz time-domain spectroscopy (THz-TDS) were used. Firstly, a high-repetition rate transient absorption (HRRTA) spectrometer was commissioned to perform pump-probe measurements with an ultraviolet pump and broadband visible probe. The performance of the HRRTA spectrometer was benchmarked using Au NPs and used to investigate the existence of a promoting vibration in MR contributing to the catalysis process, predicted to occur at THz frequencies. Weak oscillations were detected in the charge-transfer absorption band of MR bound to the non-reactive cofactor 1,4,5,6-tetra-hydro-nicotinamide adenine dinucleotide (NADH4), with a frequency of approximately 1.5 THz and provide evidence of the first direct observation of a promoting vibration in an enzyme. To complement the TA measurements, THz-TDS was also used to obtain direct measurements of the absorption at THz frequencies. Due to the challenge of studying water-based biological samples, an initial investigation was performed on a wurtzite GaN wafer, which exhibited optical phonon modes in the THz frequency range that were found to determine the dielectric response of the semi-insulating semiconductor wafer. Use of a non-polar m-plane wafer allowed the anisotropic nature to be observed and values of 9.22 ± 0.02 and 10.32 ± 0.03 for the static dielectric constants were obtained for the THz electric field polarised both perpendicular and parallel to the c-axis of the wurtzite GaN wafer, respectively. Finally, biological studies using THz-TDS were performed with measurements on Rb pellets and films revealing vibrational modes in the THz region. The sharp absorption features were not observed in FMN, despite a small difference in molecular content from Rb, and dehydration was required to reveal small amplitude absorption features. Final measurements on MR and MR-NADH4 films were carried out and evidence of absorption features in the THz frequency range were observed, however further work is required to determine the precise origin of these features.

It has become increasingly apparent that the future of next generation of electronic devices can and will rely on graphene nanoribbons. Graphene nanoribbons and sister structures showcase several key properties that can address the emerging need of terahertz science and technology, and break through the many technological limits on conventional semiconductor electronics operating in the terahertz spectrum. In this thesis, we focus on the study of the terahertz nonlinear optical response of metallic armchair graphene nanoribbons and sister structures using a k.p model and time dependent perturbation theory. We find that these nanoribbons exhibit a stronger interband optical response, and a smaller critical field strength (of the order of 10 kV/m) than does 2D single layer graphene. We demonstrate that finite ribbon size, spatial profile of the applied terahertz radiation field, polarization of the applied terahertz radiation, a small band gap opening, and application of a superlattice potential are several ways to tune the strong terahertz nonlinear optical response of metallic armchair graphene nanoribbons. The major contributions of this thesis include: 1) developes of a simpler method compared to other sophisticated methods of the terahertz nonlinear optical interband response of metallic armchair graphene nanoribbons; 2) extends the method in the characterization of various quantum size effects, elliptically polarized radiation field, small gap opening and superlattice on the terahertz optical response of these nanoribbons; 3) The versatility of the tunability showed in the terahertz nonlinear response of metallic armchair nanoribbons and sister structures will help advance the development of the nonlinear terahertz armchair graphene nanoribbon opto-electronic and photonic technology.

In recent years Terahertz (THz) time domain spectroscopy has emerged as a promising new technology with potential applications in a variety of fields, including industrial manufacturing, security screening and medical imaging. Pulsed THz systems are uniquely suited for non-destructive evaluation (NDE) of the sub-surface layers of dielectric packaging and coating materials, because they provide high dynamic range over a wide bandwidth in the far infrared portion of the electromagnetic spectrum. Often the dielectric materials of the packaging and/or surface coating layers exhibit relatively low loss and abrupt changes in the refractive index at the layer boundaries can be observed as a train of THz pulses in A-scan data. However, many practical applications of THz NDE will require fast signal acquisition to efficiently scan and evaluate many samples. The conventional processing approach shown in much of the published work in the field of THz NDE does not perform well in low signal-to-noise ratio (SNR) conditions. In addition, many samples of interest contain thin film layers and the THz pulses reflecting from the boundaries overlap on top of one another. Thus, it is not always possible to calculate the thickness of thin films from conventional time difference of arrival (TDOA) measurements. In this dissertation physics-based signal processing methods that have been historically used for radar/sonar signal processing are adapted and applied for THz NDE of layered media. Results are demonstrated with measured data from a pulsed THz system in the Northwest Electromagnetic and Acoustics Research Laboratory (NEAR-Lab) at Portland State University (PSU). This research is expected to provide an important link for THz researchers to access and apply the robust methods that have been developed over several decades for other applications. Two key contributions of this work are: 1. Development of a matched filter approach for THz NDE of thick layered media based on the maximum likelihood estimator (MLE). 2. Development of a matched field processing (MFP) approach for THz NDE of thin-film layered media, based on techniques in the underwater acoustics literature.

Le phénomène de filamentation laser apparaît spontanément dans un faisceau laser infra-rouge se propageant dans l'air si sa puissance crête excède quelques Gigawatts. À ce régime d'intensité le faisceau s'effondre sur lui même par effet Kerr et un équilibre dynamique s'installe entre la diffraction, l'effet Kerr et la défocalisation par l'air ionisé, permettant de maintenir une intensité très élevée dans le coeur du faisceau sur une très grande distance. Il donne alors naissance à une mince colonne de plasma dont la longueur peut atteindre des centaines de mètres. De nombreuses applications reposant sur l'utilisation de filaments laser générés à grande distance ont été proposées. Dans ce cas la propagation du faisceau peut être notablement affectée par les turbulences atmosphériques. Pour quantifier les effets de la turbulence sur la filamentation nous avons fait une étude expérimentale et numérique de l'influence de la turbulence de l'air sur la stabilité de pointé du faisceau filamenté et sur la distance de filamentation. Par ailleurs nous avons étudié les propriétés de la colonne de plasma dans le filament et plus particulièrement sa capacité à initier de puissants arcs électriques avec un minimum de pertes, dans l'optique d'applications telles que le captage de forts courants pour l'alimentation de trains rapides. Enfin, une étude complète de l'utilisation de filaments laser comme source de rayonnement Térahertz a été faite. Plusieurs mécanismes de génération de THz ont été mis à jour. Leurs propriétés spectrales et leur énergie permettent d'envisager le développement d'applications de spectroscopie à distance ou pour certains mécanismes, d'atteindre des régimes de physique non-linéaire dans le domaine THz.

Ce travail porte sur l'excitation et la détection d'ultrasons dans des multicouches métalliques en acoustique picoseconde afin d'engendrer des ondes de hautes fréquences (jusqu'au THz) pour pouvoir éventuellement les émettre dans un matériau. Nous avons développé des modèles permettant de calculer les réponses optiques et acoustiques de structures excitées par des impulsions lasers ultrabrèves. D'une part, nous avons étudié les effets d'interface à l'origine de l'anomalie élastique dans certains systèmes multicouches et nous avons montré que l'adoucissement observé dans les systèmes Mo/Ni est dû à des alliages interfaciaux épais de 1,3 nm. D'autre part, l'étude des multicouches périodiques a permis d'expliciter les processus d'excitation et de détection de modes résonnants dans des bandes de fréquences interdites. Enfin, nous avons réalisé une expérience de transmission à travers un substrat épais et montré qu'on pouvait transmettre des fréquences supérieures à 200 GHz.

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

Surface plasmon polaritons are electromagnetic surface waves coupled to electron plasma oscillation of metals at a metal-dielectric interface. At optical frequencies, these modes are of great interest because of their high confinement to a metal-dielectric interface. Due to the field enhancement at the interface, they have been used in different applications such as sensors, second harmonic generation and enhanced Raman scattering. Surface plasmon resonance based sensors are being used for detection of molecular adsorption such as DNA and proteins. These sensors are known to be highly sensitive and have successfully become commercialized. Terahertz (THz) frequency band of electromagnetic spectrum has attracted researchers in the last few years mostly because of sensing and imaging applications. Many important chemical and biological molecules have their vibrational and rotational resonance frequencies in the THz range that makes the THz sensing one of the most important applications of THz technology. Considering above mentioned facts, extending the concept of surface plasmon sensors to THz frequencies can result in sensitive sensors. In this work the possibility of this extension has been investigated. After reviewing optical surface plasmon polariton waves and a basic sensor configuration, surface plasmon polariton waves propagating on at metallic and doped semiconductor surfaces have been examined for this purpose. It has been shown that these waves on metallic surfaces are loosely confined to the metal-dielectric interface and doped semiconductors are also too lossy and cannot meet the requirements for sensing applications. Afterwards, it is shown that periodically patterned metallic surfaces can guide surface waves that resemble surface plasmon polariton waves. A periodically patterned metallic surface is used to guide THz surface plasmon polariton-like surface waves and a highly sensitive sensor is proposed based on that. The quasi-optical continuous wave (CW) THz radiation is coupled to this structure using the Otto's attenuated total reflection (ATR) configuration and the sensitivity of the device is discussed. A general scattering parameter based model for prism coupling has been proposed and verified. It is shown that a critical coupling condition can happen by changing the gap size between the prim and periodic surface. Details of fabrication of the periodic structure and experimental setup have also been presented.

Several novel optical devices, which were designed to manipulate terahertz waves for broadband near-field imaging, wave-guiding (invisible space), and sensing (resonator), are presented in this thesis. We developed the original working concepts of each device, and demonstrated the prototype experimentally in our lab. The working concepts of physics were investigated in experiment, in simulation and in theoretical analysis. We exploited a tapered parallel-plate waveguide (PPWG) as a novel probe for broadband near-field imaging. This imaging probe consists of two metal plates with the plate spacing gradually tapered from one end to the other. We proved that the space tapering enables this probe to propagate the broadband THz waves efficiently (with low-loss, no cut-off and nearly no dispersion) from the input end of large spacing into the narrow end of sub-wavelength spacing. Working in a reflection mode, this imaging probe is proved to be able to differentiate the dielectric features as well as topographic information on the sample. Combined with the methodology of filtered back projection, we reconstructed a two-dimensional image of a gold pattern on a GaAs chip by using this tapered PPWG probe. The smallest feature of ~100 µm is resolved by using the waves with average wavelength of 1.5 mm. We studied the phenomenon of surface plasmon-polariton in THz range on the platform of a parallel-plate waveguide (PPWG). In this thesis, we show the characterization of the waveguide mode of a finite-width parallel plate waveguide by using an improved scattering-probe technique. An abrupt waveguide mode transition was observed at a very narrow frequency range. We demonstrated that this transition frequency is determined by the material properties of the waveguide, the frequencies of the electromagnetic waves as well as the geometry of the waveguide. This result provides a good guidance for the waveguide design for THz transmission. We also exploited the capability of using the spoof surface plasmon to enhance the reflectivity of an interface between free space and a PPWG. We demonstrated that the reflection coefficient of this interface can be enhanced up to ~100 % at a designed frequency, by cutting a designed pattern of periodic rectangular groove on the output facet of the PPWG. A lateral shift and a phase shift of the reflected beam is observed in the experiment, which is a strong reminiscent of Goos-Hanchen shift. We carried out the experimental, simulation and theoretical characterizations of the lateral and phase shift. As an application, we designed and demonstrated a prototype of a band-pass THz resonator. We introduced the concept of a waveguide-based two-dimensional inhomogeneous artificial dielectric into THz range. This artificial dielectric is the space between the two metal plates of a PPWG working in TE1 mode. We designed a THz mirage device (or an invisible space device) by using ray-tracing and full-wave simulations, which contributed to the first experimental demonstration of such a device. A metal coin of size several times larger than the working wavelength can be hidden in the device without casting any shadow. This work is in collaboration with Dr. Rajind Mendis and the author of this thesis contributed to the design and characterization of the device in simulations.