The terahertz regime has until recently been some what neglected due to the difficulty of generating and measuring terahertz radiation. Terahertz time domain spectroscopy has allowed for affordable and broadband probing of this frequency regime with phase sensitive measurements (chapter 3). This thesis aims to use this tool to add to the knowledge of the interactions between electromagnetic radiation and matter specifically in regard to plasmonics. This thesis covers several distinct phenomena related to plasmonics at terahertz frequencies. The generation of terahertz radiation from metal nanoparticles is first described in chapter 4. It is shown that the field strength of the plasmon appears to relate to the strength of the generated field. It is also shown that the power dependence of the generated terahertz radiation is not consistent with the optical rectification description of this phenomenon. An alternative explanation is developed which appears more consistent with the observations. A simple model for the power dependence is derived and compared to the experimental results. In chapter 5 the parameters that make good plasmonic materials are discussed. These parameters are used to assess the suitability of semiconductors for terahertz surface plasmon experiments. The Drude permittivity of InSb is measured here, leading to a discussion of terahertz particle plasmons in chapter 6. Finite element method modelling is used to show some merits of these over optical particle plasmons. This also includes a discussion of fabrication methods for arrays of these particles. Finally, chapter 7 is a discussion of so called spoof surface plasmons. This includes some experimental work at microwave frequencies and an in depth analysis of open ended square hole arrays supported by model matching method modelling. Perfect endoscope effects are discussed and compared to superlensing. The thesis ends with a brief conclusions chapter where some of the ideas presented are brought together.

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.

A dual-wavelength mode-locked semiconductor vertical-external-cavity-surface-emitting laser is demonstrated. A semiconductor saturable absorber mirror allows for simultaneous mode locking of pulses centered at two center wavelengths with variable frequency spacing. The difference-frequency control is achieved with an intracavity etalon. Changing the finesse of the etalon enables the adjustment of the pulse duration between 6 and 35 ps. The emitted two-color pulses are modulated by a beat frequency in the terahertz range. Self-starting mode-locking with 0.8-W average output power is demonstrated.

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.

A compact optical terahertz (THz) source was demonstrated based on an efficient high-repetition-rate doubly resonant optical parametric oscillator (OPO) around 2 mu m with two type-II phase-matched KTP crystals in the walk-off compensated configuration. The KTP OPO was intracavity pumped by an acousto-optical (AO) Q-switched Nd:YVO4 laser and emitted two tunable wavelengths near degeneracy. The tuning range extended continuously from 2.068 mu m to 2.191 mu m with a maximum output power of 3.29 W at 24 kHz, corresponding to an optical-optical conversion efficiency (from 808 nm to 2 mu m) of 20.69%. The stable pulsed dual-wavelength operation provided an ideal pump source for generating terahertz wave of micro-watt level by the difference frequency generation (DFG) method. A 7.84-mm-long periodically inverted quasi-phase-matched (QPM) GaAs crystal with 6 periods was used to generate a terahertz wave, the maximum voltage of 180 mV at 1.244 THz was acquired by a 4.2-K Si bolometer, corresponding to average output power of 0.6 mu W and DFG conversion efficiency of 4.32x10(-7). The acceptance bandwidth was found to be larger than 0.35 THz (FWHM). As to the 15-mm-long GaSe crystal used in the type-II collinear DFG, a tunable THz source ranging from 0.503 THz to 3.63 THz with the maximum output voltage of 268 mV at 1.65 THz had been achieved, and the corresponding average output power and DFG conversion efficiency were 0.9 mu W and 5.86x10(-7) respectively. This provides a potential practical palm-top tunable THz sources for portable applications.

L'observation du milieu interstellaire est très importante aux fréquences mm / (sub) mm / Thz pour comprendre comment se forment les étoiles et les planètes. De telles observations dépendent des récepteurs hétérodynes. Ces instruments atteignent une résolution spectrale très élevée en convertissant un signal haute fréquence à une fréquence plus basse. Dans un récepteur hétérodyne, le signal collecté est superposé sur un signal artificiel, bien connu, monochromatique, généré par l'oscillateur local (OL), donc ce signal artificiel est plus-ou-moins la fréquence du signal du ciel. Le mélangeur produit le signal de la fréquence du battement. Cette fréquence est équivalente à la différence entre le OL et la fréquence du signal du ciel. Ainsi, le signal du ciel est traduit à une fréquence plus basse, pour qu'il soit facile à amplifier et détecter. Habituellement, les récepteurs hétérodynes ont seulement un pixel spatial avec de nombreux canaux en fréquences. Notre objectif est de développer des réseaux de centaines de pixels. Pour faire cela, certains composants de l'hétérodyne doivent être repensés radicalement, tels que l'antenne de réception et le diviseur de faisceau OL. En effet, l'antenne réceptrice est généralement constituée d'une antenne à double fentes sur une lentille, ou d'une antenne cornet. Par contre, ces antennes ne sont pas les meilleurs choix pour des réseaux de nombreux pixels car elles doivent être usinées et montées individuellement. Au lieu de cela, il est commode de développer des structures planaires qui peuvent être facilement produites toutes ensembles. En particulier, nous avons conçu et simulé des réseaux d'antennes patch, de réseaux de transmission, et de plaques de zone. Le réseau d'antennes patch consiste d'un réseau de patchs métalliques reliés par une ligne microruban et séparés du plan de masse par un substrat diélectrique. Cette configuration profite du facteur du réseau pour réduire la largeur de faisceau du signal collecté. Cependant, nos simulations nous montrent que la bande RF des réseaux d'antennes patch est étroite. Pour cette raison, nous avons analysé la possibilité d'utiliser une autre solution : le réseau de transmission. C'est un réseau de plusieurs cellules qui déphase une onde afin de transformer son front de phase de forme planaire en forme sphérique. Le but de la matrice de transmission est de focaliser le faisceau collecté vers une antenne et mélangeur à double fentes. La thés démontre qu'un effet de focalisation satisfaisant est atteint sur une ligne. Nous avons fabriqué un tel réseau de transmission et l'avons testé en laboratoire. En raison des petites dimensions de quelques millimètres, ces tests sont difficiles à réaliser. Au sein de l'erreur de mesure, la conception et les simulations sont cohérentes. Une troisième option (d'une lentille planaire) a été étudiée dans la thèse : la plaque de zone. C'est un type particulier de réseau de transmission qui ne présente que deux déphasages de 0 ° et 180 °. Le plaque de zone focalise bien, mais est peu efficace. La dernière partie de la thèse introduit un type de diviseur de faisceau particulier qui permet une division du faisceau du signal OL vers un réseau de quatre mélangeurs très serrés. Diviser le faisceau avec des angles suffisamment petits est très difficile avec les réseaux de Fourier et Dammann classiques. Pour cette raison la méthode que nous avons proposée pour concevoir un tel diviseur est très novatrice. En effet, il permet la formation de motifs de faisceaux de forme arbitraire, qui ne sont pas limités par les ordres de diffraction. Les simulations montrent des efficacités allant jusqu'à 80% qui sont très bonnes en comparaison avec les réseaux classiques. En résumé, dans cette thèse, j'ai essayé plusieurs moyens radicalement différents pour simplifier les récepteurs hétérodynes et ouvrir la voie aux grandes matrices hétérodynes avec des centaines de pixels. / The observation of the interstellar medium is very important at mm/(sub)mm/THz frequencies to understand how stars and planets form. Generally such observations rely on heterodyne receivers. These are instruments that achieve very high spectral resolution by down converting a high frequency signal towards a lower frequency one. In a heterodyne receiver the incoming signal is superimposed onto an artificial, well-known, monochromatic signal generated by the local oscillator (LO), chosen to be close to the frequency of the sky signal. The mixer produces the beat frequency signal. It has a frequency equivalent to the difference between the LO and sky signal frequency. Thus the sky signal is translated to a lower frequency, and it is easier to amplify and detect. Usually heterodyne receivers have only one spatial pixel with many frequency channels. Some prototypes have been realized recently with few pixels. Our objective is to develop arrays of hundreds of pixels. In order to do that, some components which compose the heterodyne receiver must be radically rethought, such as the receiving antenna and the LO beam divider.Indeed the receiving antenna generally consists of a double slot antenna on a lens, or a horn antenna. Such antennas are not the best choice for arrays of many pixels since they have to be machined and mounted individually. Instead it is convenient to develop planar structures which can be easily produced in bulk in a single process. In particular we designed and simulated arrays of patch antennas, transmit-arrays and zone plates. The array of patch antennas consists of an array of metallic patches connected via a microstrip line and separated from the ground plane by a dielectric substrate. This configuration takes advantage of the array factor to reduce the beamwidth of the incoming signal in place of the lens. However our simulations showed the array of patch antennas to be quite narrowband for a general purpose application, and quite difficult to realize. For this reason we also analyzed the possibility to use another solution such as the transmit-array. It is an array of several cells which provide a certain phase shift to an incoming wave in order to transform its phase front from planar to spherical. The purpose of the transmit-array is to focus the incoming beam towards a double slot antenna and a mixer placed below it. The simulations showed that a good focusing effect can be reached on a line. We fabricated such a transmit-array and tested it in the laboratory. Because of the small dimensions of a few millimeters these tests are difficult to carry out. Within the measurement error design and simulations are consistent. A third option of a planar lens was studied in the thesis: the zone plate. This is a particular kind of transmit-array which presents only two phase shift of 0° and 180°. The zone plates focus well, but are unfortunately not very efficient.The final part of the thesis introduces a particular kind of beam divider which allows beam splitting of the LO signal towards an array of four very closely packed mixers. To split the beam with such small relative angles is very difficult with the classical Fourier and Dammann grating, for this reason the method we proposed to design such a beam divider is very innovative. Indeed it allows the forming of arbitrary shaped beam patterns, which are not limited by the diffraction orders. Simulations show efficiencies up to 80% which are very good in comparison with classical gratings.In summary in this thesis I have tried several radically different approaches to simplify heterodyne receivers and made a first step towards for large heterodyne arrays with hundreds of pixels.

Ce travail porte sur la caractérisation de matériaux en couches minces par spectroscopie terahertz dépendant du temps. Dans ce but, nous avons élaboré un banc d’analyse spectroscopique dont l’émission et la détection terahertz reposent sur l’utilisation de laser femtoseconde, de semi-conducteurs, de photocommutateurs ultrarapides ou de cristaux électro-optiques. La réponse diélectrique quantitative de matériaux ferroélectriques (titanate de baryum/ - Ba1-xSrxTiO3) déposés sous forme de couches minces, a permis de mettre en évidence l’importance des modes mous de phonon par une étude en température. Enfin, le comportement électromagnétique de polymères conducteurs à base de polyaniline a été effectué notamment leur efficacité de blindage en bande millimétrique et submillimétrique. / This work concerns the characterization of thin film materials by terahertz time domain spectroscopy. For this purpose, we elaborated a terahertz setup in which the terahertz emission and terahertz detection are based on the use of femtosecond laser, semiconductors, ultrafast photoswitches or electro-optic crystals. The study of dielectric function of ferroelectrics thin film (barium titanate/-Ba1-xSrxTiO3) with temperature, give the importance of soft phonon mode. Finally, the electromagnetic behavior of conducting polymers based on polyaniline was made, in particular their shelding effectiveness in millimeter and sub-millimeter length.

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.

Le travail de thèse porte sur la conception et la réalisation de deux modulateurs optiques assistés par phonon-polaritons de surface fonctionnant en réflectivité, autour de 8.5 THz, à température ambiante. Nous avons dans un premier temps validé expérimentalement la description théorique du couplage du champ propagatif aux phonons polaritons de surface pour un réseau de GaAs grâce à des mesures de réflectivité THz résolues angulairement. Nous montrons l'importance de la géométrie de la structure pour une description quantitative du couplage. Nous avons réalisé un modulateur de réflectivité THz contrôlé optiquement. La structure est un réseau lamellaire de GaAs dopé , de période inférieure à la longueur d'onde supportant un mode composé de plasmon-phonon-polaritons de surface se propageant le long des murs du réseau. L'éclairement de la structure dans le visible modifie la fréquence de résonance THz de ce mode en créant des photo-porteurs dans les murs du réseau et permet ainsi un contrôle actif de la réflectivité.Enfin nous étudions et réalisons un modulateur de réflectivité THz contrôlé électriquement. Nous proposons une structure permettant d'exciter un mode de phonon-polaritons d'interface dans un puits quantique. Ce mode est très confiné dans le puits et présente une forte sur-intensité de champ. Cet effet original est lié à la permittivité du puits proche de zéro à la fréquence du mode d'interface. La perturbation engendrée par des transitions intersous-bandes dans le puits quantique unique permet, en appliquant une tension de l'ordre du volt, de contrôler l'intensité du couplage du champ propagatif au mode du puits, ce qui donne un contrôle actif de la réflectivité de la structure. / In this work we report the design and fabrication of two THz modulators, assisted by surface phonons polaritons. Both devices work around 8.5 THz at room temperature.We first validate experimentally the coupling of the propagating field to surface-phonons polaritons on a GaAs grating by angular resolved THz reflectivity measurements. We show that a good knowledge of the grating geometry is necessary to have a quantitative description of the coupling.We have fabricated an optically controled THz modulator made of a doped GaAs lamellar grating with subwavelength dimensions. This grating supports surface-plasmon-phonons polaritons along the grating walls. The THz resonance frequency is then modified by shining visible light on the grating, which creates photo-electrons. This allows a dynamic optical control of the THz reflectivity.Finally we present an electrically controlled THz modulator. We design a structure that allows the coupling of the propagating field to an interface-phonon polaritons in a single quantum well. This mode is confined inside the quantum well and gives a high field enhancement. This original effect is due to a near-zero dielectric function of the well at the interface-mode frequency. The perturbation introduced by intersubband transitions in the single quantum well allows, by applying a voltage of about one volt, a control of the coupling intensity between the propagating field and the interface mode, leading to a change in the structure reflectivity.

The present research is a targeted endeavor to study the underlying characteristics and novel applications of millimeter (mm) wave through terahertz (THz) spectrum of electromagnetic (EM) energy. Focused thereof are the following specific tasks broadly considered pertinent to the said EM spectral range: (i) To elucidate the material characteristics vis-à-vis the interaction with EM energy at the test frequencies; (ii) to identify biomedical applications based on the material characteristics studied and applied to biomedia; and (iii) to model the wireless communication channels supporting EM waves at the test frequency bands of interest. Commensurate with the scope as above, the objectives of the research are as follows: / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2014. / FAU Electronic Theses and Dissertations Collection