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The electric properties of materials in the THz spectrum are of significant interest for scientific research in the past two decades thanks to the development of THz-TDS systems. However, the measurement capability of the system is still bound by the low efficiency and instability of the system. In this work, a thorough assessment of the THz-TDS system is carried out in order to enhance the measurement capability of the system and provide guidelines for accurate and repeatable measurements. In Chapter 2, the operation fundamentals of THz-TDS systems including the generation and detection are reviewed. The limitations of THz-TDS systems are evaluated in the aspects of dynamic range, signal-to-noise ratio, and spectral resolution. The influence of systematic parameters are addressed and examined. In Chapter 3, a systematic characterisation of the performance of PCAs is performed. The performance of THz PCA is evaluated with respect to the intrinsic and extrinsic excitation parameters, as well as the power collection efficiency within the THz-TDS system. Performance evaluation is carried out in combination of experimental measurements and numerical modellings. Chapter 4 extensively investigates the sensitivity of the THz-TDS system regarding on misalignment of the components. An EM simulation model is built for the evaluation. Point E-field respond in frequency domain and time-domain are examined corresponding directly to the detection signal, and compared with lab measurements. The model is then extended to study the field distribution inside the system. Mode analysis of the field is conducted to discover the pattern of energy coupling related to misalignment. Chapter 5 aims to further enhance the efficiency and radiation characteristics of THz PCAs by adapting the concept of antenna array. The influence of array configuration is assessed by array factor analysis. Coupling conditions of array parameters are established. Performance dependences of THz PCAs on the array geometrics are extensively studied in theory, and tested against experiment. Chapter 6 assess the implementation of plasmonoic structures for the improvement of efficiency and power at the THz generation process. Fundamentals are discussed and structures are designed accordingly. Optimisation principles in consideration of the carrier properties are proposed and practised. Photoconductive antennas with plasmonic structures are fabricated and tested in a THz-TDS system, and the results are compared with simulation.

L’objectif de ma thèse est d’étendre les modèles électromagnétiques existants à l’Institut Fresnel pour les fréquences optiques vers le domaine des fréquences térahertz (THz), pour mieux comprendre les phénomènes physiques mis en jeu lors d’interaction onde-matière. Cette compréhension permettrait d’améliorer l’analyse des images THz acquises et de mieux définir les configurations des systèmes optiques utilisés. Ce travail est réalisé en comparant les résultats issus de la modélisation avec ceux provenant d’expériences menées par imagerie THz au sein de l’entreprise Terahertz Waves Technologies. Dans le futur, la modélisation pourrait devenir un outil prédictif pour la caractérisation de matériaux dans le domaine THz.Les ondes THz se situent entre l’infrarouge lointain et les micro-ondes dans le spectre électromagnétique allant de 0.01 mm à 3 mm (ou 100 GHz à 30 THz). Ces ondes bénéficient des avantages des ondes optiques et des micro-ondes dépendant des longueurs d’ondes utilisées. L’imagerie THz présente un fort potentiel pour la caractérisation de la matière, car ces ondes peuvent pénétrer beaucoup de matériaux qui sont opaques dans le visible et dans l’infrarouge. La détection de défauts, les délaminations, la présence d’humidité, etc…, sont un exemple des problématiques qui peuvent être investiguées grâce au rayonnement THz.Dans un premier temps, j’ai pu simuler la réponse optique d’échantillons polymères plans homogènes et isotropes avec de bons accords entre le calcul et la mesure. Ces résultats ont permis de réaliser de premières modélisations d’images en adéquation avec l’imagerie THz. L’étude est ensuite élargie aux matériaux anisotropes qui existent dans l’environnement industriel actuel ainsi qu’aux objets de forme cylindrique. Les modèles développés considèrent l’indice de réfraction complexe d’un échantillon et son épaisseur, c’est pourquoi un chapitre est dévolu à la méthode d’estimation de ces paramètres à partir de mesures issues de spectroscopie THz dans le domaine temporel mise en œuvre. / The aim of my thesis is to extend the electromagnetic models already existing at the Institut Fresnel for the optical frequencies towards the terahertz (THz) range, to have a better knowledge of the physical phenomena involved in THz light-matter interactions. This understanding would allow to improve the analysis of the THz images acquired and to have a better definition of the optical systems configurations that we use. To achieve this work, we compare the results coming from the model with those from the experiments led by THz imaging by Terahertz Waves Technologies. In the future, the modelling could become a predictive tool for the characterization of materials in the THz domain.THz waves are located between far infrared and microwaves in the electromagnetic spectrum going from 0.01 mm to 3 mm (or 100 GHz to 30 THz). These waves benefit from advantages of the optical waves and from microwaves depending on used frequencies. THz imaging presents a high potential one for the characterization on the material, because these waves can penetrate a lot of materials which are opaque in the visible and the infrared lights. Detection of defects, delaminations, the presence of humidity, etc…, are examples of the problems which can be investigated with THz light.At first, I was able to model the optical response of planar, homogenous, isotropic and polymeric samples with good agreements between the calculation and the measurement. These results allowed to realize first modellings of images which are consistent with THz imaging. Therefore, the study is enlarged to anisotropic materials which exist in the current industrial environment as well as the objects of full cylindrical shape. The developed models consider the complex refractive index of a sample and its thickness, that is why a chapter is devoted to the method of estimation of these parameters from measurements coming from THz Time-Domain Spectrocopy signals which was implemented.

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

In this thesis, we describe the development of a time-domain terahertz (THz) spectrometer driven by two different laser sources: an Er-doped femtosecond fiber laser and a mode-locked Ti:Sapphire laser. The resulting THz electromagnetic radiation was generated and detected using photoconductive antenna detection methods in both systems. In these experiments we characterized the THz power output for both the fiber laser driven system and the Ti:Sapphire laser driven system. Emphasis is given throughout this thesis on understanding the working principles behind time-domain terahertz spectroscopy, applications of THz radiation and terahertz generation as well as terahertz detection methods. We calculated the THz power output using two different methods. By using the &ldquo / Hertzian Dipole&rdquo / method we estimated the generated THz power after the generation photoconductive antenna. Using this method, we showed that the v generated power is on the order of milliwatts, which is far larger than the expected power typical for these systems. The second, &ldquo / Open-Circuit Voltage&rdquo / method, allowed us to calculate the received power on the detection photoconductive antenna. Using this method we were able to show that the THz power generated and detected in these systems is on the order of microwatts. For the mode-locked fiber laser driven spectrometer we obtained on average a ~ 4 ps (0.25 THz) pulse length which corresponded to an average power in the range of 71.8 nW - 70.54 &amp / #956 / W on a dipole antenna with a 6 &amp / #956 / m dipole gap and 44 &amp / #956 / m dipole length. Using the mode-locked Ti:Sapphire laser driven spectrometer we observed a ~ 2 ps (0.5 THz) pulse length and average power in the range of 0.54 nW &ndash / 5.12 &amp / #956 / W on a different dipole antenna with a 5 &amp / #956 / m gap and 40 &amp / #956 / m dipole length. Since these values agree with expected values for these systems we believe the &ldquo / Open-Circuit Voltage&rdquo / method is appropriate when trying to calculate the THz power.

This thesis is on the topic of investigation of the characteristics of DC Glow Discharge plasmas. Emphasis is given on characterizing the plasma electron density. The methods of generating and detecting THz pulses are described. THz transmission spectroscopy and plasma emission spectroscopy is examined. Transmission spectrum is taken for Air, gaseous Nitrogen and Argon plasmas. Moreover, emission spectrum of Air, N2 and Ar plasma analysis were done respectively. It was found that the transmission of terahertz pulses through nitrogen plasma was considerably affected compared to that of the argon plasma. Initially Drude model theory of electron conduction is employed to analyze the plasma density.

The purpose of this work was to construct and analyze a THz time domain spectroscopy (THz-TDS) system by using a nanojoule energy per pulse ultrafast laser (non-amplified ultrafast laser or oscillator) source and a non-linear optical generation method for THz generation. First a THz-TDS system, which uses photoconductive antenna (PCA) method for THz generation, was built to understand the working principles of these types of systems. This THz-TDS system which used PCA for generation and a 2mm thick &lt / 110&gt / ZnTe crystal for detection had a bandwidth up to 1 THz with a 1000:1 signal to noise ratio (S/N). Using this system, various materials were investigated to study the usefulness of the obtained bandwidth. Absorption coefficient and refractive indices of the sample materials were calculated. Results showed that the bandwidth of the system was not sufficient to obtain fingerprint properties of these materials. In order to improve the system, optical rectification method was used for THz generation. A different THz-TDS system was built with a 1mm thick &lt / 110&gt / ZnTe crystal used for the method of non-linear generation of THz radiation. Theoretical calculations of radiated intensity and electric field were done to analyze the expected bandwidth of the system. Results showed that the generation and the detection crystal thicknesses affect the obtained bandwidth of the system in that the bandwidth limiting factor is the crystal thickness and not the ultrafast laser pulse duration. Especially for detection, measurements obtained with both a 1mm thick and 2mm thick &lt / 110&gt / ZnTe crystal showed that there was not much difference in bandwidth as was predicted by theory. Also in order to increase the signal to noise ratio, the optics used in the system were optimized. It was found that by using same focal lengths for focusing and collimating optics around the generation crystal and by using a short focal length parabolic mirror, S/N could be improved. After these improvements this THz-TDS system which uses optical rectification for THz generation and electro-optic method for THz detection had a larger bandwidth up to 3 THz but with a lower 100:1 signal to noise ratio.

As the amount of study into the terahertz (THz) region of the electromagnetic spectrum steadily increases, the parallel plate waveguide has emerged as a simple and effective fixture to perform many experiments. The ability to concentrate THz radiation into a small area or volume enables us to analyze smaller samples and perform more repeatable measurements, which is essential for future research. While the fundamental physics of PPW transmission are understood mathematically, the practical knowledge of building such a fixture for the THz domain and taking measurements on it with a real system needs to be built up through experience. In this thesis, multiple PPW configurations are built and tested. These include waveguides of different lengths and opening heights, using lenses and antennas to focus and collect radiation from the input and output, and different amounts of polish on the waveguide surface. A basic resonator structure is also built and measured as a proof of concept for future research. The two most useful propagation modes through the waveguide, the lowest order transverse magnetic (TEM) and transverse electric (TE) modes, were characterized on all of the setups. Additionally, a flexible fixture was designed and measured which will allow future work in the THz field to be much more reliable and repeatable.

Ce travail de thèse a été réalisé dans le cadre d'un contrat avec l'ANR (ANR-09-VERS-013 « THID ») et porte sur le développement d'une nouvelle génération de tags Chipless à bas coûts fonctionnant dans le domaine THz, pour des applications d'identification et/ou authentification unitaire des articles commerciaux, des papiers d'identités, des personnes pour le contrôle d'accès... Les structures proposées, constituées d'un empilement périodique de couches diélectriques d'indices de réfraction différents, utilisent les propriétés particulières des cristaux photoniques 1D de présenter une réponse électromagnétique entrecoupée de bandes interdites photoniques (BIP). Toute perturbation de la périodicité de la structure engendre des pics dans les bandes interdites qui sont utilisés pour coder une information binaire. Cette structuration particulière des matériaux permet donc de manipuler précisément une signature électromagnétique. Pour des raisons liées à l'industrialisation (facilité de fabrication en masse) et aussi de coût, nous avons retenu des matériaux de base déjà couramment utilisés dans l'industrie papetière : le papier et le polyéthylène. Le choix de ces matériaux, qui doivent allier contraste d'indice élevé et faible absorption, représente une étape cruciale dans ce travail. Ainsi, à partir des résultats expérimentaux obtenus par spectroscopie THz dans le domaine temporel (THz-TDS) sur un grand nombre de matériaux, nous avons pu concevoir deux familles de tags sur la base de ces différents matériaux. Par ailleurs, nous avons développé deux méthodes de codage d'une information binaire, toutes deux basées sur l'absence ou la présence de pics dans une BIP, pics dont la position et le nombre dépendent bien évidemment des défauts de périodicité introduits. Pour des applications liées à l'identification, des capacités de codage de près de 20 bits ont été démontrées. Nous avons aussi montré que la richesse d'information contenue dans la réponse électromagnétique de ces Tags THz peut être utilisée pour les applications liées à l'authentification unitaire, en utilisant comme critère de discrimination le coefficient d'autocorrélation. Nous avons ainsi pu évaluer les performances d'un test d'authentification basé sur ce critère dans différents domaines d'analyse : temporel, fréquentiel et temps-fréquence. Nous avons montré qu'une étude du spectrogramme (combinant temps et fréquence) est ainsi bien plus pertinente qu'une étude dans les seuls domaines temporel ou fréquentiel. / This thesis work deals with the development of a new generation of low-cost Chipless tags operating in the THz frequency domain, it has been supported by the french national agency for research (ANR-09-VERS-013 « THID » ). It covers a wide area of applications such as the identification and/or unitary authentication of commercial items, identity papers, access control…To manufacture these tags, we proposed to use a periodic stack of dielectric material layers with different refractive index and whose thickness is of the order of the wavelength, commonly known as a one dimensional photonic crystal. The electromagnetic signature of such a structure exhibits photonic bandgaps (PBG), i.e. frequency windows in which light propagation is prohibited. We suggested modifying the periodicity of the crystal to create defect levels (peaks) for example in the 1st PBG to encode binary information. This particular structure allows to precisely tuned an electromagnetic signature. To ensure a mass and cost effective industrialization, we retained basic materials which are widely used in the pulp and paper industry: paper and polyethylene. The choice of these materials, which must combine high index contrast and low absorption, represents the first and a crucial step in this work. We characterize a wide range of materials using classical THz time domain spectroscopy (THz-TDS) and we propose two families of tags based on paper and polyethylene. Furthermore, we developed two methods to encode binary information, both based on the absence or presence of peaks in a PBG, peaks whose number and position depend on the introduced defects of periodicity. In a real identification test, a coding capacity of nearly 20-bit has been demonstrated. We also showed that the information contained in the electromagnetic response of these THz tags can be used for other applications related to the unitary authentication and by using the correlation coefficient as criterion for discrimination of the different signatures. Therefore, we evaluate the performance of an authentication test based on this criterion in various analysis domains: time, frequency and time-frequency. We showed that a study of the spectrogram (combining time and frequency representation) is much more relevant than a study in the only time or frequency domain.