Thesis (Ph.D.)--University of Delaware, 2007. / Principal faculty advisor: James Kolodzey, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Ralph, Stephen; Committee Member: Citrin, David; Committee Member: Cressler, John; Committee Member: Denison, Douglas; Committee Member: Mukhopadhyay,Saibal. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (M.S.E.C.E.)--University of Delaware, 2007. / Principal faculty advisor: Keith W. Goossen, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

L’ANR Terametadiel a pour but de concevoir, fabriquer et caractériser un dispositif à base de métamatériaux tout diélectriques fonctionnant à 10GHz et un autre, fonctionnant à 0,35THz. Le but est de démontrer l'existence d’un indice de réfraction négatif avec de tels dispositifs, en se servant de technologies céramiques de pointe pour un contrôle dimensionnel rigoureux. Les travaux de thèse ont été structurés autour des matériaux diélectriques et du procédé de fabrication additive céramique utilisé : la stéréolithographie.La stéréolithographie utilise la polymérisation UV initiée par un laser UV pour élaborer couche par couche une pièce céramique en 3D. Les taux de conversion des monomères sont évalués grâce à différentes techniques de spectroscopie qui ont été mises en places au laboratoire, et qui sont comparées (FTIR et Raman). Une deuxième partie est consacrée à la synthèse et aux caractérisations structurales, microstructurales et électriques des matériaux diélectriques d’intérêt pour la conception de métamatériaux : le SrTiO3 et le BaxSr(1-x)TiO3 (BST). Les propriétés électriques obtenues en fonction de différentes méthodes de mises en forme des échantillons sont discutées à moyenne fréquence (1kHz) et au THz pour le SrTiO3. Pour pouvoir utiliser les poudres diélectriques synthétisées avec le procédé de stéréolihtographie, nous avons dû les adapter. Plusieurs procédés d’enrobage ont été envisagés et leurs résultats sont présentés, sans réussir à apporter de modification significative de l’absorption. D'autres procédés et matériaux sont envisagés pour la suite des travaux. / This work aims to design, to fabricate and to characterize all-dielectricmetamaterials relying on Mie resonance of high-permittivity ceramic resonators, to be able to control the propagation of terahertz waves. As artificial components, metameterials canexhibit electromagnetic properties that are apparently unreachable. The aim of our work is focused on two steps of the metamaterial elaboration : the synthesis of ceramic dielectric powders by co-precipitation and the construction of a three-dimensional metamaterial produced by stereolithography process. The ceramic dielectric materials must present ahigh permittivity and few losses. The dielectric ceramics SrTiO3 (ST) and BaxSr(1-x)TiO3 (BST) are identified as eventual materials for the elaboration of metamaterial. The co-precipitation via the oxalic route leads to produce a high purity ceramic powder with homogeneous particle size and composition. Stereolithography is a 3D additive manufacturing process that makes it possible the elaboration of complex architectures of ceramic parts with a high resolution, thanks to a layer by layer procedure and a local photopolymerisation of ceramic paste with a UV laser beam. The ceramic system consists in a suspension of ceramic particles in oligomers/monomers with a photoinitiator.The composition of these suspensions requires suitable rheological properties to obtain a homogenous thin film deposition, photo-reactivity properties to UV light to obtain good mechanical properties of the ceramic part after curing and high dimensional resolution.

Compared to the history of science, spectroscopy at terahertz frequencies is a relatively recent development. Terahertz instruments were initially large and inefficient due to the characteristics of available technology. With progress in materials science and miniature circuit manufacturing techniques, we may fabricate micro-meter scale devices to generate and detect terahertz radiation. However, the complete spectroscope apparatus remains large due to the use of components such as lenses and mirrors designed in the far-field optical regime. A truly miniature terahertz spectroscope would be designed without lenses and mirrors to enable a wide range of inexpensive and pervasive applications in diverse fields such as medicine, materials identification, and security. We present detailed evaluation of some candidate structures for a quasi-optical device and design criteria for a quasi-TEM transmission-line based device. Quasi-TEM transmission lines are inherently broadband and therefore suited for use in a spectroscope. / Graduate

La gamme des ondes terahertz (THz) se situe à l'interface des domaines électronique et optique. Malgré un potentiel d'applications élevé, elle souffre d'un manque de dispositifs performants. Dans ce cadre, cette thèse se concentre sur l'étude fondamentale et la réalisation de nouvelles fonctionnalités associées à différentes sources THz, en utilisant la spectroscopie THz dans le domaine temporel (TDS). Cet outil puissant permet de mesurer le profil temporel d'un champ électrique THz et est utilisé pour explorer l'émission THz de lasers à cascade quantique (LCQ) et de graphène.Dans une première partie, la réponse ultrarapide de LCQs est étudiée. Un contrôle de la phase du champ électrique de LCQs THz via la technique "d'injection seeding" est réalisé puis optimisé. Il nous permet de mesurer le profil temporel de l'émission laser. A l'appui de cette expérience et de simulations, une description quantitative de la dynamique du gain est faite. Ces informations sont critiques pour la production d'impulsions courtes. Une modulation rapide du gain de LCQ est ensuite réalisée et conduit à la génération d'impulsions courtes (durée ~ 15 ps) en régime de blocage de modes. Ces études permettent notamment d'envisager les LCQs comme sources puissantes pour la TDS. Dans une seconde partie, nous montrons que le graphène peut émettre un rayonnement THz sous excitation optique par un effet non linéaire d'ordre 2. Cette émission résulte d'un transfert de quantité de mouvement des photons aux électrons du graphène ("photon drag"). Elle permet ainsi d'explorer des propriétés subtiles du graphène, telles que de très faibles différences de comportement entre les électrons et trous photogénérés. / The terahertz (THz) range is a region of the electromagnetic spectrum which lies at the limit between the electronic and optical domain. Currently, THz applications suffer from the lack of sources and detectors. In this context, this thesis focuses on the fundamental study and the development of new functionalities of different THz sources, usingTHz time-domain spectroscopy (TDS) as a base. This powerful tool enables to acquire the temporal profile of a THz electric field and is used to explore the THz emission properties of quantum cascade lasers (QCLs) and graphene.In the first part, the ultrafast response of QCLs is investigated. A phase control of the electric field of THz QCLs via injection seeding is realised and optimised. This enables the measurement of the amplitude and temporal profile of the laser emission. Throughthese experiments and simulations, a quantitative description of the gain dynamics can be accessed. This information is critical for modelocking. Finally, a fast modulation of the gain of QCLs is realized and leads to short pulses generation (15 ps) in a modelocked regime. These studies open the way for using QCLs as powerful sources in TDS.In the second part, THz radiation generation from graphene under optical excitation is demonstrated by a second order non-linear process. The THz emission results from themomentum transfer from the photons to the electrons of graphene (photon drag). As well as broadband THz generation, novel bandstructure properties of graphene can be explored such as the different dynamics between the photogenerated electrons and holes.

A guided-wave THz System-on-Chip (TSoC) is emerging as an attractive alternative to the routine free-space THz systems to reduce physical bulk, propagation loss, pulse dispersion and cost of free-space THz systems. Recently, our research group succeeded in demonstrating a novel waveguided TSoC based on the coplanar stripline (CPS) transmission lines on a 1 µm-thin Silicon Nitride membrane. The novelty of this membrane-based platform was bonding the transmitter and receiver directly on the transmission line to eliminate the radiation loss by the routine THz optics. Besides, the delicate thin-membrane dramatically reduces the dielectric loss of the platform which results in low-loss and low-dispersion THz-bandwidth pulses. This Ph.D. dissertation presents the first end-to-end TSoC components that were designed and fabricated using the CPS transmission lines on 1 µm-thin Si3N4 membranes. These components are integrated into a TSoC by bending or connecting different impedance CPS transmission-line sections. We demonstrate four passive TSoC components: THz low-pass filter (TLPF), THz power divider (TPD), THz apodized Bragg grating (TABG) and THz branch-line coupler (TBLC). One of the most significant gains from this work is the assurance that more complex TSoCs can be designed and fabricated using this membrane-platform based on the strong agreement between simulation and experimental results. / Graduate / 2021-12-01

Doctor en Ingeniería Eléctrica / Los fotomezcladores Verticalmente Iluminados (VI) de Onda Viajera (TW) de Portadores Unipolares (UTC) son fuentes continuas de radiación de THz. Este dispositivo usa la conversión heterodino para generar señales de onda milimétrica. Este dispositivo genera además una corriente distribuida para incrementar su capacidad de manejar mayores cantidades de corriente y además eliminar la limitación de constante RC. Este trabajo se divide en simulaciones electromagnéticas de alta frecuencia y simulaciones de dispositivos semiconductores. Los estudios de dispositivos semiconductores se enfocan en el modelado numérico del fenómeno de transporte de portadres proveyendo una descripción cualitativa y cuantitativa del transporte de portadores en fotodiodos UTC. Como resultado del análisis de semiconductor, resultados de brecha de energía, espacio de carga, densidad electrónica, velocidad del electrón, todos ellos bajos diferentes valores de potencia de iluminación son presentados en esta sección. Una curva de responsividad versus potencia óptica se muestra también. Esta tesis desarrolla además simulaciones electromagnéticas de alta frecuencia para estudiar la propagación de la onda electromagnética a lo largo del dispositivo VI-TW-UTC. Los fotodiodos VI-TW-UTC ultra-rápidos requieren una capa base altamente dopada que hace de conexión conductora entre el fondo de la estructura mesa y los contactos metálicos de la capa base. Tal estructura se denomina mesa vertical p-i-n o de Uni-Portador. La capa base dopada tiene una fuerte influencia en las perdidas de THz. Por lo tanto, simulaciones electromagnéticas de alta frecuencia fueron ejecutadas en HFSS y CST Microwave Studio para estudiar las pérdidas de THz. El dispositivo VI-TW-UTC fue modelado como una línea de transmisión cuasi-coplanar (Q-CPW). Posteriormente, las pérdidas de THz fueron calculadas indirectamente a través de los parámetros de dispersión S21. Las simulaciones muestran un valle de baja pérdida cerca de la conductividad 5×〖10〗^4 Sm-1, en medio de un rango de conductividad de excesiva absorción de THz haciendo este valor la mejor elección para el rango de frecuencia de 0 a 2000 GHz. Adicionalmente, estructuras de Mushroom-CPW y Wall-CPW se desarrollaron y simularon en la presente tesis para comparar sus pérdidas de THz. Un modelo analítico describiendo la potencia entregada a la entrada de antena del fotomezclador se desarrolló. El modelo analítico tiene como variables de entrada la curva de responsividad versus potencia óptica y la absorción de THz. Como resultado, la conductividad de la capa base muy alta es necesaria para alcanzar una potencia de THz razonablemente alta. / Esta tesis de investigación fue financiada por la Comisión Nacional de Investigación Científica y Tecnológica CONICYT

Terahertz (THz) band represents the unused frequency band between the microwave and optical bands and lies in the range of frequencies between 0.1 to 10 THz. As a result, the THz signal generation can be done using electronic or photonic circuits. Moreover, the channel gain has hybrid features from both microwave and optical bands allowing to reap the benefits of each band. Adopting such a technology can mitigate the spectrum scarcity and introduce a substantial solution to other systems such as visible light communications. Despite of the generous bandwidth, the THz communications suffer from high attenuation that increases with adopted frequency similar to the microwave frequency band. Furthermore, THz communications are subject to a different type of attenuation called Molecular Absorption, that depends on the chemical nature of the ambiance air. Thus, THz transmitters need to use extra power and high antenna gains to overcome signal loss and compensate the short distance range limitation. In this thesis, we investigate the pathloss model to compute the overall attenuation faced by the THz wave for different frequencies and weather conditions. Then, we use the THz technology to support the operation of uplink networks using directional narrow beams. We optimize the uplink communication network resource represented in the frequency bands and the assigned power in order to minimize the total power consumption while achieving a specific quality of service. Furthermore, we investigate the impact of weather conditions and the system’s requirements in order to guarantee a better performance.

This paper presents and evaluates a new terahertz metamaterial absorber (MMA) for sensing applications. Because of its unique properties, metamaterial-based sensors are widely employed in a variety of applications. The reported structure comprises of two identical metallic patches, a dielectric spacer and a ground metal plane. The finite element approach has been utilized to simulate and analyse the design. It is found that the MMA offered a prominent resonant peak with near 100% absorbance at frequency 4.5 THz due to the resultant effect of coupling between the two identical patches. In addition, surface current distribution, absorption mechanism and structural parametric analysis has also been investigated. The peak is designated as 'A', with a line width of 0.02 THz and a quality factor (Q-factor) of 225, which is sensitive to the refractive index of the environment (RI). As a result of its highly sensitive sensing capabilities, the proposed design can be employed as a sensor for refractive index, having 1.6 THz per refractive index unit (RIU) sensitivity and figure of merit (FoM) of 80 in terms of change in RI of the environment. The majority of biomedical samples have RI of 1.3 to 1.36, which is worth highlighting. Thus, biomedical applications may be possible with the suggested sensor.