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The emergence of 6G and beyond networks is set to enable a range of novel services such as personalized highly immersive experiences, holographic teleportation, and human-like intelligent robotic applications. Such applications require a set of stringent sensing, communication, control, and intelligence requirements that mandate a leap in the design, analysis, and optimization of today's wireless networks. First, from a wireless communication standpoint, future 6G applications necessitate extreme requirements in terms of bidirectional data rates, near-zero latency, synchronization, and jitter. Concurrently, such services also need a sensing functionality to track, localize, and sense their environment. Owing to its abundant bandwidth, one may naturally resort to terahertz (THz) frequency bands (0.1 − 10 THz) so as to provide significant wireless capacity gains and enable high-resolution environment sensing. Nonetheless, operating a wireless system at the THz band is constrained by a very uncertain channel which brings forth novel challenges. In essence, these channel limitations lead to unreliable intermittent links ergo the short communication range and the high susceptibility to blockage and molecular absorption. Second, given that emerging wireless services are "intelligence-centric", today's communication links must be transformed from a mere bit-pipe into a brain-like reasoning system. Towards this end, one can exploit the concept of semantic communications, a revolutionary paradigm that promises to transform radio nodes into intelligent agents that can extract the underlying meaning (semantics) or significance in a data stream. However, to date, there has been a lack in holistic, fundamental, and scalable frameworks for building next-generation semantic communication networks based on rigorous and well-defined technical foundations. Henceforth, to panoramically develop the fully-fledged theoretical foundations of future 6G applications and guarantee affluent corresponding experiences, this dissertation thoroughly investigates two thrusts. The first thrust focuses on developing the analytical foundations of THz systems with a focus on network design, performance analysis, and system optimization. First, a novel and holistic vision that articulates the unique role of THz in 6G systems is proposed. This vision exposes the solutions and milestones necessary to unleash THz's true potential in next-generation wireless systems. Then, given that extended reality (XR) will be a staple application of 6G systems, a novel risk and tail-based performance analysis is proposed to evaluate the instantaneous performance of THz bands for specific ultimate virtual reality (VR) services. Here, the results showcase that abundant bandwidth and the molecular absorption effect have only a secondary effect on the reliability compared to the availability of line-of-sight. More importantly, the results highlight that average metrics overlook extreme events and tend to provide false positive performance guarantees. To address the identified challenges of THz systems, a risk-oriented learning-based design that exploits reconfigurable intelligent surfaces (RISs) is proposed so as to optimize the instantaneous reliability. Furthermore, the analytical results are extended to investigate the uplink freshness of augmented reality (AR) services. Here, a novel ruin-based performance is conducted that scrutinizes the peak age of information (PAoI) during extreme events. Next, a novel joint sensing, communication, and artificial intelligence (AI) framework is developed to turn every THz communication link failure into a sensing opportunity, with application to digital world experiences with XR. This framework enables the use of the same waveform, spectrum, and hardware for both sensing and communication functionalities. Furthermore, this sensing input is intelligently processed via a novel joint imputation and forecasting system that is designed via non-autoregressive and transformed-based generative AI tools. This joint system enables fine-graining the sensing input to smaller time slots, predicting missing values, and fore- casting sensing and environmental information about future XR user behavior. Then, a novel joint quality of personal experience (QoPE)-centric and sensing-driven optimization is formulated and solved via deep hysteretic multi-agent reinforcement learning tools. Essentially, this dissertation establishes a solid foundation for the future deployment of THz frequencies in next-generation wireless networks through the proposal of a comprehensive set of principles that draw on the theories of tail and risk, joint sensing and communication designs, and novel AI frameworks. By adopting a multi-faceted approach, this work contributes significantly to the understanding and practical implementation of THz technology, paving the way for its integration into a wide range of applications that demand high reliability, resilience, and an immersive user experience. In the second thrust of this dissertation, the very first theoretical foundations of semantic communication and AI-native wireless networks are developed. In particular, a rigorous and holistic vision of an end-to-end semantic communication network that is founded on novel concepts from AI, causal reasoning, transfer learning, and minimum description length theory is proposed. Within this framework, the dissertation demonstrates that moving from data-driven intelligence towards reasoning-driven intelligence requires identifying association (statistical) and causal logic. Additionally, to evaluate the performance of semantic communication networks, novel key performance indicators metrics that include new "reasoning capacity" measures that could go beyond Shannon's bound to capture the imminent convergence of computing and communication resources. Then, a novel contrastive learning framework is proposed so as to disentangle learnable and memoizable patterns in source data and make the data "semantic-ready". Through the development of a rigorous end-to-end semantic communication network founded on novel concepts from communication theory and AI, along with the proposal of novel performance metrics, this dissertation lays a solid foundation for the advancement of reasoning-driven intelligence in the field of wireless communication and paves the way for a wide range of future applications. Ultimately, the various analytical foundations presented in this dissertation will provide key guidelines that guarantee seamless experiences in future 6G applications, enable a successful deployment of THz wireless systems as a versatile band for integrated communication and sensing, and build future AI-native semantic communication networks. / Doctor of Philosophy / To date, the evolution of wireless networks has been driven by a chase for data rates, i.e., higher download or upload speeds. Nonetheless, future 6G applications (the generation succeeding today's fifth generation 5G), such as the metaverse, extended reality (encompassing augmented, mixed, and virtual reality), and fully autonomous robots and vehicles, necessitate a major leap in the design and functionality of a wireless network. Firstly, wireless networks must be able to perform functionalities that go beyond communications, encompassing control, sensing, and localization. Such functionalities enable a wide range of tasks such as remotely controlling a device, or tracking a mobile equipment with high precision. Secondly, wireless networks must be able to deliver experiences (e.g. provide the user a sense of immersion in a virtual world), in contrast to a mere service. To do so, extreme requirements in terms of data rate, latency, reliability, and sensing resolution must be met. Thirdly, intelligence must be native to wireless networks, which means that they must possess cognitive and reasoning abilities that enable them to think, act, and communicate like human beings. In this dissertation, the three aforementioned key enablers of future 6G experiences are examined. Essentially, one of the focuses of this dissertation is the design, analysis, and optimization of wireless networks operating at the so-called terahertz (THz) frequency band. The THz band is a quasi-optical (close to the visible light spectrum) frequency band that can enable wireless networks to potentially provide the extreme speeds needed (in terms of communications) and the high-resolution sensing. However, such frequency bands tend to be very susceptible to obstacles, humidity, and many other weather conditions. Therefore, this dissertation investigates the potential of such bands in meeting the demands of future 6G applications. Furthermore, novel solutions, enablers, and optimization frameworks are investigated to facilitate the successful deployment of this frequency band. To provide wireless networks with their reasoning ability, this dissertation comprehensively investigates the concept of semantic communications. In contrast to today's traditional communication frameworks that convert our data to binary bits (ones and zeros), semantic communication's goal is to enable networks to communicate meaning (semantics). To successfully engineer and deploy such networks, this dissertation proposes a novel suite of communication theoretic tools and key performance indicators. Subsequently, this dissertation proposes and analyzes a set of novel artificial intelligence (AI) tools that enable wireless networks to be equipped with the aforementioned cognitive and reasoning abilities. The outcomes of this dissertation have the potential to transform the way we interact with technology by catalyzing the deployment of holographic societies, revolutionizing the healthcare via remote augmented surgery, and facilitating the deployment of autonomous vehicles for a safer and more efficient transportation system. Additionally, the advancements in wireless networks and artificial intelligence proposed in this dissertation could also have a significant impact on various other industries, such as manufacturing, education, and defense, by enabling more efficient and intelligent systems. Ultimately, the societal impact of this research is far-reaching and could contribute to creating a more connected and advanced world.

In dieser Arbeit wird die Solvatationsdynamik einer solvatochromen molekularen Sonde diskutiert, und zwar als Methode für den Erhalt von lokalen IR-THz-Spektren von komplexen Systemen. Durch Femtosekundenanregung wird die Ladungsverteilung der Sonde verändert, und als Folge davon wird ein elektrisches Feld induziert. Zu diesem Zeitpunkt wirkt die im Lösungsmittel gelöste Sonde als Lichtquelle mit THz-Frequenzen. Da durch die Anregung das Gleichgewicht des Systems gestört wird, reorganisieren sich die Lösungsmittelmoleküle, sodass ein neues Gleichgewicht im angeregten Zustand entsteht. Die Bewegung der Lösungsmittelmoleküle ist (in gemittelter Form) als Stokes-Verschiebung des Fluoreszenz-Bandes beobachtbar. Durch eine geeignete Transformation der zeitaufgelösten Stokes-Verschiebung erhält man ein lokales IR-THz-Spektrum. Das Sondenmolekül wirkt daher auch als ein Detektor. Der Vorteil eines solchen "molekularen Spektrometers" ist sein mikroskopischer Aufenthaltsort, der u.a. sehr wichtig wird, wenn Messungen in Wasser durchgeführt werden: In diesem Fall macht eine intensive Absorption durch das Lösungsmittel das Eindringen von externen THz Strahlen tief in die Probe unmöglich. / Solvation dynamics of a solvatochromic molecular probe is discussed as a method to yield local IR-THz spectra of complex systems. After femtosecond excitation, the charge distribution of the probe is altered and, as a consequence, an electric field is generated. At this stage the solute acts as a light source with THz frequencies. Since by excitation the equilibrium of the system is perturbed, solvent molecules reorganize such that a new equilibrium is created in the excited state. This motion of solvent molecules can be seen (in an averaged form) by recording the Stokes shift of the fluorescence band. By an appropriate transformation of the time-resolved Stokes shift, a local IR-THz spectrum is obtained. The probe molecule therefore also acts as a detector. The advantage of such a “molecular spectrometer” is its locality, which becomes important when measurements are made in water. In this case, intense absorption by the solvent makes impossible the penetration of external THz beams deep into the sample.

Afin de développer de nouvelles applications aux couches minces d’oxydes fonctionnels, il est nécessaire de comprendre les corrélations entre leurs modes de croissance, leur microstructure, leur structure à l’interface avec le substrat, et leurs contraintes et propriétés physiques. Pour cela, une étude par spectroscopie infrarouge et THz des systèmes modèles films/substrats a été exécutée, et confrontée à des calculs théoriques, en particulier sur des couches épitaxiales de SrTiO3/Si(001), VO2/Gd2O3/Si(111) et des couches d’alumine sur alliage d’aluminium. Les caractéristiques vibrationnelles des couches minces sont ici étudiées dans l’infrarouge moyen et lointain sur la ligne AILES du Synchrotron SOLEIL, et simulées à l’aide de la Théorie de la Fonctionnelle de la Densité (DFT), permettant ainsi la première détermination de la structure cristalline de ces couches. Ainsi, une comparaison entre la structure bidimensionnelle et tridimensionnelle des matériaux est effectuée. L’effet des contraintes dans les couches est évalué grâce aux variations des énergies de vibration par rapport au matériau massif. L’influence des conditions expérimentales de l’épitaxie dans la structure locale interatomique de couches minces de SrTiO3/Si(001) est évaluée. D’autre part, la nature de l’interface STO-Si peut être caractérisée par les modes de vibration du réseau cristallin. Enfin, la transition métal-isolant (MIT) des couches minces de VO2 sur des substrats de Gd2O3/Si(111) est étudié par spectroscopie IR ; les variations de propriétés optiques et diélectriques pendant la transition, ainsi que les changements d’intensité des modes de vibration, indiquent que la transition est entraînée par une corrélation électronique et une basse température. La phase monoclinique M1 de VO2 est un isolant de Mott. Ce résultat peut aider à un meilleur contrôle des MIT de couches minces de VO2 pour de futures applications. / In order to understand the relations between growth, microstructure, interface structure, strain, and physical properties in functional oxide thin films for further applications, a study of infrared and THz spectroscopy combined with theoretical calculation has been performed on the films/substrates model systems, in particular epitaxial SrTiO3/Si(001), VO2/Gd2O3/Si(111) films and alumina/alloy films. The vibrational characteristics of the crystal structure of films have been investigated in the mid and far infrared ranges on the AILES beamline at Synchrotron SOLEIL. This experimental vibrational study has been combined with Density Functional Theory (DFT) simulation to allow for the first measure of the crystalline structure of these thin films. The 2-dimensional lattice modification compared with the bulk materials has been discussed. The strain effect in the films can be evaluated on the phonon shifts compared with the crystal spectrum. The influences of epitaxial conditions on the local interatomic structure of SrTiO3/Si(001) thin films have been estimated. The nature of STO-Si interface can be characterized by the phonon modes. The metal–insulator transition (MIT) of VO2 thin films on Gd2O3/Si(111) substrate have been studied by IR spectroscopy. The variations of optical and dielectric properties during the MIT, as well as the phonon intensities, indicate that the MIT is driven by electron correlation and the low temperature M1 monoclinic phase of VO2 is a Mott insulator. This result may help to better understand and control the MITs of VO2 thin films in the device applications.

In dieser Arbeit wird die ultraschnelle Dynamik von Leitungsbandelektronen in Halbleitermaterialien mit Hilfe nichtlinearer Terahertz-Spektroskopie erforscht. Insbesondere wird n-dotiertes Galliumarsenid bei mittleren Dotierdichten zwischen 10^(16) cm^(-3) und 10^(17) cm^(-3) untersucht. Für die Erzeugung intensiever THz Strahlung wurde eine neuartige Quelle entwickelt, die THz Transienten mit nur einer Oszillationsperiode und maximalen Feldamplituden von mehr als 400 kV/cm liefert. Diese THz-Quelle benutzt ultrakurze optische Laserpulse aus einem Ti:Saphir Oszillator. Zusätzlich wurde ein neuartiger zwei-Farben Anrege-Abtast Experimentierplatz aufgebaut, der zweidimensionale, zeitaufgelöste Messungen im mittleren und fernen Infrarotbereich ermöglicht. Feldionisation flacher, neutraler Störstellen im Galliumarsenid-Gitter mittels intensiver, ultrakurzer THz Impulse und die anschliessende kohärente, strahlende Rekombination von Elektronen in die Störstellen-Grundzustände bei Raumtemperatur wird gezeigt. Der superradiante Zerfall der nichtlinearen Polarisation führt zur Abstrahlung eines kohärenten Signals mit Lebensdauern von über einer Pikosekunde. Solche nichtlinearen Signale, die 10-fache Lebensdauern im Vergleich zum linearen Fall aufweisen, wurden in dieser Arbeit zum ersten Mal gemessen. Bei niedrigen Temperaturen und THz Feldstärken unter 5 kV/cm werden Rabi-Oszillationen an Übergängen in flachen Störstellen demonstriert. Zum ersten Mal konnte die polare Elektron-LO-Phonon Wechselwirkung im quantenkinetischen Regime direkt gemessen werden. Die quasi-instantane Beschleunigung von Leitungsbandelektronen im polaren Galliumarsenid-Gitter und die anschließende Messung der Transmission im mittleren Infrarot-Bereich, zeigen eine Modulation der Transmission entlang der Anrege-Abtast Verzögerung mit der Frequenz des LO Phonons. Diese Oszillation ist ein direktes Maß der relativen Phase zwischen der Elektronenbewegung und der umgebenden Phonon Wolke. Quantenkinetische Modellrechnungen reproduzieren vollständig die beobachteten Effekte. / In this thesis, the ultrafast dynamics of conduction band electrons in semiconductors are investigated by nonlinear terahertz (THz) spectroscopy. In particular, n-doped gallium arsenide samples with doping concentrations in the range of 10^16cm^(-3) to 10^17 cm^(-3) are studied. A novel source for the generation of intense THz radiation is developed which yields single-cycle THz transients with field amplitudes of more then 400 kV/cm. The THz source uses ultrashort optical laser pulses provided by a Ti:sapphire oscillator. In addition, a two-color THz-pump mid-infrared-probe setup is implemented, which allows for two-dimensional time-resolved experiments in the far-infrared wavelength range. Field ionization of neutral shallow donors in gallium arsenide with intense, ultrashort THz pulses and subsequent coherent radiative recombination of electrons to impurity ground states is observed at room temperature. The superradiant decay of the nonlinear polarization results in the emission of a coherent signal with picosecond lifetimes. Such nonlinear signals, which exhibit a lifetime ten times longer than in the linear regime are observed for the first time. At low temperatures and THz field strengths below 5 kV/cm, Rabi flopping on shallow donor transitions is demonstrated. For the first time, the polar electron-LO phonon interaction is directly measured in the quantum kinetic transport regime. Quasi-instantaneous acceleration of conduction band electrons in the polar gallium arsenide lattice by the electric field of intense THz pulses and subsequent probing of the mid-infrared transmission reveals a modulation of the transmission along the THz-mid-infrared delay coordinate with the frequency of the LO phonon. These modulations directly display the relative phase between the electron motion and its surrounding virtual phonon cloud. Quantum kinetic model calculations fully account for the observed phenomena.