As visual display technologies move closer to producing true three dimensional displays, pixel technologies need to be ever smaller and more functional to keep pushing the boundaries. Plasmonic metasurfaces have been shown to control the phase, amplitude and/or polarisation of incoming electromagnetic radiation. Nano-fabrication advancements have resulted in the fabrication of the building blocks of such metasurfaces at nano-scale dimensions, allowing the surfaces to interact with visible light, opening up applications in visual displays. As pixel sizes shrink, smaller colour filters will be required. The excitation of plasmonic resonances in metallic nano-structure arrays have resulted in colour filters an order of magnitude smaller than what is currently commercially available. As colour filters, plasmonic metasurfaces offer numerous advantages over pigment-based colour filters used in modern commercial liquid crystal (LC) displays, including environmental, size and longevity factors. Furthermore, exploiting the wavelength and polarisation dependant scattering of nano-structures, optical components, including lenses, waveplates and holograms containing sub-wavelength pixels have been demonstrated in the visible wavelength spectrum. The metasurfaces are able to mould optical wavefronts into arbitrary shapes with sub-wavelength resolution by introducing spatial variations in the optical response of the light scatterers. The applications demonstrated so far are, on the whole, static devices, that is to say their optical properties may not be altered post fabrication. To realise the full potential of plasmonic metasurfaces to visual applications the devices must be made active. By activating structural colour surfaces, not only may pixel densities potentially be increased simply by removing the need for separate red, green and blue filters, but a new class of high definition ultra-thin display devices may be accessible, whilst the dynamic manipulation of the wavelength and polarisation properties of nano-scattering elements would open up the possibilities to create sub-wavelength holographic pixels. This thesis investigates ways to activate static metasurfaces for colour, flat optic, and holographic applications. First, methods of dynamic control of the structural colour of plasmonic nano-hole arrays are investigated. By combining nano-hole arrays with liquid crystals, transmissive electrically tunable LC-nanohole pixels operating across the visible spectrum with un-polarised input light are experimentally demonstrated. An output analyser in combination with a nematic LC layer enables pixel colour to be electronically controlled through an applied voltage across the device, where LC re-orientation leads to tunable mixing of the relative contributions from the plasmonic colour input. Furthermore, exploiting the strong surface anchoring effects between an aluminium surface and LC molecules a twisted nematic LC cell, using a metallic grating as a combined colour filter, electrode and alignment layer, was shown to act a variable amplitude colour filter. The colour of these pixels was improved greatly utilising a grating-insulator-grating structure unique to this work. Second, a new process for fabricating aluminium nano-rod structures embedded in an elastomeric medium, with high spatial accuracy, is presented. The process is used to create nano-rod plasmonic resonator arrays whose optical properties may be altered by mechanical deformation. The pattern transfer process is further utilised to create dynamic optical elements, including nano-rod arrays for colour filters, tunable focal length Fresnel zone plates and photon sieves, and stretchable holograms with dynamic replay fields.

Plasmonic nanoparticles feature remarkable optical and electronic properties in consequence of the excitation of conduction band electrons by visible light, which leads to collective oscillations. This so called localized surface plasmon resonance (LSPR) is utilized in the fields of photovoltaics, sensing, catalysis and optoelectronics. Especially, the emergence of optical metasurfaces—subwavelength structured surfaces with properties typically not occurring for homogeneous materials—has attracted significant attention for the applications mentioned above. However, their fabrication is usually complex and the materials often lack in situ tunability. Here, a colloidal approach is demonstrated for the preparation of optical metasurfaces with tunable properties. They are based on plasmonic gold nanoparticles, which were coated with three different shell materials to provide three different functionalities when coupled to plasmonic mirrors: i) Dye-labeled silica coatings exhibit strong enhancement of their fluorescent properties, as shown in this extensive single particle study. ii) Hydrogel shells are applied to receive switchable electric and magnetic properties in response to swelling of the gel. iii) Electrochromic polymer coatings facilitate the preparation of anti-reflective metasurfaces that feature tunable efficiency by changing the pH or applying a voltage. In addition, mechano-tunable plasmonic lattices are demonstrated. The material is based on self-assembled gold nanoparticles, which are embedded in a transparent elastomer matrix and feature pronounced surface lattice resonances (SLR). These tunable resonances could be applied for lasing, strain sensing, or controlling catalytic reactions. / Plasmonische Nanopartikel besitzen bemerkenswerte optische und elektronische Eigenschaften, die sie für Anwendungen in Bereichen der Katalyse, Sensorik, Optoelektronik, sowie der Nanooptik prädestinieren. Ihre Eigenschaften beruhen auf der Anregung von Leitungsbandelektronen zu kollektiven Oszillationen durch sichtbares Licht. Diese sogenannte Oberflächenplasmonenresonanz ist insbesondere für optische Metaoberflächen von Interesse, also Materialien mit strukturierten Oberflächen im Größenbereich unterhalb der sichtbaren Wellenlängen, welche Charakteristika aufweisen, die bei homogenen Materialien typischerweise nicht auftreten. Sie werden allerdings häufig mit aufwendigen Methoden hergestellt und sind in situ nicht justierbar. In dieser Arbeit werden kolloidale Ansätze zur Herstellung plasmonischer Metaoberflächen mit einstellbaren optischen und elektronischen Eigenschaften vorgestellt. Das Konzept basiert auf der Verwendung von plasmonischen Goldkernen, die mit drei unterschiedlichen funktionellen Schalen beschichtet und anschließend mit plasmonischen Spiegeln gekoppelt wurden: i) Farbstoffmarkierte Silicapartikel zeigen starke Fluoreszenz-verstärkung, wie in dieser ausführlichen Einzelpartikelstudie nachgewiesen wird. ii) Hydrogelbeschichtungen werden verwendet um schaltbare elektrische und magnetische Eigenschaften mittels Quellung zu erzeugen. iii) Elektrochrome Polymerhüllen fungieren als Antireflexschicht auf Goldoberflächen, deren Extinktion sich mittels Anlegen einer Spannung oder durch pH-Änderungen einstellen lässt. Neben diesen Ansätzen werden mechanisch einstellbare plasmonische Gitterstrukturen vorgestellt. Die selbstassemblierten und in transparentem Elastomer eingebetteten Goldnanopartikel weisen eine ausgeprägte Oberflächengitterresonanz auf. Diese kann für sensorische Zwecke in den Bereichen der Mikromechanik und der Katalyse, sowie für abstimmbare Laser verwendet werden.

<p dir="ltr">This thesis presents novel methodologies to integrate AI-driven and physics-inspired methodologies into photonic inverse design, setting new benchmarks for high-performance photonic devices in different branches of photonics. By blending advanced computational techniques with the foundational principles of electromagnetism, this research tackles key challenges in optimizing device efficiency, robustness, and functionality. The aim is to propel photonic technology beyond its current capabilities, offering transformative solutions for a range of novel applications.</p><p dir="ltr">The first major contribution focuses on adjoint-based topology optimization for on-chip single-photon coupling. We developed an adjoint topology optimization scheme to design high-efficiency couplers between photonic waveguides and single-photon sources (SPSs) in hexagonal boron nitride (hBN). This algorithm addresses fabrication constraints and SPS location uncertainties, achieving a remarkable average coupling efficiency of 78%. A library of designs is generated for different positions of the hBN flake containing an SPS relative to a silicon nitride (SiN) waveguide. These designs are then analyzed using dimensionality reduction techniques to investigate the relationship between device geometry and performance, infusing the design process with deep physical intuition and insight.</p><p dir="ltr">The second key advancement is presented through a neural network-based inverse design framework specifically developed for optimizing single-material, variable-index multilayer films. This neural network-driven technique, supported by a differentiable analytical solver, enables the realistic design and fabrication of these multilayer films, achieving high performance under ideal conditions. The approach also addresses the challenge of bridging the gap between these ideal designs and practical devices, which are subject to growth-related imperfections. By incorporating simulated systematic and random errors—reflecting actual deposition challenges—into the optimization process, we demonstrate that the neural network, initially trained to produce the ideal device, can be reconfigured to create designs that compensate for systematic deposition errors. This method remains effective even when random fabrication inconsistencies are present. The results provide a practical and experimentally viable strategy for developing single-material multilayer film stacks, ensuring reliable performance across a wide range of real-world applications.</p><p dir="ltr">The final cornerstone of this research investigates the two-stage inverse design of superchiral dielectric metasurfaces. We propose a two-stage inverse design scheme for dielectric lossless metasurfaces with central superchiral hot spots. By leveraging the excitation of high-quality factor modes with low mode volumes, we achieve up to 19,000-fold enhancements of optical chirality. This method extends the local density of field enhancements for non-chiral fields into the chiral regime and significantly surpasses previous enhancements in superchiral field generation. Our results open new avenues in chiral spectroscopy and chiral quantum photonics, exemplifying the powerful synergy of AI techniques and physics-based design principles in creating highly innovative and functional photonic structures.</p><p dir="ltr">Collectively, the methodologies developed in this thesis signify a major advancement in the field of photonic inverse design. By merging AI-driven techniques with rigorous physics-based optimization frameworks, this research paves the way for the next generation of photonic devices.</p>

Ces dernières années, les métamatériaux acoustiques sont largement étudiés pour leur intérêt dans la réalisation de divers types de contrôle des ondes à une échelle sub-longueur d’onde. En particulier, les métasurfaces acoustiques ont montré leur capacité à manipuler des ondes en limites de milieux de propagation via les processus de réflexion, de transmission et de réfraction. Contrairement au régime linéaire qui concerne l’immense majorité des travaux sur les métamatériaux acoustiques, les études sur les propriétés non linéaires des métamatériaux, de surcroit des métasurfaces, restent peu nombreuses, malgré la possibilité de générer des phénomènes acoustiques riches et variés. Les principaux freins au développement des métamatériaux non linéaires sont l'efficacité généralement faible de la réponse non linéaire et le manque de contrôle sur cette non-linéarité. Les travaux de recherche présentés ici ont donc pour objectif de concevoir des architectures de métasurfaces élastiques, permettant un contrôle des ondes acoustiques dans le régime non linéaire. En particulier l’effet de conversion d’une onde fondamentale vers son deuxième harmonique est étudié dans le processus de réflexion et de transmission unidirectionnelle. Cela nécessite le design de la non-linéarité élastique, qui est réalisé à base de modélisations discrètes de systèmes masses-ressorts et d'architectures composées d'éléments tournants. Les métasurfaces ainsi conçues, résonantes et à non-linéarité contrôlée, permettent de générer des effets non linéaires acoustiques inhabituels, potentiellement intéressants pour la manipulation d'ondes acoustiques. / In recent years, acoustic metamaterials have proven to be of great interest for their ability to achieve a variety of wave control at sub-wavelength scale. In particular, acoustic metasurfaces have shown their ability to manipulate waves from the boundaries of propagation media, via the reflection, transmission and refraction processes. Unlike the linear regime which has been extensively investigated in acoustic metamaterials, studies of the nonlinear acoustic properties of metamaterials, especially nonlinear acoustic metasurfaces, are quite scarce, despite the possibility to lead to a rich and diverse set of non-trivial acoustic phenomena. The key limitations in the development of nonlinear acoustic metamaterials are the typically weak efficiency of their nonlinear response together with the lack of control on this nonlinearity. This PhD research is thus dedicated to the design of nonlinear elastic metamaterial and metasurface architectures, enabling acoustic wave control in the nonlinear regime. Specifically, the conversion effect from a fundamental wave to its second harmonic is studied through the one-dimensional scattering process (reflection and transmission) by metasurfaces. This requires the elastic nonlinearity management, realized via the discrete modeling of lumped-element systems and architectures made of rotating units. Such designed metasurfaces, resonating and with harnessed nonlinearity, can create unusual nonlinear acoustic effects, potentially interesting for wave control. This research open the path to a more systematic study of nonlinear acoustic wave manipulation by metamaterials.

Les antennes multifaisceaux (AMFs) sont cruciales pour les applications de télécommunications par satellite modernes et futures, civiles et militaires. La partie basse du spectre électromagnétique est saturée alors que de larges bandes de fréquences sont disponibles dans la bande Ka, dans laquelle des missions à très-haut débit ont émergées au cours de la dernière décennie. La tendance consiste à réduire la taille des spots pour les couvertures multi-spots afin de diminuer le prix des satellites. Ainsi des antennes d’ouverture de plus en plus grande électriquement sont requises, induisant des ruptures technologiques majeures. Les lentilles de Luneburg insérées dans un guide d’ondes à plans parallèles (GOPP) deux plaques métalliques parallèles (PMPs) sont des solutions attractives pour illuminer les AMFs, puisqu’elles peuvent aboutir à des formateurs de faisceaux de bande et champ de visée larges, pertes et coûts faibles, et simples à concevoir, réaliser et intégrer. Les travaux de cette thèse portent sur le développement de nouvelles méthodes d’implémentation et sur la conception de AMFs à base de lentille de Luneburg. La réalisation de la lentille de Luneburg est connue pour être un défi technologique majeur. Un état de l’art des méthodes de réalisation est fourni. Ensuite, deux nouvelles méthodes sont proposées, ainsi qu’une méthode et des outils de conception. La première méthode de réalisation consiste en une matrice périodique et régulière de plots métalliques de taille inférieure à la longueur d’onde, et où la séparation du GOPP varie. La hauteur des plots et la séparation du GOPP contrôlent la valeur de l’indice de réfraction équivalente. L’antenne à 9 faisceaux tout métal conçue, fabriquée et mesurée, comporte 8314 plots et présente d’excellentes performances, notamment meilleures que sa version à séparation de plaques constante. La seconde méthode de réalisation consiste en une matrice périodique et régulière de trous circulaires de taille inférieure à la longueur d’onde réalisés sur un des deux revêtements cuivrés d’un substrat diélectrique plus une plaque métallique supérieure séparée du plan des trous par une couche d’air d’épaisseur fixe. L’antenne à 5 faisceaux conçue comporte 2696 trous et présente de très bonnes performances comparés à ces semblables dans la littérature. / Multi-beam antennas (MBAs) are crucial to modern and future, civilian and military satellite telecommunications applications. The low part of the electromagnetic spectrum is congested, while wide band of frequencies are available in the Ka-band, in which broadband missions have emerged in the last decade. The trend is reducing the size of spots in multi-beam coverage to reduce the cost of satellites, hence more electrically large antennas are needed, with major technological breakthrough as a consequence. Luneburg lenses in parallel-plate waveguide (PPW) are attractive solutions to excite MBAs, since they could lead to wide band and field-of-view, low loss and cost, easy to design, manufacture and accommodate Beam Forming Networks. This PhD deals with the development of novel implementations and the design of broadband, low loss and wide field-of-view Luneburg lens based MBAs. The implementation of the Luneburg lens is known as a major technological challenge. A state-of-the art of the implementation techniques is presented. Then two novel implementations of Luneburg lens in PPW environment are proposed, like design method, process and tools. The first implementation consists of a periodic and regular array of subwavelength vertical metal posts, where the PPW spacing is variable. The post height and PPW spacing modulate the equivalent refractive index. The all-metal 9-beams antenna designed, manufactured and measured, has 8314 posts and shows excellent performances, better than the traditional constant PPW spacing version. The second implementation consists of periodic and regular array of subwavelength circular holes etched on the copper cladding of a dielectric substrate with an air gap between the holes plane and the PPW top plate. The radius of the holes control the equivalent index. The 5-beams antenna designed has 2696 holes and shows very good performances as compared to similar devices in literature.

<p>Plasmonics with the capability to harness electromagnetic waves at a nanoscale can be utilized for multitude of applications in ultra-compact miniature optical devices. Plasmonic metasurfaces which are artificially designed sub-wavelength structures have gained unprecedented interest in being able to engineer and effectively modulate the amplitude and phase of the incident wave. Introducing randomness to such plasmonic metasurfaces can also advance possibilities for extraordinary wave manipulation. Hence, by exploiting the plasmonic response of the ordered and disordered metasurfaces, we can design high performance devices for nanoscale optics.</p> <p>Aiming to provide a holistic solution to the current device limitations and bio-compatibility, my research focuses on non-toxic and environment-friendly coloration using plasmonic disordered metasurfaces. These structures generate a broad range of long-lasting colors in reflection that can be applied to real-life artistic or technological applications with a spatial resolution on the order of 0.3 mm or less. Moreover, my research also deals with the possibility of even concentrating energy in the smallest phase-space volume in optics in the form of coherent radiation through designing nanolasers. The study of carrier dynamics and photophysics of the gain media can be extremely beneficial towards the practicability of these lasers. This work elucidates the evolution of different competing mechanisms for coherent lasing. The dynamic study and experimental demonstration of these devices and respective materials can therefore provide a novel aspect to fundamental and applied research.</p>

Wireless sensors are used widely for condition monitoring in electric machines. The metal enclosure of an electric motor restricts the signal from sensors to radiate outside. The signal from the metal cavity needs to be guided to the only opening in the enclosure, through a narrow gap between the stator and the rotating rotor. Gap waveguide technology is proposed as a solution by texturing the stator surface with electromagnetic band gap (EBG) structures. Arrays of periodic holey structures are used to realize the metasurface waveguide. Two Bravais lattice structures – square and hexagonal, are explored for guiding waves along a desired path in a parallel plate waveguide. Simulations are carried out to study the influence of various dimensions of the unit cells. A waveguide with hexagonal hole-type unitcell is designed and manufactured for experimental verification. The possibility of extending the same technology to cylindrical surface is confirmed by simulations. / Trådlösa sensorer används allmänt för tillståndsövervakning i elektriska maskiner. Metallhöljet hos en elektrisk motor begränsar signalen från sensorerna från att stråla utåt. Signalen från metallhåligheten behöver styras till den enda öppningen i höljet, genom ett smalt mellanrum mellan statorn och den roterande rotorn. Gap-vågledarteknik föreslås som en lösning genom att strukturera statorytan med elektromagnetiska bandgap-strukturer (EBG). Arrayer av periodiskt håliga strukturer används för att realisera metayt-vågledare. Två Bravais gitterkonstruktioner –kvadratiska och sexkantiga, undersöks för styrning av vågor längs en önskad väg i en parallellplattvågledare. Simuleringar utförs för att studera påverkan av olika dimensioner hos enhetscellerna. En vågledare med hexagonal håltypsenhetscell är konstruerad och tillverkad för experimentell verifiering. Möjligheten att utvidga samma teknik till cylindrisk yta bekräftas genom simuleringar.

Yes / This paper presents a novel on-chip antenna using standard CMOS-technology based on metasurface implemented on two-layers polyimide substrates with a thickness of 500 μm. The aluminium ground-plane with thickness of 3 μm is sandwiched between the two-layers. Concentric dielectric-rings are etched in the ground-plane under the radiation patches implemented on the top-layer. The radiation patches comprise concentric metal-rings that are arranged in a 3 × 3 matrix. The antennas are excited by coupling electromagnetic energy through the gaps of the concentric dielectric-rings in the ground-plane using a microstrip feedline created on the bottom polyimide-layer. The open-ended feedline is split in three-branches that are aligned under the radiation elements to couple the maximum energy. In this structure, the concentric metal-rings essentially act as series left-handed capacitances CL that extend the effective aperture area of the antenna without affecting its dimensions, and the concentric dielectric rings etched in the ground-plane act as shunt left-handed inductors LL, which suppress the surface-waves and reduce the substrates losses that leads to improved bandwidth and radiation properties. The overall structure behaves like a metasurface that is shown to exhibit a very large bandwidth of 0.350–0.385 THz with an average radiation gain and efficiency of 8.15dBi and 65.71%, respectively. It has dimensions of 6 × 6 × 1 mm3 that makes it suitable for on-chip implementation. / This work is partially supported by RTI2018-095499-B-C31, Funded by Ministerio de Ciencia, Innovación y Universidades, Gobierno de España (MCIU/AEI/FEDER,UE), and innovation programme under grant agreement H2020-MSCA-ITN-2016 SECRET-722424 and the fnancial support from the UK Engineering and Physical Sciences Research Council (EPSRC) under grant EP/E022936/1. / Research Development Fund Publication Prize Award winner, March 2020

L'objectif général est de développer un nouvel outil numérique dédié à la focalisation en 3D de l'énergie en zone de champ très proche par un système antennaire. Cet outil permettra de définir la distribution spatiale complexe des champs dans l'ouverture rayonnante afin de focaliser l'énergie sur un volume quelconque en zone de champ réactif. L'hybridation de cet outil avec un code de calcul dédié à l'analyse rapide d‘antennes SIW par la méthode des moments permettra de synthétiser une antenne SIW ad-hoc. Les structures antennaires sélectionnées seront planaires comme par exemple les antennes RLSA (Radial Line Slot Array). Les dimensions de l'antenne (positions, dimensions et nombre de fentes) seront définies à l'aide des outils décrits ci-dessus. Les résultats numériques ainsi obtenus seront validés d'abord numériquement par analyse électromagnétique globale à l'aide de simulateurs commerciaux, puis expérimentalement en ondes millimétriques (mesure en zone de champ très proche). Pour atteindre ces objectifs, nous avons défini quatre tâches principales : Développement d'un outil de synthèse de champ dans l'ouverture rayonnante (formulation théorique couplée à une méthode dite des projections alternées) ; développement d'un outil de calcul rapide (sur la base de traitements par FFT) du champ électromagnétique rayonné en zone de champ proche par une ouverture rayonnante, et retro-propagation ; hybridation de ces algorithmes avec un code de calcul (méthode des moments) en cours de développement à l'IETR et dédié à l'analyse très rapide d'antennes en technologie SIW ; conception d'une preuve ou plusieurs preuves de concept, et validations numérique et expérimentale des concepts proposés. / With the demand for near-field antennas continuously growing, the antenna engineer is charged with the development of new concepts and design procedures for this regime. From the microwave and up to terahertz frequencies, a vast number of applications, especially in the biomedical domain, are in need for focused or shaped fields in the antenna proximity. This work proposes new theoretical methods for near-field shaping based on different optimization schemes. Continuous radiating planar apertures are optimized to radiate a near field with required characteristics. In particular, a versatile optimization technique based on the alternating projection scheme is proposed. It is demonstrated that, based on this scheme, it is feasible to achieve 3-D control of focal spots generated by planar apertures. Additionally, with the same setup, also the vectorial problem (shaping the norm of the field) is addressed. Convex optimization is additionally introduced for near-field shaping of continuous aperture sources. The capabilities of this scheme are demonstrated in the context of different shaping scenarios. Additionally, the discussion is extended to shaping the field in lossy stratified media, based on a spectral Green's functions approach. Besides, the biomedical applications of wireless power transfer to implants and breast cancer imaging are addressed. For the latter, an extensive study is included here, which delivers an outstanding improvement on the penetration depth at higher frequencies. The thesis is completed by several prototypes used for validation. Four different antennas have been designed, based either on the radial line slot array topology or on metasurfaces. The prototypes have been manufactured and measured, validating the overall approach of the thesis.