Dans le domaine de l'imagerie infrarouge, on distingue deux types de technologies : les détecteurs refroidis, coûteux et très performants, et les détecteurs thermiques, bas coût et moins encombrants. Dans les deux cas le système optique associé au détecteur représente une part importante du coût total du fait de la production unitaire de lentilles et du besoin en une résolution de plus en plus importante pour suivre la diminution du pas pixel et l'augmentation du format des détecteurs. Dès lors il est intéressant d'explorer des solutions pour diminuer le coût et l'encombrement du système optique tout en maintenant ou en améliorant les performances optiques. Dans ce contexte, ce travail de thèse s'intéresse à l'utilisation d'optiques sub-lambda ou métasurfaces optiques en matériau diélectrique au sein de systèmes d'imagerie infrarouge. De telles optiques sont peu encombrantes et fabriquées par des moyens issus de l'industrie de la microélectronique ce qui permet d'envisager une fabrication collective donc une diminution des coûts. Ces objets sont obtenus en structurant un substrat plan avec des motifs de taille inférieure à la longueur d'onde. La géométrie de ces motifs, dans des matériaux à forts indices de réfraction et transparents dans l'infrarouge comme le silicium, permettent de modifier les propriétés d'une onde optique : sa polarisation, sa phase, sa dispersion et la transmission. Cependant il est complexe de contrôler tout ces paramètres tout en prenant compte des limites technologiques des outils de fabrication. Ce travail s'est donc orienté vers la conception de systèmes optiques mêlant lentilles réfractives et lames sub-lambda de correction de front d'onde. Pour ce faire nous avons (i) développé un outil de simulation mêlant calculs électromagnétiques par la méthode RCWA, pour rendre compte du comportement d'une optique sub-lambda, et conception optique pour la partie réfractive. Nos optiques sub-lambda ont dès lors des dimensions millimétriques à centimétriques pour être couplées à des lentilles réfractives au sein de systèmes d'imagerie, et notre méthode permet de simuler efficacement de tels systèmes optiques. Dans un second temps nous avons (ii) développé des procédés de fabrication de prototypes d'optiques sub-lambda, notamment pour la correction d'aberration sphérique dans le LWIR (bande 8-14µm de longueur d'onde). (iii) Enfin la caractérisation de nos systèmes optiques a permis de valider notre modèle et de démontrer une forte amélioration de la FTM d'un système optique aberrant associé à une lame sub-lambda de correction de front d'onde (FTM multipliée par 3 à 25 cycles par millimètre). Nos derniers résultats montrent une amélioration sur une bande 8-12µm de nos systèmes optiques et ouvrent la voie vers la conception d'optiques sub-lambda large bande au sein de systèmes d'imagerie infrarouge. / In the field of infrared imaging, there are two main types of detectors : cooled detectors, with great sensitivity but expensive, and uncooled detectors, exhibiting precise temperature measurement at moderate cost. In both technologies, the optical systems associated with the detectors represent an important part of the overall cost because of the unitary fabrication process of infrared lenses and the need of more resolved imaging system to follow the shrinkage of the pixel and the increasing array format. Thus, it is important to search for cost effective and low footprint optical solutions exhibiting a high level of performance for infrared imaging systems. In this thesis work we study how dielectric subwavelength structures, or metasurfaces, can adress these issues in infrared systems. Such devices can be made using microelectronics based collective fabrication process, which are cost effective compared to molded infrared optics. Subwavelength optics can be made with silicon, which is transparent in long wave infrared (LWIR) imaging and exhibiting a high refractive index. By designing the geometry of resonators with subwavelength dimensions, one can control light properties like its polarization, phase, transmission and dispersion. However as it is challenging to control all those parameters, even more with fabrication process limitations, we first propose to mix refractive lenses with subwavelength phase blades which correct wavefront errors. (i) We first developed a time effective simulation method mixing electromagnetic calculations with RCWA, for the subwavelength part of the optical system, and classical optical design for the refractive optics. It is worth noting that our subwavelength optics have millimetric to centimetric dimensions to be coupled with refractive lenses, and our method allows us to simulate the overall system. (ii) Then we developed the fabrication process for prototyping subwavelength optics, mainly for spherical aberration correction in LWIR imaging systems. (iii) Finally, we conducted optical characterisations of our systems to validate our model. Our subwavelength optics show an important improvement of the MTF (more than 3 times better at 25 cycles per millimeter) of an optical infrared system by correcting its spherical aberration. Our last results show a improvement of the image quality on a large bandwith (8-12µm) paving the way to large bandwidth subwavelength optics in infrared imaging systems.

abstract: Articially engineered two-dimensional materials, which are widely known as metasurfaces, are employed as ground planes in various antenna applications. Due to their nature to exhibit desirable electromagnetic behavior, they are also used to design waveguiding structures, absorbers, frequency selective surfaces, angular-independent surfaces, etc. Metasurfaces usually consist of electrically small conductive planar patches arranged in a periodic array on a dielectric covered ground plane. Holographic Articial Impedance Surfaces (HAISs) are one such metasurfaces that are capable of forming a pencil beam in a desired direction, when excited with surface waves. HAISs are inhomogeneous surfaces that are designed by modulating its surface impedance. This surface impedance modulation creates a periodical discontinuity that enables a part of the surface waves to leak out into the free space leading to far-eld radia- tion. The surface impedance modulation is based on the holographic principle. This dissertation is concentrated on designing HAISs with Desired polarization for the pencil beam Enhanced bandwidth Frequency scanning Conformity to curved surfaces HAIS designs considered in this work include both one and two dimensional mod- ulations. All the designs and analyses are supported by mathematical models and HFSS simulations. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

L’objectif de ce travail est le développement de dispositifs de contrôle de la diffusion des ondes acoustiques à l’aide de surfaces hétérogènes, pour des applications à l’acoustique urbaine. Pour remplir cet objectif, deux méthodes sont employées. La première à l’aide d’une métasurface, la direction d’une onde réfléchie est contrôlée. La seconde étude concerne l’influence d’un traitement inhomogène aux parois d’un guide ouvert sur les effets conjoints ou compétitifs d’absorption, de confinement et de rayonnement de l’onde. Nous montrons expérimentalement la possibilité de dissimuler un objet disposé sur un mur pour une onde acoustique audible. Pour y parvenir, une métasurface composée de différents résonateurs de Helmholtz est conçue et est réalisée de façon à être la plus fine possible. Ces travaux sont réalisés dans le domaine fréquentiel mais également dans le domaine temporel, ce qui permet de mettre en avant le caractère large bande de la métasurface. Il est démontré numériquement et expérimentalement que la direction des ondes réfléchies peut être contrôlée. Enfin la dernière partie est consacrée à l’influence d’une paroi hétérogène sur la propagation d’une onde acoustique à l’intérieur d’une rue. Une rue pouvant être assimilée à un guide d’onde ouvert engendre donc des modes de propagation complexes, dus aux pertes par rayonnement. La présence d’un matériau poreux sur les parois d’un guide vient perturber fortement la localisation spatiale des modes, ce qui les rend plus ou moins fuyants. / The aim of this thesis is to develop a scheme for controling the propagation of acoustic waves using heteregenous surfaces. Its results can be applied in the field of urban acoustic. The thesis is composed of two sections, each of them employing a different method. The first section focuses on controling the direction of a reflected wave, using a metasurface. The second concentrates on the influence of an inconsistent treatment to the side of an open waveguide on the wave joint and competitive effects of absorption, confinement and radiation. Part one provides experimental evidence that it is possible to conceal an object placed on a wall from an audible acoustic wave. To prove it, the thinest possible metasurface was constructed with Helmholtz resonators. The experimental results were compared to a numerical study realized with finite elements. This work was made in both temporal and frequency domains, allowing to point out the wide frequency characteristics of the metasurface. The numerical and experimental results show that the direction of a reflected wave can, indeed, be controled. Part two analyse the impact of a heterogeneous wall on the spreadinf of an acoustic wave in a street. Due to radiation losses, the street produces complex ways of propagation. The presence of a porous material on a waveguide‘ side deeply disrupt the spatial location of these waves, making them more or less fleeting. In particular, depending on the position of the material in the street, certain waves will be more confined to the inside of the street, radiating less towards the open external environment. They are consequently, less cushioned.

abstract: Since the advent of High Impedance Surfaces (HISs) and metasurfaces, researchers have proposed many low profile antenna configurations. HISs possess in-phase reflection, which reinforces the radiation, and enhances the directivity and matching bandwidth of radiating elements. Most of the proposed dipole and loop element designs that have used HISs as a ground plane, have attained a maximum directivity of 8 dBi. While HISs are more attractive ground planes for low profile antennas, these HISs result in a low directivity as compared to PEC ground planes. Various studies have shown that Perfect Electric Conductor (PEC) ground planes are capable of achieving higher directivity, at the expense of matching efficiency, when the spacing between the radiating element and the PEC ground plane is less than 0.25 lambda. To establish an efficient ground plane for low profile applications, PEC (Perfect Electric Conductor) and PMC (Perfect Magnetic Conductor) ground planes are examined in the vicinity of electric and magnetic radiating elements. The limitation of the two ground planes, in terms of radiation efficiency and the impedance matching, are discussed. Far-field analytical formulations are derived and the results are compared with full-wave EM simulations performed using the High-Frequency Structure Simulator (HFSS). Based on PEC and PMC designs, two engineered ground planes are proposed. The designed ground planes depend on two metasurface properties; namely in-phase reflection and excitation of surface waves. Two ground plane geometries are considered. The first one is designed for a circular loop radiating element, which utilizes a circular HIS ring deployed on a circular ground plane. The integration of the loop element with the circular HIS ground plane enhances the maximum directivity up to 10.5 dB with a 13% fractional bandwidth. The second ground plane is designed for a square loop radiating element. Unlike the first design, rectangular HIS patches are utilized to control the excitation of surface waves in the principal planes. The final design operates from 3.8 to 5 GHz (27% fractional bandwidth) with a stable broadside maximum realized gain up to 11.9 dBi. To verify the proposed designs, a prototype was fabricated and measurements were conducted. A good agreement between simulations and measurements was observed. Furthermore, multiple square ring elements are embedded within the periodic patches to form a surface wave planar antenna array. Linear and circular polarizations are proposed and compared to a conventional square ring array. The implementation of periodic patches results in a better matching bandwidth and higher broadside gain compared to a conventional array. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2020

Cette thèse a pour but de mettre en avant les récentes avancées dans le domaine des métasurfaces. Ces structures ont été utilisées dans le but d’améliorer les performances des antennes classiques ou de concevoir de nouveaux concepts d’antenne. Les travaux menés s’inscrivent dans le cadre d’une collaboration avec des partenaires industriels qui sont Airbus Safran Lunchers, Airbus Group Innovations et le CNES. La thèse est organisée en deux parties. La première partie est consacrée aux métasurfaces utilisées comme des surfaces partiellement réfléchissantes (SPR) pour concevoir des antennes à cavité Fabry-Perot. Un modèle analytique permettant de prédire le dépointage du faisceau d’antenne par une modulation de la phase sur la SPR a été développé. Ensuite, un nouveau concept de métasurface permettant de réaliser du dépointage de faisceau est proposé. Il consiste à appliquer un gradient de phase en faisant varier l’indice effectif le long du substrat diélectrique de la SPR. La deuxième partie de cette thèse est quant à elle consacrée à la conception d’une métasurface active permettant d’émuler plusieurs fonctions. Dans un premier temps, la métasurface est utilisée comme un réflecteur présentant une reconfigurabilité fréquentielle et angulaire. Ensuite cette métasurface est utilisée comme polariseur reconfigurable où une polarisation linéaire de l'onde incidente est convertie en polarisation circulaire. Enfin, la dernière étude concerne l’utilisation de la métasurface active pour la réalisation d’une antenne à réflecteur cylindro-parabolique et à réflecteur dièdre reconfigurables. / This thesis aims at highlighting recent advances in the field of metasurfaces. These structures have been used to improve the performances of conventional antennas or to design new antenna concepts. The work has been carried in the framework of a collaboration with industrial partners, namely Airbus Safran Launchers, Airbus Group Innovations and CNES. The manuscript is organized into two parts. The first part is devoted to metasurfaces used as partially reflecting surfaces (PRS) to design Fabry-Perot cavity antennas. In this part, an analytical model allowing to predict the beam steering angle by a phase modulation along the PRS is developed. Then, a new concept of metasurface allowing to steer the main antenna beam is proposed. It consists in applying a phase gradient by varying the effective index of the substrate that constitutes the PRS. The second part of this thesis is devoted to the design of an active metasurface that allows emulating different functionalities. First, the metasurface is utilized as a reflector with frequency and steering reconfigurability characteristics. Then, this metasurface is used as a reconfigurable polarizer where linearly polarized incident waves are converted into circularly polarized ones. Finally, the last study concerns the use of the active metasurface for the design of reconfigurablecylindro-parabolic and corner reflector antennas.

Circular polarization is valuable for many electromagnetic radiation applications such as wireless and satellite communication, radars, RFID, global positioning systems, etc. Many efforts have been made to manipulate and control polarization by using linear-to-linear or linear-to-circular transmission or reflection polarization converters. Most of the existing linear-to-circular single-layer polarizers have been found to be narrowband. Some attempts have been made to improve the bandwidth of these polarizers including using multiple layered structures at the expense of a bulkier device. There was, however, still a requirement for thin single-layer linear-to-circular polarizers with enhanced bandwidth. The purpose of this research was to design two thin single-layer linear-to-circular polarizers, one for transmission and the other for reflection, with enhanced bandwidth. A thin single-layer linear-to-circular transmission polarizer with a 3 dB axial ratio bandwidth of 34% is presented. The bandwidth of this polarizer is significantly better than that of previously published polarizers of the same type. The unit cell of the polarizer consists of an I-shaped strip and a perpendicular linear strip printed on the one side of a thin dielectric substrate and two additional capacitive coupling strips printed on the other side of the substrate. Experimental results were found to agree well with the simulated results. A thin single-layer reflective linear-to-circular polarizer with a 3 dB axial ratio bandwidth of 57% is also presented. The unit cell of the polarizer consists of an I-shaped strip and a perpendicular linear strip printed on the one side of a substrate and a ground plane on the other side of the substrate. Experimental results for this polarizer were also found to agree well with the simulated results. / Dissertation (MEng)--University of Pretoria, 2018. / Electrical, Electronic and Computer Engineering / MEng / Unrestricted

This Master’s dissertation focuses on engineering artificial nanostructures, namely, arrays of metamolecules on a substrate (metasurfaces), with the goal to achieve the desired linear and nonlinear optical responses. Specifically, a simple analytical model capable of predicting optical nonlinearity of an individual metamolecule has been developed. The model allows one to estimate the nonlinear optical response (linear polarizability and nonlinear hyperpolarizabilities) of a metamolecule based on the knowledge of its shape, dimensions, and material. In addition, a new experimental approach to measure hyperpolarizability has also been investigated. As another research effort, a 2D plasmonic metasurface with the collective behaviour of the metamolecules known as hybrid plasmonic-Fabry-Perot cavity and surface lattice resonances was designed, fabricated and optically characterized. We experimentally discovered a novel way of coupling the microcavity resonances and the diffraction orders of the plasmonic metamolecule arrays with the low-quality plasmon resonance to generate multiple sharp resonances with the higher quality factors. Finally, we experimentally observed and demonstrated a record ultra-high-Q surface lattice resonance from a plasmonic metasurface. These novel results can be used to render highly efficient nonlinear optical responses relying on high optical field localization, and can serve as the stepping stone towards achieving practical artificial nanophotonic devices with tailored linear and nonlinear optical responses.

<p>Advances in additive manufacturing technologies enable the rapid, high-throughput generation of mechanically soft microelectromechanical devices with tailored designs for many applications spanning from optical to biomedical applications. These devices can be softly interfaced with biological tissues and mechanically fragile systems, which enables to open up a whole new range of applications. However, the scalable production of these devices faces a significant challenge due to the complexity of the microfabrication process and the intolerable thermal, chemical, and mechanical conditions of their flexible polymeric substrates. To overcome these limitations, I have developed a set of advanced additive manufacturing technologies enabling (1) mechanics-driven manufacturing of quasi-three-dimensional (quasi-3D) nanoarchitectures with arbitrary substrate materials and structures; (2) repetitive replication of quasi-3D nanoarchitectures for infrared (IR) bandpass filtering; (3) electrochemical reaction-driven delamination of thin-film electronics over wafer-scale; (4) rapid custom printing of soft poroelastic materials for biomedical applications. </p> <p>First, I have developed a new mechanics-driven nanomanufacturing method enabling large-scale production of quasi-3D plasmonic nanoarchitectures that are capable of controlling light at nanoscale length. This method aims to eliminate the need for repetitive uses of conventional nanolithography techniques that are time- and cost-consuming. This approach is innovative and impactful because, unlike any of the conventional manufacturing methods, the entire process requires no chemical, thermal, and mechanical treatments, enabling a large extension of types of receiver substrate to nearly arbitrary materials and structures. Pilot deterministic assembly of quasi-3D plasmonic nanoarrays with imaging sensors yields the most important advances, leading to improvements in a broad range of imaging systems. Comprehensive experimental and computational studies were performed to understand the underlying mechanism of this new manufacturing technique and thereby provide a generalizable technical guideline to the manufacturing society. The constituent quasi-3D nanoarchitectures achieved by this manufacturing technology can broaden considerations further downscaled plasmonic metamaterials suggest directions for future research.</p> <p>Second, I have developed mechanics-driven nanomanufacturing that provides the capability to repetitively replicate quasi-3D plasmonic nanoarchitectures even with the presence of an extremely brittle infrared-transparent spacer, such as SU-8, thereby manipulating IR light (e.g., selectively transmitting a portion of the IR spectrum while rejecting all other wavelengths). Comprehensive experimental and computational studies were performed to understand the underlying nanomanufacturing mechanism of quasi-3D plasmonic nanoarchitectures. The spectral features such as the shape of the transmission spectrum, peak transmission and full width at half maximum (FWHM), etc. were studied to demonstrate the bandpass filtering effect of the assembled quasi-3D plasmonic nanoarchitecture.</p> <p>Third, I have developed an electrochemical reaction-driven transfer printing method enabling a one-step debonding of large-scale thin-film devices. Conventional transfer printing methods have critical limitations associated with an efficient and intact separation process for flexible 3D plasmonic nanoarchitectures or bio-integrated electronics at a large scale. The one-step electrochemical reaction-driven method provides rapid delamination of large-scale quasi-3D plasmonic nanoarchitectures or bio-integrated electronics within a few minutes without any physical contact, enabling transfer onto the target substrate without any defects and damages. This manufacturing technology enables the rapid construction of quasi-3D plasmonic nanoarchitectures and bio-integrated electronics at a large scale, providing a new generation of numerous state-of-art optical and electronic systems.</p> <p>Lastly, I have developed a new printing method enabling the direct ink writing (DIW) of multidimensional functional materials in an arbitrary shape and size to rapidly prototype stretchable biosensors with tailored designs to meet the requirement of adapting the geometric nonlinearity of a specific biological site in the human body. Herein, we report a new class of a poroelastic silicone composite that is exceptionally soft and insensitive to mechanical strain without generating significant hysteresis, which yields a robust integration with living tissues, thereby enabling both a high-fidelity recording of spatiotemporal electrophysiological activity and real-time ultrasound imaging for visual feedback. Comprehensive <i>in vitro</i>, <i>ex vivo</i>, and <i>in vivo</i> studies provide not only to understand the structure-property-performance relationships of the biosensor but also to evaluate infarct features in a murine acute myocardial infarction model. These features show a potential clinical utility in the simultaneous intraoperative recording and imaging on the epicardial surface, which may guide a definitive surgical treatment.</p>

Au cours des dernières années, des efforts importants ont été déployés pour développer des métasurfaces (MSs) électromagnétiques avec la possibilité de changer de manière abrupte les propriétés de la lumière. Ces avancées ont ouvert une nouvelle gamme de possibilités pour contrôler la lumière en utilisant des dispositifs optiques ultra-minces. Dans ce contexte, et plus spécifiquement dans le spectre visible, les applications en bio-imagerie s’avèrent particulièrement intéressantes. Une technique qui est particulièrement bien adaptée à l'étude de molécules proches d'une membrane cellulaire est la microscopie à fluorescence par réflexion interne (TIRFM), qui repose sur un champ évanescent d'excitation. Dans ce cas la lumière incidente est totalement réfléchie sur une interphase (typiquement verre/eau) en raison de son angle d'incidence élevé. À ce jour, la TIRFM est généralement mise en œuvre à l'aide d'objectifs volumineux de grande ouverture numérique et de petit champ de vision.Dans ce travail de thèse, nous réalisons de substrats pour la microscopie TIRF à base de métasurfaces constituées de réseaux périodiques de structures asymétriques fabriquées en dioxyde de titane (TiO2) sur du verre borosilicaté. Ces structures, aussi petites que 48 nm, ont été optimisées à l’aide de simulations numériques "Rigorous coupled-wave analysis” (RCWA) dans le but de coupler de 50 à 90% de la lumière incidente dans le premier ordre de diffraction avec des angles élevés (θ > 63deg). Le fait de pouvoir utiliser des objectifs de faible grossissement et d'avoir une grande zone de champ évanescent fournit des conditions TIRF uniques qui ne sont pas accessibles par les méthodes traditionnelles. De plus, ces structures sont compatibles avec la lithographie par nanoimpression UV, ce qui permet d’envisager une fabrication à bas coût et à grande échelle. Outre la conception, et la fabrication, dans cette thèse nous aboutissons à une preuve de principe de la microscopie TIRF basée sur des métasurfaces en milieu biologique en imageant notamment des membranes fluorescentes de cellules souches. Ces métasurfaces permettent ainsi l’implémentation TIRFM à contraste élevé et à faible photo-blanchissement compatible avec des microscopes à champ large peu coûteux. / In recent years there has been a significant effort to push electromagnetic metasurfaces with the ability to abruptly change light properties into visible wavelengths. These advancements have opened a new range of possibilities to reshape light using ultra-thin optical devices and there is one field that is starting to gather attention: bioimaging. One technique particularly well suited for the study of molecules near a cell membrane is Total Internal Reflection Fluorescence (TIRF) microscopy, which relies on an evanescence field created by light being totally internally reflected within a glass substrate due to its high incidence angle. As of today, TIRF is generally implemented using bulky high-NA, small field of view oil objectives.In this project we present the realization of metasurface-based TIRF microscopy substrates consisting of periodic 2D arrays of asymmetric structures fabricated in titanium dioxide on borosilicate glass. These patterns, as small as 48nm, were optimized through rigorous coupled-wave analysis to couple 50-90% of the incoming normally incident light into the first diffraction order, which outputs at an angle that suffices total internal reflection in water and eliminates the requirement for high NA objectives or prisms to achieve TIRF. Being able to utilize lower-magnification air objectives and having a large evanescence field area provide unique TIRF conditions not accessible by traditional methods. Additionally, these structures are compatible with soft UV nanoimprint lithography, for cost-effective scale production, to give TIRF’s high contrast, low photodamage and low photobleaching capabilities to inexpensive wide-field microscopes.

Metasurfaces are arrays of subwavelength meta-atoms that shape waves in a compact and planar form factor. During recent years, metasurfaces have gained a lot of attention due to their compact form factor, easy integration with other devices, multi functionality and straightforward fabrication using conventional CMOS techniques. To provide and evaluate an efficient metasurface, an optimized design, high resolution fabrication and accurate measurement is required. Analysis and design of metasurfaces require accurate methods for modeling their interactions with waves. Conventional modeling techniques assume that metasurfaces are locally periodic structures excited by plane waves, restricting their applicability to gradually varying metasurfaces that are illuminated with plane waves. In this work, we will first provide a novel technique that enables the development of accurate and general models for 1D metasurfaces. This approach can be easily extended to 2D metasurfaces. Due to the remarkable importance of accurate characterization of metasurfaces, we will provide a rigorous method to characterize 1D metasurfaces. Finally, we will provide an accurate approach to fabricate and characterize 2D metasrufaces.