The capability of light goes well beyond illumination, yet it is so underused in our lives because the control of light still largely relies on clumsy bulk lenses. Less than 10 years ago, a type of revolutionary devices made of nanometer scale optical elements – metasurfaces – was invented to control the light propagation and its energy dissipation with arbitrary degree of freedom, at unprecedentedly small volumes (although some would argue that the advent of metasurfaces came in the 1990s). Vast diversity of new discoveries has since been made possible, and many more existing applications have seen significant performance enhancement with the aid of metasurfaces.<div><br><div> <div>In the scope of this work, I explore the use of a variety of metasurfaces to address several existing real-world challenges: sensing, optical heating, and data storage. Among these, three metasurfaces involve the world’s first two-dimensional material, graphene. I first investigate the graphene plasmonic resonator, which have been shown to be extremely sensitive single-molecule sensors. Graphene also has many intriguing properties in photodetection applications, such as lightweight, ultra-wide detection band, and ultrafast response speed. I have used two different metasurfaces to enhance the intrinsically low responsivity (sensitivity) of graphene photodetectors. Amidst the discussion of graphene photodetectors, I show the characterization result of plasmonic heating of metasurfaces, an essential process of the graphene photo-responsivity enhancement. Lastly, I present a multi-functional metasurface which can be used in optical steganography, encryption, and data storage. The proposed metasurface is compatible with large scale parallel readout, which outperforms current Blu-ray technology in both storage capacity and readout speed</div></div></div>

Nowadays, metamaterials have found their places in different branches of wave physics ranging from electromagnetics to acoustic waves. Acoustic metamaterials are sub-wavelength structures in which their effective acoustic properties are dominated by their structural shape rather than their constitutive materials. In recent years, acoustic metamaterials have gained increasing interest due to numerous promising applications such as sub-wavelength imaging, perfect absorption, acoustic cloaking, etc. The focus of the work herein is to leverage acoustic metamaterial/metasurface structures to manipulate the acoustic wavefront to pave the road for future applications of the metamaterials. In the first part of the work, the metamaterial structure is introduced, which can be leveraged for better manipulation of the transmitted wave by modulating both phase and amplitude. Initially, a general bound on the transmission phase/amplitude space for the case of arbitrary metasurface has been presented and subsequently, the necessary condition for the complete modulation of the transmitted wave is investigated. Next, a horn-like space coiling metamaterial is introduced, which satisfied the aforementioned condition and enabled us to simultaneously modulate both the phase and amplitude of the transmitted wave. Furthermore, our initial efforts toward designing a metamaterial capable of real-time phase modulation with relatively constant amplitude will be discussed. In the second part of this work, a novel metamaterial-based methodology is presented for the design of the air-permeable acoustic silencer. In this work, the concept of the bilayer-transverse metamaterial is introduced, and its functionality for silencing the acoustic wave is demonstrated. Furthermore, it is shown that the methodology presented herein essentially does not limit the ratio of the open area, and ultra-open metamaterial silencers may be designed. Eventually, based on the presented methodology, the ultra-open metamaterial featuring nearly 60% open area is designed, and silencing capacity of about 94% at the targeted frequency is experimentally realized. In the last part of this work, the behavior of a locally resonant class of acoustic metamaterial in the non-Rayleigh regime has been explored. Elaborately, it is demonstrated that in the case of spherical inclusion in a matrix material, large variation in the effective acoustic impedance emerges near the inclusion’s Eigenmode. Eventually, the potential application of this novel phenomenon in the non-destructive evaluation (NDE) and ultrasound imaging is discussed. / 2020-08-09T00:00:00Z

This thesis will explore the ultrafast modulation and optical tunability of plasmonic filters in the terahertz (THz) spectral region. First, the principles and functional design of THz metasurfaces are explored through plasmonic surface lattice resonance interactions and lumped-element circuit models. We will then describe the methodology of generating and detecting THz radiation through the nonlinear processes of optical rectification and electrooptic sampling, respectively. Next, the implementation of a THz time-domain spectroscopy technique is discussed in the context of pump-probe measurements and time-domain resonance analysis. We then show how THz probed materials can be characterized in terms of a temporal and spectral analysis. We will demonstrate how this time-domain technique can allow us to characterize the interaction of plasmonic resonators with optically active substrates and 2D nanomaterials. A completely tunable THz plasmonic notch resonance is modulated utilizing a static and dynamic method of optical tunability in silicon. Active tunability is also demonstrated in a graphene-based plasmonic resonator through the hot carrier multiplication effect. The significance of this work lies in the application of designing controllable devices for future THz communication technologies.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20380号 / 工博第4317号 / 新制||工||1669(附属図書館) / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 北野 正雄, 教授 山田 啓文, 教授 松尾 哲司 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The area of nanophotonics has been the focus of researchers recently due to its high potential to overcome the limitations of scaling in electronic devices. One of the most popular devices in this field is the metasurface. A metasurface consists of a periodic or aperiodic array of spaced units called ’meta-atoms’, where the interaction between these neighboring elements provide unprecedented properties that cannot be obtained using a a regular array of antennas. By tuning the shape and structure of the meta-atoms, electromagnetic wave interaction with the metasurface can be manipulated to achieve a plethora of response characteristics. For active applications that require tunability of the response, a passive metasurface cannot be used to adapt to the varying operating conditions. Tunability of metasurfaces can then be achieved by using phase-changing materials. This type of materials can attain different optical properties by applying external stimulus such as heat, electric current, or laser pulses. The change in the optical properties would be beneficial for applications requiring reconfigurability or adaptation. In this thesis, I demonstrate the employment of volatile (Vanadium Dioxide) and non-volatile (Germanium Antimony Telluride) examples of phase-change materials to design reconfigurable metasurfaces operating at different bands in the infrared regime. I show metallic and dielectric-based structures that employ volatile and non-volatile phase-change materials, as well as apply physics such as plasmonics and bound states in the continuum to design and optimize metasurface structures for energy and biosensing applications. / Thesis / Doctor of Philosophy (PhD) / This thesis proposes methods to design and optimize reconfigurable and adaptive metasurfaces for energy harvesting, radiative cooling, and biosensing applications in the infrared range. The concept of phase-change metasurfaces is highlighted where a phase-change material (PCM) is employed to provide the tunable response. The properties of the PCM can be modified using several excitation methods such as thermal, electric, and laser excitation. The details of material selection, geometry configuration, as well as optimization procedures are demonstrated. Target applications for the study is harvesting from Earth’s ambient radiation around 10.6µm, adaptive cooling of spacecraft in the mid-infrared band 2.5 − 25µm, and trace biomarkers detection in the amide-I and amide-II bands (5.5−6.9µm). Full-wave numerical analysis was conducted using COMSOL Multiphysics software. Optimization was carried out using global optimization techniques implemented using Matlab and Python. The results show innovative designs for switchable absorbers, new approach for modeling of phase-change metasurfaces using deep learning, and employment of the physics of bound states in the continuum for the first time to implement a robust biosensing device. The results of this thesis would help advance the field of reconfigurable nanophotonics and related integrated applications.

Yes / A novel 50μm Silicon-based on-chip antenna is presented that combines metamaterial (MTM) and substrate integrated waveguide (SIW) technologies for integration in THz circuits operating from 0.28 to 0.30 THz. The antenna structure comprises a square patch antenna implemented on a Silicon substrate with a ground-plane. Embedded diagonally in the patch are two T-shaped slots and the edges of the patch is short-circuited to the ground-plane with metal vias, which convert the structure into a substrate integrated waveguide. This structure reduces loss resulting from surface waves and Silicon dielectric substrate. The modes in the structure can be excited through two coaxial ports connected to the patch from the underside of the Silicon substrate. The proposed antenna structure is essentially transformed to exhibit metamaterial properties by realizing two T-shaped slots, which enlarges the effective aperture area of the miniature antenna and significantly enhances its impedance bandwidth and radiation characteristics between 0.28 THz to 0.3 THz. It has an average gain and efficiency of 4.5dBi and 65%, respectively. In addition, it is a self-isolated structure with high isolation of better than 30dB between the two ports. The on-chip antenna has dimensions of 800×800×60 μm3. / H2020-MSCA-ITN-2016 SECRET-722424 and the financial support from the UK Engineering and Physical Sciences Research Council (EPSRC) under grant EP/E0/22936/1

Yes / A metasurface based decoupling structure that is composed of a square-wave slot pattern with exaggerated corners that is implemented on a rectangular microstrip provides high-isolation between adjacent patch antennas for Synthetic Aperture Radar (SAR) and Multi-Input-Multi-Output (MIMO) systems. The proposed 1×2 symmetric array antenna integrated with the proposed decoupling isolation structure is designed to operate at ISM bands of X, Ku, K, and Ka. With the proposed mutual coupling suppression technique (i) the average isolation in the respective ISM bands listed above is 7 dB, 10 dB, 5 dB, and 10 dB; and (ii) edge-to-edge gap between adjacent radiation elements is reduced to 10 mm (0.28λ). The average antenna gain improvement with the metasurface isolator is 2 dBi. / H2020-MSCA-ITN-2016 SECRET-722424 and the financial support from the UK Engineering and Physical Sciences Research Council (EPSRC) under grant EP/E0/22936/1

Cette thèse étudie le façonnage de champs électromagnétiques micro-ondes dans des cavités présentant des conditions aux limites reconfigurables. Le dispositif expérimental s'appuie sur une metasurface électroniquement reconfigurable qui couvre partialement les parois d'une cavité et qui permet ainsi de contrôler la façon dont les ondes y sont réfléchies. Le premier chapitre explore des aspects fondamentaux. D’abord, une étude paramétrique du façonnage d'un champ d'ondes électromagnétiques monochromatique et stationnaire en cavité est proposée en fonction d'un degré de contrôle introduit. Selon la valeur de ce paramètre, il est possible de concentrer de l'énergie en un endroit donné de la cavité de façon prédictible, de reconfigurer totalement cette cavité, ou bien de décider d'obtenir une résonance à une fréquence qui n'en supportait pas auparavant. Ensuite, l’imposition d’un comportement chaotique à une cavité de géométrie régulière est démontrée et une application au brassage des modes en chambre réverbérante est donnée. Dans la suite, la possibilité d’ajuster le couplage antenne-cavité est abordée, et une adaptation parfaite et dynamiquement configurable de l’impédance est proposée. Le reste du premier chapitre considère des champs transitoires. Dans un premier temps, la focalisation spatio-temporelle d’une impulsion fortement réverbérée dans une cavité en utilisant uniquement le contrôle spatial des ondes offert par la metasurface est démontrée, puis le lien avec le couplage entre les dégrées de liberté spatiaux et temporels du milieu de propagation est fait. Enfin, un dispositif permettant la reconfiguration répétée des conditions aux limites d'une cavité en un laps de temps inférieur au temps de vie des photons est réalisé, et des résultats préliminaires sont montrés. Dans le deuxième chapitre, des applications aux systèmes de communication sans fil multi-utilisateurs sont proposées. D’abord, dans la limite d’un bas facteur de qualité de la cavité, il est montré qu’un formalisme matriciel permet de décrire l’impact de la metasurface sur le champ. Cette matrice, mesurée sans information de phase, permet alors de focaliser le champ sur une ou plusieurs positions simultanément. Ensuite, la possibilité d’obtenir une diversité de canaux optimale (orthogonalité des canaux) en façonnant idéalement le désordre d’un milieu de propagation à l'aide de metasurfaces est établie. Finalement, le formalisme matriciel est utilisé afin d’introduire un concept de calcul analogique réalisé par le milieu désordonné en façonnant le front d’onde incident. Il est dès lors conclu qu’avec une infrastructure standard de Wi-Fi dans une maison, en combinaison avec une metasurface simple, cette idée peut être implémentée. Le concept est enfin transposé au domaine optique avec une fibre multimode. Au cours du troisième chapitre, quelques applications du façonnage d'ondes en milieux réverbérants aux capteurs des environnements connectés sont étudiées. D’abord, la possibilité de concentrer des champs électromagnétiques ambients sur des circuits redresseurs afin d’obtenir des tensions de sortie utiles est démontrée. De plus, grâce aux non-linéarités intrinsèques du redresseur, ceci est possible même sans avoir un retour direct du redresseur sur l’intensité du champ incident. Ensuite, un détecteur de mouvement hors ligne de vue et « intelligent » est proposé, qui profite d’un co-design de sa couche physique et du traitement de données. Enfin, il est démontré que même des objets non-coopératifs dans un environnement complexe peuvent être localisés grâce à leur contribution à la diffusion des ondes dans ledit milieu. L’équivalence d’utiliser la diversité fréquentielle ou bien le façonnage d’ondes dans ce contexte est établie. / In this thesis, the shaping of microwave fields in chaotic cavities with tunable boundary conditions is studied experimentally. The experiments leverage a metasurface reflect-array that partially covers the cavity walls to tune the reverberation of waves inside the cavity. The first chapter explores several fundamental aspects. First, the achievable degree of control over stationary monochromatic wave fields is thoroughly investigated, and various regimes are identified, ranging from partial control over the wave field up to the limiting case of discrete resonances that can be tuned at wish. Next, the possibility to convert a cavity of regular geometry into one displaying chaotic characteristics by modulating the boundary conditions is examined and an application to non-mechanical mode-stirring in reverberation chambers is given. Then, the ability to tune the coupling between an antenna inside a cavity and the cavity itself is studied, revealing the opportunity of achieving (dynamically tunable) perfect impedance matching. The chapter goes on to consider spatio-temporal wave fields, and the re-focusing of such transient fields at a desired instant with the purely spatial control of the metasurface is demonstrated; moreover, the interplay of spatial and temporal degrees of freedom is addressed. Finally, an experimental platform enabling the rapid modulation of cavity boundary conditions within the photon lifetime is presented. The second chapter considers applications to multi-user wireless communication systems. First, it is shown that a matrix formalism to capture the impact of the metasurface on the wave field can be formulated in the regime of low reverberation, and even without access to phase information focusing on a single as well as on multiple targets is demonstrated. Second, it is shown that the channel diversity, which dominates the achievable capacity of information transfer, can be optimized by tweaking the environment’s disorder; perfectly orthogonal channels are obtained without any software or hardware efforts on the transmit or receive side, and the benefits of the implied minimal cross-talk are illustrated for the scenario of wirelessly transmitting a full-color image. Third, the matrix formalism is leveraged to propose a scheme of analog computation that counter-intuitively uses a disordered instead of a carefully tailored propagation medium, by appropriately shaping the incident wave front. A proof-of-concept demonstration suggests that combining ubiquitous Wi-Fi hardware in an indoor environment with a simple metasurface is sufficient to implement the concept. Finally, the concept is also implemented in the optical domain using a multimode fiber. The third chapter outlines a few applications for sensors in context-aware environments. First, it is shown that by shaping ambient wave fields, they may be concentrated on harvesting devices to increase the output voltage available for sensor powering; moreover, the non-linear nature of the harvesting device enables to do so without direct feedback from the target, using indirect feedback from the second harmonic. Second, a smart around-the-corner motion detector for complex environments is presented, enjoying a co-design of hardware and processing software by using a dynamic metasurface aperture; the latter is essentially a small (but still electrically large) disordered cavity with tunable boundaries that leaks tunable random radiation patterns that couple differently to the environment’s modes. Third, it is shown that objects may be precisely localized in complex environments even if they are non-cooperative by establishing signatures of their location that leverage their scattering contribution; this is demonstrated both with a frequency diverse and a wavefront shaping scheme, and the equivalence of the respective degrees of freedom is established.

Metapovrchy sú nanoštruktúrované povrchy vytvorené za účelom špecifického ovládania propagácie svetla. Predstavujú revolúciu v oblastiach ultratenkých optických prvkov a nanofotonických obvodov. Zakomponovaním laditeľných dielektrických materiálov do metapovrchov sa otvára možnosť aktívne ovládať ich optické vlastnosti aj po tom, čo boli vyrobené. Oxid vanadičitý (VO2) takéto ladenie umožňuje vďaka svojej fázovej premene už pri teplote okolo 67°C a preto sa radí k najsľubnejším z laditeľných dielektrických materiálov. Nakoľko je možné postupnú fázovú premenu vo VO2 vybudiť opticky a lúč svetla je možné fokusovať do stopy s veľkosťou pár stoviek nanometrov, laditeľné metapovrchy obsahujúce VO2 by mohli byť ladené postupne a dokonca s nanometrovým rozlíšením. V tejto práci skúmame fázu a amplitúdu svetla po prechode VO2 nanoštruktúrami usporiadanými do metapovrchu navrhnutého pre viditeľnú zložku elektromagnetického žiarenia. Výskum fáze a amplitúdy je založený na numerických simuláciách VO2 nanoštruktúr (stavebných kameňov metapovrchov), ktoré sú následne overené experimentálnymi výsledkami. VO2 nanoštruktúry vykazujú taktiež Mieho dielektrické rezonancie, ktoré sú v závere tejto práce využité v postupne laditeľnom metapovrchu fungujúcom vo viditeľnej oblasti. Okrem termálneho ladenia je možné vyrobený metapovrch ovládať taktiež opticky, čo dokazuje možnosť postupného ladenia na nanometrových rozmeroch.

Metasurfaces, introduced as a compact 2D alternative of metamaterials, have developed into a vast field in recent times for light manipulation at an ultra-compact scale. Metasurface applications have found a place in the literature for compact alternatives to lens, holograms, polarizers, color filters. Plasmonic metasurfaces consisting of noble metals such as gold and silver provide light confinement on an unprecedented scale. Gold and silver grown conventionally on transparent substrates are polycrystalline, and exhibit losses and limit performance of the device. Moreover, these materials have a lower damage threshold and melting point. To circumvent the lower melting point and damage thresholds, new materials, and material growing techniques need to be researched. <br>In the first part of this work, a metasurface for color holography with an epitaxially grown silver thin film on a transparent substrate is shown. The demonstrated metasurface has been the first ever epitaxial silver metasurface that operated in the transmission mode. This plasmonic hologram has also been the thinnest metasurface hologram operating in transmission mode at the time of its reporting. The holographic image of all three basic color components of red, green, and blue has been demonstrated in the transmission mode. The control of color has been achieved by resonant sub-wavelength slits and the phase can be manipulated through altering slit orientation. This amplitude and phase control pave the way to applications of ultra-compact polychromatic plasmonic metasurfaces for advanced light manipulation. In the second part, we explore temperature rise due to the optical absorption in plasmonic structures. Titanium Nitride based metasurfaces structures are fabricated, that work in harsh environmental conditions and high temperature. A time domain thermo reflectance technique for rapid measurement of temperature is explored. Finally, a practical design prototype for thermo-photovoltaic (TPV) emitters using plasmonic metasurfaces is fabricated and characterized.<br><br>