Plasmonic metasurfaces provide a novel platform for designing and implementing optical functional devices with distinguished advantages of their compactness and ultrathin footprint over traditional optical elements. The constituent metallic structures, or so-called "meta-atoms" or "meta-molecule" can interact with light at a subwavelength scale and introduce local modulations over multiple degrees of freedom like amplitude, phase, polarization, etc. The specific functions of the devices are then realized by assembling those meta-atoms together to form a planar interface with predesigned distributions. In this thesis we mainly studied nonlinear plasmonic metasurfaces made of gold meta-atoms for second harmonic generations (SHG). These metasurfaces work in the near infrared regime, and exhibit spin-controlled nonlinear responses due to the nonlinear geometric Pancharatnam-Berry phase-based designs. Firstly, a quasicrystal metasurface was demonstrated to modulate the far-field second harmonic radiations based on both the local symmetry of the meta-atoms and the global symmetry of the lattice those meta-atoms adhere to. Our designs of the nonlinear optical quasicrystal metasurfaces are based on the well-known Penrose tiling and the newly found bronze-mean hexagonal quasiperiodic tiling. The optical diffraction behaviors are studied in both linear and nonlinear regimes to reveal the effects of local and global symmetries on the far-field radiations. Secondly, a polarization manipulation metasurface was designed to encode a grayscale image into the polarization profiles of the generated second harmonic waves. We use single meta-atoms to manipulate the polarization directions of the second harmonic waves into predefined directions. With homogenous intensity profiles, the vectorial second harmonic beam can encode and decode information securely. At last, we utilized the state-of-the-art nano-kirigami technology to design and fabricate a three-dimensional plasmonic metasurface, which exhibits giant nonlinear circular dichroism in second harmonic generations. The second harmonic generations from the metasurface is much stronger when pumping by right circularly polarized fundamental waves than left circularly polarized ones. Broadband near-unity nonlinear circular dichroism was observed and numerical models were developed to explain the phenomenon. We believe that our works presented in this thesis enriched the study of plasmonic metasurfaces in the nonlinear optical regimes, and may be used to design novel nonlinear light sources, encryption applications, chiroptical devices, etc.

Plasmonic metasurfaces provide a novel platform for designing and implementing optical functional devices with distinguished advantages of their compactness and ultrathin footprint over traditional optical elements. The constituent metallic structures, or so-called "meta-atoms" or "meta-molecule" can interact with light at a subwavelength scale and introduce local modulations over multiple degrees of freedom like amplitude, phase, polarization, etc. The specific functions of the devices are then realized by assembling those meta-atoms together to form a planar interface with predesigned distributions. In this thesis we mainly studied nonlinear plasmonic metasurfaces made of gold meta-atoms for second harmonic generations (SHG). These metasurfaces work in the near infrared regime, and exhibit spin-controlled nonlinear responses due to the nonlinear geometric Pancharatnam-Berry phase-based designs. Firstly, a quasicrystal metasurface was demonstrated to modulate the far-field second harmonic radiations based on both the local symmetry of the meta-atoms and the global symmetry of the lattice those meta-atoms adhere to. Our designs of the nonlinear optical quasicrystal metasurfaces are based on the well-known Penrose tiling and the newly found bronze-mean hexagonal quasiperiodic tiling. The optical diffraction behaviors are studied in both linear and nonlinear regimes to reveal the effects of local and global symmetries on the far-field radiations. Secondly, a polarization manipulation metasurface was designed to encode a grayscale image into the polarization profiles of the generated second harmonic waves. We use single meta-atoms to manipulate the polarization directions of the second harmonic waves into predefined directions. With homogenous intensity profiles, the vectorial second harmonic beam can encode and decode information securely. At last, we utilized the state-of-the-art nano-kirigami technology to design and fabricate a three-dimensional plasmonic metasurface, which exhibits giant nonlinear circular dichroism in second harmonic generations. The second harmonic generations from the metasurface is much stronger when pumping by right circularly polarized fundamental waves than left circularly polarized ones. Broadband near-unity nonlinear circular dichroism was observed and numerical models were developed to explain the phenomenon. We believe that our works presented in this thesis enriched the study of plasmonic metasurfaces in the nonlinear optical regimes, and may be used to design novel nonlinear light sources, encryption applications, chiroptical devices, etc.

Flat-optics enable the miniaturization of many traditional bulk photonic devices routinely used in optical modulation and detection in telecommunication systems, biosensing and microscopic imaging in biomedical research, and light detection and ranging (LiDAR) used in automobile, military and surveillance applications. The backbone of typical flat-optic devices are the metasurfaces comprising structured nanoparticle lattices embedded in flat layer of traditional dielectric or semiconductor optical materials. The metasurface lattices can create optical resonances by exploiting different aspects of the light-matter interaction, e.g., light absorption, radiation, scattering and diffraction by the nanoparticles array. Such resonances are essential for the efficient optical interactions performed by the flat-optic devices, for example, enhancing nonlinear second-harmonic generation for optical frequency modulators, or enhancing light absorption in photodetectors. This Ph.D. dissertation reports the mechanisms of exciting and tailoring the metasurface lattice-controlled resonances using metal nanoparticle arrays. Exhibiting localized surface plasmon effects, metal particles can dramatically enhance the light field intensity under resonance conditions. Nevertheless, by nature, metal particles concurrently exhibit high absorption, radiation, and scattering losses, which cannot be sufficiently suppressed by the localized surface plasmon resonances. Almost two decades ago, researchers theoretically estimated that the benefits of the plasmonic field enhancement could still be harnessed by suppressing the scattering loss by organizing such lossy metal particles in a periodic lattice formation. In contrast to the low-Q localized resonances, such an engineered lattice arrangement could excite high-Q nonlocalized resonances, which are now often called as lattice plasmon or surface lattice resonances. Notwithstanding, the efforts on the experimental validation of such a concept were not succeeding as per expectation in terms of the resonance Q-factors. Thus, prior to the work accomplished in this dissertation, it was largely believed by the photonics community that, it is the 'lossy' plasmonic metal particles that do not allow to excite the high-Q resonances as per the minimum requirements in the practical flat optic applications. As a primary contribution to my Ph.D. dissertation, we successfully debunk that myth. In our work, we systematically proved that the non-localized lattice resonances can still be excited in 'lossy' metal nanoparticle arrays. Precisely, we improved both the design of the metasurface lattices and their fabrication and characterization techniques to eventually observe the high-Q lattice resonances as per the theoretical prediction. Our primary success later inspired us to analyze the systems more profoundly to make them suitable for different types of practical applications, which ultimately resulted in additional secondary successful projects described in my Ph.D. dissertation. The success of these projects would allow us in the future to utilize the nonlocalized plasmonic metasurface lattice-controlled resonances in a diverse range of flat integrated photonics applications, such as free-space light modulation and detection, which may rely on the nonlinear or electro-optical light-matter interaction in the flat thin-film region. We believe that the outcome of this dissertation will pave the way to designing and manufacturing efficient flat meta-optic devices for real-life applications, particularly in the telecommunication and medical sectors for the utmost betterment of human civilization.

Metals interact very differently with light than with radio waves and finite conductivities and losses often limit the way that RF concepts can be directly transferred to higher frequencies. Plasmonic materials are investigated here for various optical applications, since they can interact, confine and focus light at the nanoscale; however, regular plasmonic devices are severely limited by frequency dispersion and absorption, and confined signals cannot travel along plasmonic lines over few wavelengths. For these reasons, novel concepts and materials should be introduced to successfully manipulate and radiate light in the same flexible way we operate at lower frequencies. In line with these efforts, optical metamaterials exploit the resonant wave interaction of collections of plasmonic nanoparticles to produce anomalous light effects, beyond what naturally available in optical materials and in their basic constituents. Still, these concepts are currently limited by a variety of factors, such as: (a) technological challenges in realizing 3-D bulk composites with specific nano-structured patterns; (b) inherent sensitivity to disorder and losses in their realization; (c) not straightforward modeling of their interaction with nearby optical sources. In this study, we develop a novel paradigm to use single-element nanoantennas, and composite nanoantenna arrays forming two-dimensional metasurfaces and three-dimensional metamaterials, to control and manipulate light and its polarization at the nanoscale, which can possibly bypass the abovementioned limitations in terms of design procedure and experimental realization. The final design of some of the metamaterial concepts proposed in this work was inspired by biological species, whose complex structure can exhibit superior functionalities to detect, control and manipulate the polarization state of light for their orientation, signaling and defense. Inspired by these concepts, we theoretically investigate and design metasurfaces and metamaterial models with the help of fully vectorial numerical simulation tools, and we are able to outline the limitations and ultimate conditions under which the average optical surface impedance concept may accurately describe the complex wave interaction with planar plasmonic metasurfaces. We also experimentally explore various technological approaches compatible with these goals, such as the realization of lithographic single-element nanoantenna and nanoantenna arrays with complex circuit loads, periodic arrays of plasmonic nanoparticles or nanoapertures, and stacks of rotated plasmonic metasurfaces. At the conclusion of this effort, we have theoretically analyzed, designed and experimentally realized and characterized the feasibility of using discrete metasurfaces to realize phenomena and performance that are not available in natural materials, oftentimes inspired by the biological world. / text

In this letter, we propose all-dielectric Huygens' metasurface structures to construct high numerical aperture flat lenses and beam deflecting devices. The designed metasurface consists of two-dimensional array of all dielectric nanodisk resonators with spatially varying radii, thereby introducing judiciously designed phase shift to the propagating light. Owing to the overlap of Mie-type magnetic and electric resonances, high transmission was achieved with rigorous design analysis. The designed flat lenses have numerical aperture value of 0.85 and transmission values around 80%. It also offers easy fabrication and compatibility with available semiconductor technology. This spectrally and physically scalable, versatile design could implement efficient wavefront manipulation or beam shaping for high power laser beams, as well as various optical microscopy applications without requiring plasmonic structures that are susceptible to ohmic loss of metals and sensitive to the polarization of light.

Creating compact, efficient, highly-controllable optical systems has been one of the central goals of optics and photonics research. Integrated photonics provides a powerful platform for manipulating light efficiently and flexibly by guiding light in waveguide circuits on chip. Among the rich family of integrated photonic devices, integrated optical modulators and wavefront generators are two types of components for a great many applications such as optical communications, VR/AR displays, and LiDAR. Current approaches to creating these two types of devices – integrated optical modulators based on waveguides, active wavefront transceivers based on phased arrays, and passive wavefront transceivers based on grating couplers or integrated metasurfaces – suffer from large footprint, high power consumption, low beam quality, and limited controllability. It is desirable to improve the performance of such devices by exploring new device physics and architectures.In this thesis, we propose and investigate several novel approaches for efficient optical modulation and wavefront manipulation using integrated photonics. First, we show that efficient optical phase modulation can be achieved using a micro-resonator operating in the strongly over-coupled regime. Theoretical analysis, simulations, and experimental demonstrations of thermally tuned silicon nitride adiabatic micro-ring resonators operating at the visible and near-infrared wavelengths are conducted. Compared with traditional waveguide-based devices, our resonator-based phase modulators operating at the visible wavelengths showed order-of-magnitude reductions in both device footprint and power consumption. Through a statistical study of the device performance, our adiabatic micro-ring device architecture also showed significantly improved robustness against fabrication variations when compared with the regular micro-ring architecture. Second, we invent a new category of integrated wavefront-shaping devices – leaky-wave metasurfaces – that possess the simple form factor of a grating coupler and the capability of complete wavefront manipulation over all the four optical degrees of freedom: amplitude, phase, polarization ellipticity, and polarization orientation. The working principle of the leaky-wave metasurfaces is based on symmetry-broken photonic crystal slabs supporting quasi-bound states in the continuum (q-BICs). We extended the mechanism of q-BICs excited by free-space planewaves into q-BICs excited by guided waves, and developed a semi-analytical model describing the mapping between the four structural parameters and the four optical parameters of a meta-unit. We experimentally demonstrated multiple leaky-wave metasurface devices that convert light confined in an optical waveguide to an arbitrary optical pattern in free space, realizing custom polarization control, phase-amplitude control, and complete wavefront control, and validating the theory and capability of this platform. Lastly, we explore strategies to optimize the beam quality and efficiency of integrated optical phased arrays. We show that a two-dimensional disordered hyperuniform array layout is promising for generating a radiation pattern with high directionality with performance surpassing uniform arrays, constrained random arrays, and non-redundant arrays. We experimentally demonstrated a passive 32-channel phased array operating at the blue wavelength that showed a high percentage of power in the main beam and suppressed side lobes. We further propose and discuss the use of efficient, resonator-based modulators in phased arrays to improve the system compactness, power efficiency, and scalability. The approaches we investigated in this thesis provide a concrete set of solutions for interfacing free-space optics and integrated photonics. These two platforms have traditionally been studied by investigators from different subfields of optics and have led to commercial products addressing different needs. Our work suggests new ways to create “hybrid” systems consisting of partly integrated photonics and partly free-space optics and utilize the best of both worlds to address many emerging applications such as quantum optics, optogenetics, sensor networks, inter-chip communications, and holographic displays.

Yes

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24626号 / 工博第5132号 / 新制||工||1981(附属図書館) / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 田中 勝久, 教授 三浦 清貴, 教授 藤田 晃司 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Des propriétés électromagnétiques nouvelles peuvent être réalisées à l'aide d'une méta-surface à travers des structures artificielles. La permittivité et la perméabilité effectives d'une méta-surface peuvent être conçues de façon flexible et même accordées de sorte à présenter des réponses électromagnétiques pouvant être très différentes de celles de leurs homologues naturels, ce qui conduit à des propriétés améliorées voire parfois à un comportement extraordinaire. Cette thèse porte sur la conception, la fabrication et l'expérimentation de méta-surfaces micro-fluidiques pour le contrôle de propriétés des ondes électromagnétiques. Leur réalisation est basée sur des technologies relevant de la photolithographie et de la micro-fluidique, mises en œuvre sur des substrats souples d'épaisseur sub-longueur d'onde. Plus spécifiquement, nous avons exploité l'incorporation de divers matériaux dans un réseau de canaux micro-fluidiques, y compris des diélectriques liquides, un métal liquide et un métal solide pour manipuler davantage les réponses électromagnétiques des méta-surfaces correspondantes, telles que l'absorption, la transmission et la chiralité. La première partie de la thèse présente une méta-surface très absorbante sur une ultra-large bande spectrale et. Elle est constituée d'un réseau de résonateurs formés de gouttelettes d'eau noyées dans le matériau diélectrique souple, le PDMS; l’absorption mesurée est presque parfaite sur les bandes Ku, K et Ka. La seconde partie de la thèse porte sur un absorbeur agile et indépendant de l'angle dans la gamme Térahertz ; il s’agit d’une méta-surface à base de métal liquide, où un réseau de puits métalliques liquides dont la hauteur est contrôlée de façon continue, ce qui brise la limitation d'accordabilité dans le plan 2D. La troisième partie de la thèse porte sur une méta-surface chirale active. La méta-surface peut être commutée de achiral à chiral en déformant la structure en spirale initialement plane vers une géométrie 3D. Cette fonctionnalité peut manipuler la transmission hyperfréquence de symétrique à asymétrique sous incidence avant et arrière. En conclusion, l'optimisation de l'absorption, de la transmission et de la chiralité d’ondes électromagnétiques a été réalisée grâce à des méta-surfaces micro-fluidiques, qui semblent ainsi présenter un important potentiel applicatif dans divers domaines tels que la technologie furtive, l'imagerie et la communication optique / Novel and tailored electromagnetic properties can be realized using a metasurface through artificially designed structures. The effective permittivity and permeability of a metasurface can be flexibly designed and even tuned so as to exhibit electromagnetic responses that can be very different from those of their natural counterparts, leading to enhanced properties and sometimes to extra-ordinary behaviour. This thesis focuses on the design, fabrication and experimentation of meta-liquid-based metasurfaces for electromagnetic wave control and modulation. These metasurfaces are based on the use of both photolithography-based microfabrication and microfluidic technologies implemented onto thin and flexible substrates of sub-wavelength thickness. More specifically, the incorporation within a microfluidic channel network of various materials, including liquid dielectric material, liquid metal and solid metal have been exploited to further manipulate the electromagnetic responses of the related metasurfaces, such as the absorption, transmission and chirality. The first part of the thesis reports an ultra-broadband and wide-angle absorbing material by water-resonator-based metasurface. It consists of an array of water droplets embedded in the soft dielectric material, PDMS; it exhibited an almost perfect absorptivity over the Ku, K and Ka bands. The second part of the thesis focuses on a frequency-agile and wide-angle absorber in terahertz by liquid-metal-based metasurface, where a liquid-metal-pillar array can be continuously controlled in the vertical direction hence breaking the tuning limitation in the 2D plane. The third part of the thesis focuses on an active chiral metasurface. The metasurface can be switched from achiral to chiral by changing the spiral structure from planar pattern to 3D pattern. This functionality can manipulate the microwave transmission from symmetric to asymmetric under forward and backward incidence. In conclusion, tunability on the absorption, transmission and chirality have been realized through microfluidic metasurfaces, which appear having high potential applications in various areas such as stealth technology, imaging system, and optical communication, to name a few

Metasurfaces which emerged as two-dimensional counterparts of metamaterials, facilitate the realization of arbitrary phase distributions using large arrays with subwavelength and ultra-thin features. Even if metasurfaces are ultra-thin, they still effectively manipulate the phase, amplitude, and polarization of light in transmission or reflection mode. In contrast, conventional optical components are bulky, and they lose their functionality at sub-wavelength scales, which requires conceptually new types of nanoscale optical devices. On the other hand, as the optical systems shrink in size day by day, conventional bulky optical components will have tighter alignment and fabrication tolerances. Since metasurfaces can be fabricated lithographically, alignment can be done during lithographic fabrication, thus eliminating the need for post-fabrication alignments. In this work, various types of metasurface applications are thoroughly investigated for robust wavefront engineering with enhanced characteristics in terms of broad bandwidth, high efficiency and active tunability, while beneficial for application. Plasmonic metasurfaces are not compatible with the CMOS process flow, and, additionally their high absorption and ohmic loss is problematic in transmission based applications. Dielectric metasurfaces, however, offer a strong magnetic response at optical frequencies, and thus they can offer great opportunities for interacting not only with the electric component of a light field, but also with its magnetic component. They show great potential to enable practical device functionalities at optical frequencies, which motivates us to explore them one step further on wavefront engineering and imaging sensor platforms. Therefore, we proposed an efficient ultra-thin flat metalens at near-infrared regime constituted by silicon nanodisks which can support both electric and magnetic dipolar Mie-type resonances. These two dipole resonances can be overlapped at the same frequency by varying the geometric parameters of silicon nanodisks. Having two resonance mechanisms at the same frequency allows us to achieve full (0-2π) phase shift on the transmitted beam. To enable the miniaturization of pixel size for achieving high-resolution, planar, compact-size focal plane arrays (FPAs), we also present and explore the metasurface lens array-based FPAs. The investigated dielectric metasurface lens arrays achieved high focusing efficiency with superior optical crosstalk performance. We see a magnificent application prospect for metasurfaces in enhancing the fill factor and reducing the pixel size of FPAs and CCD, CMOS imaging sensors as well. Moreover, it is of paramount importance to design metasurfaces possessing tunable properties. Thus, we also propose a tunable beam steering device by combining phase manipulating metasurfaces concept and liquid crystals. Tunability feature is implemented by nematic liquid crystals infiltrated into nano holes in SiO2. Using electrically tunable nematic liquid crystals, dynamic beam steering is achieved