Showing papers on "Metamaterial published in 2020"

Physics-informed neural networks for inverse problems in nano-optics and metamaterials.

Abstract: In this paper, we employ the emerging paradigm of physics-informed neural networks (PINNs) for the solution of representative inverse scattering problems in photonic metamaterials and nano-optics technologies. In particular, we successfully apply mesh-free PINNs to the difficult task of retrieving the effective permittivity parameters of a number of finite-size scattering systems that involve many interacting nanostructures as well as multi-component nanoparticles. Our methodology is fully validated by numerical simulations based on the finite element method (FEM). The development of physics-informed deep learning techniques for inverse scattering can enable the design of novel functional nanostructures and significantly broaden the design space of metamaterials by naturally accounting for radiation and finite-size effects beyond the limitations of traditional effective medium theories.

Tunable and Active Phononic Crystals and Metamaterials

Abstract: Phononic crystals (PCs) and metamaterials (MMs) can exhibit abnormal properties, even far beyond those found in nature, through artificial design of the topology or ordered structure of unit cells. This emerging class of materials has diverse application potentials in many fields. Recently, the concept of tunable PCs or MMs has been proposed to manipulate a variety of wave functions on demand. In this review, we survey recent developments in tunable and active PCs and MMs, including bandgap and bandgap engineering, anomalous behaviors of wave propagation, as well as tunable manipulation of waves based on different regulation mechanisms: tunable mechanical reconfiguration and materials with multifield coupling. We conclude by outlining future directions in the emerging field.

Structured graphene metamaterial selective absorbers for high efficiency and omnidirectional solar thermal energy conversion

Abstract: An ideal solar-thermal absorber requires efficient selective absorption with a tunable bandwidth, excellent thermal conductivity and stability, and a simple structure for effective solar thermal energy conversion. Despite various solar absorbers having been demonstrated, these conditions are challenging to achieve simultaneously using conventional materials and structures. Here, we propose and demonstrate three-dimensional structured graphene metamaterial (SGM) that takes advantages of wavelength selectivity from metallic trench-like structures and broadband dispersionless nature and excellent thermal conductivity from the ultrathin graphene metamaterial film. The SGM absorbers exhibit superior solar selective and omnidirectional absorption, flexible tunability of wavelength selective absorption, excellent photothermal performance, and high thermal stability. Impressive solar-to-thermal conversion efficiency of 90.1% and solar-to-vapor efficiency of 96.2% have been achieved. These superior properties of the SGM absorber suggest it has a great potential for practical applications of solar thermal energy harvesting and manipulation. Here, the authors demonstrate a selective solar thermal absorber with wavelength selectivity, arising from metallic trench-like structures, using broadband dispersionless ultrathin graphene metamaterial film, with excellent thermal conductivity.

Nonreciprocity in acoustic and elastic materials

Abstract: The law of reciprocity in acoustics and elastodynamics codifies a relation of symmetry between action and reaction in fluids and solids. In its simplest form, it states that the frequency-response functions between any two material points remain the same after swapping source and receiver, regardless of the presence of inhomogeneities and losses. As such, reciprocity has enabled numerous applications that make use of acoustic and elastic wave propagation. A recent change in paradigm has prompted us to see reciprocity under a new light: as an obstruction to the realization of wave-bearing media in which the source and receiver are not interchangeable. Such materials may enable the creation of devices such as acoustic one-way mirrors, isolators and topological insulators. Here, we review how reciprocity breaks down in materials with momentum bias, structured space-dependent and time-dependent constitutive properties, and constitutive nonlinearity, and report on recent advances in the modelling and fabrication of these materials, as well as on experiments demonstrating nonreciprocal acoustic and elastic wave propagation therein. The success of these efforts holds promise to enable robust, unidirectional acoustic and elastic wave-steering capabilities that exceed what is currently possible in conventional materials, metamaterials or phononic crystals. Nonreciprocal acoustic and elastic wave propagation may enable the creation of devices such as acoustic one-way mirrors, isolators and topological insulators. This Review presents advances in the creation of materials that break reciprocity and realize robust, unidirectional acoustic and elastic wave steering.

A Comprehensive Survey on “Various Decoupling Mechanisms With Focus on Metamaterial and Metasurface Principles Applicable to SAR and MIMO Antenna Systems”

Abstract: Nowadays synthetic aperture radar (SAR) and multiple-input-multiple-output (MIMO) antenna systems with the capability to radiate waves in more than one pattern and polarization are playing a key role in modern telecommunication and radar systems. This is possible with the use of antenna arrays as they offer advantages of high gain and beamforming capability, which can be utilized for controlling radiation pattern for electromagnetic (EM) interference immunity in wireless systems. However, with the growing demand for compact array antennas, the physical footprint of the arrays needs to be smaller and the consequent of this is severe degradation in the performance of the array resulting from strong mutual-coupling and crosstalk effects between adjacent radiating elements. This review presents a detailed systematic and theoretical study of various mutual-coupling suppression (decoupling) techniques with a strong focus on metamaterial (MTM) and metasurface (MTS) approaches. While the performance of systems employing antenna arrays can be enhanced by calibrating out the interferences digitally, however it is more efficient to apply decoupling techniques at the antenna itself. Previously various simple and cost-effective approaches have been demonstrated to effectively suppress unwanted mutual-coupling in arrays. Such techniques include the use of defected ground structure (DGS), parasitic or slot element, dielectric resonator antenna (DRA), complementary split-ring resonators (CSRR), decoupling networks, P.I.N or varactor diodes, electromagnetic bandgap (EBG) structures, etc. In this review, it is shown that the mutual-coupling reduction methods inspired By MTM and MTS concepts can provide a higher level of isolation between neighbouring radiating elements using easily realizable and cost-effective decoupling configurations that have negligible consequence on the array’s characteristics such as bandwidth, gain and radiation efficiency, and physical footprint.

Spacetime Metamaterials—Part I: General Concepts

Abstract: This article deals with the general concepts underpinning spacetime metamaterials and related systems. It first introduces spacetime metamaterials as a generalization of (bianisotropic) metamaterials, presented in the holistic perspective of direct and inverse spacetime scattering, where spacetime variance and dispersion offer unprecedented medium diversity despite some limitations related to the uncertainty principle. Then, it describes the fundamental physical phenomena occurring in spacetime systems, such as frequency transitions, nonreciprocity, Fizeau dragging, bianisotropy transformation, and superluminality, allowed when the medium moves perpendicularly to the direction of the wave. Next, it extends some principles and tools of relativity physics, particularly a medium-extended version of the spacetime (or Minkowski) diagrams, elaborates a general strategy to compute the fields scattered by spacetime media, and presents a gallery of possible spacetime media, including the spacetime step discontinuity, which constitutes the building brick of any spacetime metamaterial. Finally, the conclusion section provides a list of 16 items that concisely summarizes the key results and teachings of the overall document. The second part establishes the theory and overviews some current and potential applications of spacetime metamaterials.

A Comprehensive Survey of “Metamaterial Transmission-Line Based Antennas: Design, Challenges, and Applications”

Abstract: In this review paper, a comprehensive study on the concept, theory, and applications of composite right/left-handed transmission lines (CRLH-TLs) by considering their use in antenna system designs have been provided. It is shown that CRLH-TLs with negative permittivity (e <; 0) and negative permeability (μ <; 0) have unique properties that do not occur naturally. Therefore, they are referred to as artificial structures called “metamaterials”. These artificial structures include series left-handed (LH) capacitances (C L ), shunt LH inductances (L L ), series right-handed (RH) inductances (LR), and shunt RH capacitances (CR) that are realized by slots or interdigital capacitors, stubs or via-holes, unwanted current flowing on the surface, and gap distance between the surface and ground-plane, respectively. In the most cases, it is also shown that structures based on CRLH metamaterial-TLs are superior than their conventional alternatives, since they have smaller dimensions, lower-profile, wider bandwidth, better radiation patterns, higher gain and efficiency, which make them easier and more cost-effective to manufacture and mass produce. Hence, a broad range of metamaterial-based design possibilities are introduced to highlight the improvement of the performance parameters that are rare and not often discussed in available literature. Therefore, this survey provides a wide overview of key early-stage concepts of metematerial-based designs as a thorough reference for specialist antennas and microwave circuits designers. To analyze the critical features of metamaterial theory and concept, several examples are used. Comparisons on the basis of physical size, bandwidth, materials, gain, efficiency, and radiation patterns are made for all the examples that are based on CRLH metamaterialTLs. As revealed in all the metematerial design examples, foot-print area decrement is an important issue of study that have a strong impact for the enlargement of the next generation wireless communication systems.

Projection micro stereolithography based 3D printing and its applications

Abstract: Projection Micro Stereolithography (PμSL) is a high-resolution (up to 0.6 μm) 3D printing technology based on area projection triggered photopolymerization, and capable of fabricating complex 3D architectures covering multiple scales and with multiple materials. This paper reviews the recent development of the PμSL based 3D printing technologies, together with the related applications. It introduces the working principle, the commercialized products, and the recent multiscale, multimaterial printing capability of PμSL as well as some functional photopolymers that are suitable to PμSL. This review paper also summarizes a few typical applications of PμSL including mechanical metamaterials, optical components, 4D printing, bioinspired materials and biomedical applications, and offers perspectives on the directions of the further development of PμSL based 3D printing technology.

Spacetime Metamaterials—Part II: Theory and Applications

Abstract: The overall article presents the authors’ vision of the emerging field of spacetime metamaterials, and related systems, in a cohesive and pedagogical perspective, systematically building up the physics, modeling, and applications of these media upon the foundation of their pure-space and pure-time counterparts. Following the first part, dealing with the general concepts underpinning spacetime metamaterials, this part establishes the theory of spacetime metamaterials and overviews some of their current and potential applications. It first describes the scattering phenomenology of a spacetime interface, the building brick of any spacetime metamaterial, and deduces the corresponding electromagnetic boundary conditions. Upon this basis, it derives the spacetime interface scattering (Fresnel-like) coefficients and frequency transitions, and subsequently generalizes time reversal to spacetime compansion (compression and expansion). Then, it illustrates the new physics of spacetime metamaterials with the examples of spacetime mirrors and cavities, the inverse prism and chromatic birefringence, and spacetime crystals. Next, it discusses various applications—categorized as frequency multiplication and mixing, matching and filtering, nonreciprocity and absorption, cloaking, electromagnetic processing, and radiation. Finally, the conclusion section provides a list of eight items that concisely summarizes the key results of this article, in completion to the list related to the general concepts in Part I.

Tunneling-induced negative permittivity in Ni/MnO nanocomposites by a bio-gel derived strategy

Abstract: One-dimensional wires are the most common building blocks in metamaterials. In this study, zero-dimensional nanoparticles connected by tunneling networks were used to construct metamaterials, thus providing a more flexible alternative for designing the geometrical configuration of metamaterials, particularly in nanodevices. The composites with nickel nanoparticles@MnO were prepared by a bio-gel derived strategy. Nickel nanoparticles were not connected geometrically, but the conductive network had been already formed, which was a tunneling-dominated percolative phenomenon demonstrated by the first-principles calculation. Negative permittivity was achieved in the composites, as the low-frequency plasmonic state could be generated in the tunneling nickel-networks. At the same time, negative susceptibility was observed due to the diamagnetism of the tunneling current loops. Electromagnetic simulations indicate that the composites have the potential for electromagnetic shielding (only 0.25 mm in thickness). It is believed that this study not only fills up the research gap in the influence of the tunneling effect on negative electromagnetic parameters but also opens up another way of preparing metamaterials by using zero-dimensional nanoparticles instead of one-dimensional wires.

Malus-metasurface-assisted polarization multiplexing

Abstract: Polarization optics plays a pivotal role in diffractive, refractive, and emerging flat optics, and has been widely employed in contemporary optical industries and daily life. Advanced polarization manipulation leads to robust control of the polarization direction of light. Nevertheless, polarization control has been studied largely independent of the phase or intensity of light. Here, we propose and experimentally validate a Malus-metasurface-assisted paradigm to enable simultaneous and independent control of the intensity and phase properties of light simply by polarization modulation. The orientation degeneracy of the classical Malus's law implies a new degree of freedom and enables us to establish a one-to-many mapping strategy for designing anisotropic plasmonic nanostructures to engineer the Pancharatnam-Berry phase profile, while keeping the continuous intensity modulation unchanged. The proposed Malus metadevice can thus generate a near-field greyscale pattern, and project an independent far-field holographic image using an ultrathin and single-sized metasurface. This concept opens up distinct dimensions for conventional polarization optics, which allows one to merge the functionality of phase manipulation into an amplitude-manipulation-assisted optical component to form a multifunctional nano-optical device without increasing the complexity of the nanostructures. It can empower advanced applications in information multiplexing and encryption, anti-counterfeiting, dual-channel display for virtual/augmented reality, and many other related fields.

Information Metamaterials: bridging the physical world and digital world

Abstract: Over the past 5 years, digital coding and programmable metamaterials have been developed rapidly since their first exhibition in 2014. The iconic feature of the digital coding metamaterial is using digital codes like “0” and “1” to represent the distinct electromagnetic (EM) responses. This seemingly trivial progress has successfully reform the design theory from the effective medium to coding patterns, bridging the physical world and digital information world. More interestingly, beyond the simple coding on the parameters or patterns, the digital coding metamaterials are more intend to introduce the concept of direct interactions and operations of digital information within EM fields, to realize information processing, transmission or recognition. To accurately exhibit the informational specialties, we classify the coding metamaterials, digital metamaterials and programmable metamaterials, as well as other information-operating metamaterials, as information metamaterials. In this review article, we firstly introduce the digital coding concept, working mechanism, and related design methods. Then, three important theories including the scattering pattern calculation, convolution operation, and entropy of digital coding metamaterials, are discussed in details. Finally we introduce several system-level works based on the information metamaterials, such as the new-architecture wireless communication systems and reprogrammable imaging systems, to show the powerful manipulation capabilities of information metamaterials. As the next generation of information metamaterials, two proof-of-concept smart metamaterials and their advanced architectures are discussed. In the summary, the development track of information metamaterials and future trends are presented.

Leveraging of MEMS Technologies for Optical Metamaterials Applications

Abstract: In recent years, metamaterials with artificially engineered sub-wavelength structure have shown great advancement in numerous interesting electromagnetic (EM) properties such as artificial magnetism,[1,2] negative refractive index,[3–7] metalenses,[8–13] wavelength selective absorption,[14–21] slow light behavior,[22–28] and chirality.[29–32] To actively control the metamaterial, various efforts have been developed such as the optically pumped photoconductive materials,[33,34] electrically controlled refractive index of liquid crystals,[35,36] biasing of doped semiconductor devices[37–40] or graphene,[41–43] thermally controlled refractive index of materials,[44,45] conductivity control in phase change materials,[46,47] magnetically controlled active materials,[48,49] and so on.[50–53] However, the intrinsic frequencydependent property of these materials hinders the spectral scalability. Some exotic materials are not complementary metal-oxide-semiconductor (CMOS) compatible and require bulky equipment for external stimulus, which limits commercialization and miniaturization. On the other hand, the most ideal and straightforward method for the reconfiguration is to geometrically modify the parameters of the unit cell, which is also called the meta-atom that determines the property of metamaterials. Furthermore, in terms of feature size, the microelectromechanical system (MEMS) and micro/ nanofluidics enable micro/nanoscale mechanical manipulation and are suitable for meta-atom in terahertz (THz) and IR region, which brings the diversified applications in metamaterial functional device. The advancement in MEMS and micro/nanofluidics offers a wide palette of actuators and liquid channels to enable both in-plane and out-of-plane reconfigurations with varying performance characteristics that could be realized based on the application requirements, ranging from fundamental functions, such as the modulation of intensity, frequency, bandwidth, and electromagnetically induced transparency (EIT) phenomenon, to more sophisticated devices, such as tunable waveplate, logic operation, and resonant cloaking. Beyond tunability, novel chemical sensing platforms in terms of gas, liquid, and thin film sensing of biomolecules can be realized through metamaterials resonators or the hybrid sensing platforms Tunable metamaterial devices have experienced explosive growth in the past decades, driving the traditional electromagnetic (EM) devices to evolve into diversified functionalities by manipulating EM properties such as amplitude, frequency, phase, polarization, and propagation direction. However, one of the bottlenecks of these rapidly developed metamaterials technologies is limited tunability caused by the intrinsic frequencydependent property of exotic tunable material. To overcome such limitation, the microelectromechanical system (MEMS) enabling micro/nanoscale manipulation is developed to actively control “meta-atom” in terahertz and infrared region, which brings frequency-scalable tunability and complementary metal-oxide-semiconductor-compatible functional metadevices. Beyond tunability, novel chemical sensing platforms of molecular identification and dynamic monitoring of the biochemical process can be achieved by integrating micro/nanofluidics channels with metamaterial resonators. Additionally, incorporating metamaterial absorbers with MEMS resonators brings another research interest in MEMS zero-power devices and radiation sensors. Furthermore, moving from 2D metasurfaces to 3D metamaterials, enhanced EM properties like novel resonance mode, giant chirality, and 3D manipulation reinforce the application in biochemical and physical sensors as well as functional meta-devices, paving the way to realize multi-functional sensing and signal processing on a hybrid smartsensor microsystem for booming healthcare, environmental monitoring, and the Internet of Things applications.

Broadband frequency translation through time refraction in an epsilon-near-zero material.

Abstract: Space-time duality in paraxial optical wave propagation implies the existence of intriguing effects when light interacts with a material exhibiting two refractive indexes separated by a boundary in time. The direct consequence of such time-refraction effect is a change in the frequency of light while leaving the wavevector unchanged. Here, we experimentally show that the effect of time refraction is significantly enhanced in an epsilon-near-zero (ENZ) medium as a consequence of the optically induced unity-order refractive index change in a sub-picosecond time scale. Specifically, we demonstrate broadband and controllable shift (up to 14.9 THz) in the frequency of a light beam using a time-varying subwavelength-thick indium tin oxide (ITO) film in its ENZ spectral range. Our findings hint at the possibility of designing (3 + 1)D metamaterials by incorporating time-varying bulk ENZ materials, and they present a unique playground to investigate various novel effects in the time domain. Here, the authors present an experimental demonstration of adiabatic frequency conversion using the concept of time boundary by exploiting the properties of an ITO film operating near its epsilon-near-zero frequency. They demonstrate a large and controllable shift up to 14.9 THz.

Inverse-designed spinodoid metamaterials

Abstract: After a decade of periodic truss-, plate-, and shell-based architectures having dominated the design of metamaterials, we introduce the non-periodic class of spinodoid topologies. Inspired by natural self-assembly processes, spinodoid metamaterials are a close approximation of microstructures observed during spinodal phase separation. Their theoretical parametrization is so intriguingly simple that one can bypass costly phase-field simulations and obtain a rich and seamlessly tunable property space. Counter-intuitively, breaking with the periodicity of classical metamaterials is the enabling factor to the large property space and the ability to introduce seamless functional grading. We introduce an efficient and robust machine learning technique for the inverse design of (meta-)materials which, when applied to spinodoid topologies, enables us to generate uniform and functionally graded cellular mechanical metamaterials with tailored direction-dependent (anisotropic) stiffness and density. We specifically present biomimetic artificial bone architectures that not only reproduce the properties of trabecular bone accurately but also even geometrically resemble natural bone.

Review of Metasurface Antennas for Computational Microwave Imaging

Abstract: This article covers recent advances in the fusion of metasurface antenna design and computational imaging (CI) concepts for the realization of imaging systems that are planar, fast, and low cost. We start by explaining the operation of metamaterial antennas which can generate diverse radiation patterns. Their advantages and distinctions from previous antennas are elucidated. We then provide an intuitive overview of the CI framework and argue that metamaterial antennas are a near ideal platform for implementing such schemes at microwave frequencies. We describe two metamaterial antenna implementations: frequency diverse and electronically reconfigurable. The tradeoffs governing the design and operation of each architecture are examined. We conclude by examining the outlook of metamaterial antennas for microwave imaging and propose various future directions.

A Review of Acoustic Metamaterials and Phononic Crystals

Abstract: As a new kind of artificial material developed in recent decades, metamaterials exhibit novel performance and the promising application potentials in the field of practical engineering compared with the natural materials. Acoustic metamaterials and phononic crystals have some extraordinary physical properties, effective negative parameters, band gaps, negative refraction, etc., extending the acoustic properties of existing materials. The special physical properties have attracted the attention of researchers, and great progress has been made in engineering applications. This article summarizes the research on acoustic metamaterials and phononic crystals in recent decades, briefly introduces some representative studies, including equivalent acoustic parameters and extraordinary characteristics of metamaterials, explains acoustic metamaterial design methods, and summarizes the technical bottlenecks and application prospects.

Antireflection temporal coatings

Abstract: It is known that complete transmission of waves through the interface between two different media can be achieved by proper impedance matching between them. One of the most common techniques for such reflectionless propagation is the quarter-wave impedance transformer, where an additional slab of material with proper material parameters and carefully engineered dimensions is added between the two media, minimizing reflections. Metamaterials, with properly designed spatial inhomogeneity, have exhibited unprecedented ability to tailor and manipulate waves, and recently temporal metamaterials have also gained much attention, enabling spatiotemporal control of wave propagation. Here a temporal analogue of the quarter-wave impedance transformer technique, which we name “antireflection temporal coating,” is proposed using time-dependent materials. The proposed technique is demonstrated, analytically and numerically, using metamaterials with a time-dependent permittivity. Comparison with the conventional (spatial) impedance-matching technique is shown, demonstrating that both impedance matching and frequency conversion are achieved with our proposed temporal version. As an illustrative example, the present technique is also applied to match two waveguides with different cross sections, demonstrating an example of scenarios where it may be applied.

4D Printing Auxetic Metamaterials with Tunable, Programmable, and Reconfigurable Mechanical Properties

Abstract: Auxetic mechanical metamaterials, which expand transversally when axially stretched, are widely used in flexible electronics and aerospace. However, these chiral metamaterials suffer from three severe limitations as a typical auxetic metamaterials: narrow strain range, non-tunable mechanical behaviors, and fixed properties after fabrication. In this work, 4D printing chiral metamaterials with tunable, programmable, and reconfigurable properties are developed. The deformation mode transforms from bending dominated to stretching dominated under large deformation, leading to the stress–deformation (σ–λ) behavior of the auxetic metamaterials similar to those of the biomaterials (“J”shaped), such as tissues or organs. The programmability and reconfigurability of the developed chiral metamaterials allow mechanical behavior to change between different biomaterials with high precision. Furthermore, scaffolds with personalized mechanical properties as well as configurations and metamaterials-based light-emitting diode integrated devices demonstrate potential applications in tissue engineering and programmable flexible electronics.

A dual-band metamaterial absorber for graphene surface plasmon resonance at terahertz frequency

Abstract: In this paper, we present a dual-band metamaterial absorber for graphene surface plasmon resonance at terahertz frequency. We use the finite difference time domain (FDTD) method to study the absorption characteristics of the homocentric graphene ring and disk nanostructure. These simulation results show that the change of the geometrical parameters and the substrate thickness of the nanostructure can change the absorption characteristics and the emergence of dual-band absorption peaks. Moreover, we study the field distribution of nanodisks with different radius in detail. By changing the Fermi level of graphene, the wavelength of their absorption peaks can be adjusted flexibly. In addition, the proposed dual-band absorber also shows a good angle tolerance for both TE and TM polarizations. By calculation the surface-filled water (n = 1.332) and 25% aqueous glucose solution (n = 1.372) for the metamaterial absorber, the sensitivities of mode I and mode II are 5.0 μm/RIU and 15.0 μm/RIU. These research results will have broad application prospects for sensing and spatial light modulators.

Guided transition waves in multistable mechanical metamaterials.

Abstract: Transition fronts, moving through solids and fluids in the form of propagating domain or phase boundaries, have recently been mimicked at the structural level in bistable architectures. What has been limited to simple one-dimensional (1D) examples is here cast into a blueprint for higher dimensions, demonstrated through 2D experiments and described by a continuum mechanical model that draws inspiration from phase transition theory in crystalline solids. Unlike materials, the presented structural analogs admit precise control of the transition wave’s direction, shape, and velocity through spatially tailoring the underlying periodic network architecture (locally varying the shape or stiffness of the fundamental building blocks, and exploiting interactions of transition fronts with lattice defects such as point defects and free surfaces). The outcome is a predictable and programmable strongly nonlinear metamaterial motion with potential for, for example, propulsion in soft robotics, morphing surfaces, reconfigurable devices, mechanical logic, and controlled energy absorption.

Low-loss metasurface optics down to the deep ultraviolet region

Abstract: Shrinking conventional optical systems to chip-scale dimensions will benefit custom applications in imaging, displaying, sensing, spectroscopy, and metrology. Towards this goal, metasurfaces-planar arrays of subwavelength electromagnetic structures that collectively mimic the functionality of thicker conventional optical elements-have been exploited at frequencies ranging from the microwave range up to the visible range. Here, we demonstrate high-performance metasurface optical components that operate at ultraviolet wavelengths, including wavelengths down to the record-short deep ultraviolet range, and perform representative wavefront shaping functions, namely, high-numerical-aperture lensing, accelerating beam generation, and hologram projection. The constituent nanostructured elements of the metasurfaces are formed of hafnium oxide-a loss-less, high-refractive-index dielectric material deposited using low-temperature atomic layer deposition and patterned using high-aspect-ratio Damascene lithography. This study opens the way towards low-form factor, multifunctional ultraviolet nanophotonic platforms based on flat optical components, enabling diverse applications including lithography, imaging, spectroscopy, and quantum information processing.

Continuous angle-tunable birefringence with freeform metasurfaces for arbitrary polarization conversion

Abstract: Birefringence occurs when light with different polarizations sees different refractive indices during propagation. It plays an important role in optics and has enabled essential polarization elements such as wave plates. In bulk crystals, it is typically constrained to linear birefringence. In metamaterials with freeform meta-atoms, however, one can engineer the optical anisotropy such that light sees different indices for arbitrary-linear, circular, or elliptical-orthogonal eigen-polarization states. Using topology-optimized metasurfaces, we demonstrate this arbitrary birefringence. It has the unique feature that it can be continuously tuned from linear to elliptical birefringence, by changing the angle of incidence. In this way, a single metasurface can operate as many wave plates in parallel, implementing different polarization transformations. Angle-tunable arbitrary birefringence expands the scope of polarization optics, enables compact and versatile polarization operations that would otherwise require cascading multiple elements, and may find applications in polarization imaging, quantum optics, and other areas.

Multidimensional manipulation of wave fields based on artificial microstructures

Abstract: Artificial microstructures, which allow us to control and change the properties of wave fields through changing the geometrical parameters and the arrangements of microstructures, have attracted plenty of attentions in the past few decades. Some artificial microstructure based research areas, such as metamaterials, metasurfaces and phononic topological insulators, have seen numerous novel applications and phenomena. The manipulation of different dimensions (phase, amplitude, frequency or polarization) of wave fields, particularly, can be easily achieved at subwavelength scales by metasurfaces. In this review, we focus on the recent developments of wave field manipulations based on artificial microstructures and classify some important applications from the viewpoint of different dimensional manipulations of wave fields. The development tendency of wave field manipulation from single-dimension to multidimensions provides a useful guide for researchers to realize miniaturized and integrated optical and acoustic devices.

Ultralow-loss geometric phase and polarization shaping by ultrafast laser writing in silica glass

Abstract: Polarization and geometric phase shaping via a space-variant anisotropy has attracted considerable interest for fabrication of flat optical elements and generation of vector beams with applications in various areas of science and technology. Among the methods for anisotropy patterning, imprinting of self-assembled nanograting structures in silica glass by femtosecond laser writing is promising for the fabrication of space-variant birefringent optics with high thermal and chemical durability and high optical damage threshold. However, a drawback is the optical loss due to the light scattering by nanograting structures, which has limited the application. Here, we report a new type of ultrafast laser-induced modification in silica glass, which consists of randomly distributed nanopores elongated in the direction perpendicular to the polarization, providing controllable birefringent structures with transmittance as high as 99% in the visible and near-infrared ranges and >90% in the UV range down to 330 nm. The observed anisotropic nanoporous silica structures are fundamentally different from the femtosecond laser-induced nanogratings and conventional nanoporous silica. A mechanism of nanocavitation via interstitial oxygen generation mediated by multiphoton and avanlanche defect ionization is proposed. We demonstrate ultralow-loss geometrical phase optical elements, including geometrical phase prism and lens, and a vector beam convertor in silica glass.

Investigation of graphene supported terahertz elliptical metamaterials

Abstract: Based on a graphene asymmetric sdouble ellipse (ADE) structure, the tunable Fano resonance characteristics in the terahertz range have been symmetrically studied, including the effects of graphene Fermi levels, asymmetrical degrees, operation frequencies and structural parameters The results show that due to the strong coupling between the incident THz waves and graphene ADE structures, a strong Fano peak can be observed by arranging double ellipse asymmetrically The maximum peak of Fano resonance reaches 0918, and its Q-factor is about 20 As the asymmetrical degree of the ADE structure increases, the Fano peak value increases, and the amplitude modulation depth reaches about 75% As graphene level increases, the amplitude and frequency modulation depths reaches more than 31% and 11%, respectively These results are very helpful to design novel tunable graphene devices with high Q-factor devices in the future, such as sensors, modulators, and filters

Serrodyne Frequency Translation Using Time-Modulated Metasurfaces

Abstract: Temporally modulated metamaterials have attracted significant attention recently due to their nonreciprocal and frequency converting properties. Here, a transparent, time-modulated metasurface, which functions as a serrodyne frequency translator, is reported at $X$ -band frequencies. With a simple biasing architecture, the metasurface provides electrically tunable transmission phase that covers 360°. A sawtooth waveform is used to modulate the metasurface, allowing Doppler-like frequency translation. Modal analysis of an analogous time-modulated medium is performed to gain insight into the operation of the metasurface-based serrodyne frequency translator. Two such metasurfaces can be cascaded together to achieve magnetless devices that perform either phase or amplitude nonreciprocity.

Graded elastic metasurface for enhanced energy harvesting

Abstract: In elastic wave systems, combining the powerful concepts of resonance and spatial grading within structured surface arrays enable resonant metasurfaces to exhibit broadband wave trapping, mode conversion from surface (Rayleigh) waves to bulk (shear) waves, and spatial frequency selection. Devices built around these concepts allow for precise control of surface waves, often with structures that are subwavelength, and utilise Rainbow trapping that separates the signal spatially by frequency. Rainbow trapping yields large amplifications of displacement at the resonator positions where each frequency component accumulates. We investigate whether this amplification, and the associated control, can be used to create energy harvesting devices; the potential advantages and disadvantages of using graded resonant devices as energy harvesters is considered. We concentrate upon elastic plate models for which the A 0 mode dominates, and take advantage of the large displacement amplitudes in graded resonant arrays of rods, to design innovative metasurfaces that trap waves for enhanced piezoelectric energy harvesting. Numerical simulation allows us to identify the advantages of such graded metasurface devices and quantify its efficiency, we also develop accurate models of the phenomena and extend our analysis to that of an elastic half-space and Rayleigh surface waves.