Metamaterial System Research Articles

A polarization-insensitive, wide-angle dual-band tunable graphene metamaterial perfect absorber with T-shaped strips and square ring

Metamaterial perfect absorbers play an essential role in many optoelectronic devices. In this paper, a dual-band tunable metamaterial perfect absorber based on graphene is proposed. The simulation results present that under normal incidence two absorption peaks of 99.9% and 99.9% occur at the frequencies 1.69 THz and 4.30 THz, respectively. Impedance matching theory is employed to elaborate this dual-band perfect absorption phenomenon. While at oblique incidence, the absorption of the absorber remains more than 90% over a wide incident angle from 0° to 75° for the transverse electric (TE) polarization and 60° for the transverse magnetic (TM) polarization separately. Furthermore, it is also independent to the polarization angles. In addition, the effects of different geometrical parameters and the chemical potential of graphene on the resonant frequencies are investigated in detail. The two peaks of the absorber can be dynamically tuned by the variation of the chemical potential of graphene. Due to its good performances, the designed metamaterial perfect system has great potential applications in biosensing, photodetectors, stealth, and imaging devices.

Design of Broadband Plasmon‐Induced Transparency Hybrid Metamaterial Based on the Interaction of the Metal and Dielectric Resonances

AbstractIn this paper, a broadband plasmon‐induced transparency (PIT) hybrid metamaterial system (HMS) is realized and numerically demonstrated via the interference between different resonance modes of metal and dielectric resonators. The employed PIT HMS supports a strong near‐fired coupling between unit cells consisting of eight TiO2 nested split rings (dark plasmonic resonators) surrounded by four metal cut wires (bright plasmonic resonators) embedded in the intermediate dielectric layer, and a broad transparency window which is higher than 0.9 is obtained from 0.61 to 0.76 THz with a relative bandwidth of 21.9%. In the transparent window, the drastic change of phase leads to high group delay, and the remarkable slow light phenomenon can be clearly observed. The maximum of the group delay and group index can reach 528 ps and 2880, respectively. The localization distributions of the electromagnetic field intuitively exhibit the generation and action process of destructive interference. The theoretical investigation of the two‐oscillator model further confirms the effectiveness and consistency of the simulation results. In addition, the fourfold symmetric of the proposed PIT HMS supports the liberation from the dependence of incident polarization form. Such an implementation opens a new path for the designing of filtering, switching, and data storing devices.

3D Imaging Method Based on Digital Coding Metasurface With Unknown Mutual Coupling

Three-dimensional (3D) imaging based on metamaterial radar system is a cutting-edge technique which elevates the hardware burden in traditional real-aperture radar and synthetic aperture radar (SAR), particularly with the enhanced systematic degree of freedom (DOF) by the emerging technique of digtal coding metasurface. This paper proposes a single-channel real-aperture 3D imaging method based on digital coding metasurface, which is simple and low-cost. The proposed system can generate random radiation field, and the target scene can be obtained by sparse reconstruction as in microwave correlation imaging (MCI) system. In this paper, we first establish the system signal model. Then, we propose the index of the DOF of the radiation field, and propose the accordingly optimization method by optimizing the digital code. And in practical applications, the coupling between metamaterial units introduces additional signal components, which affects the quality of the sparse recovery. In order to solve the coupling problem, we take the coupling coefficients into account in the priori assumptions for all the parameters in the sparse Bayesian learning, and use the maximum posterior probability method to estimate the unknown parameters. Iteratively, the MCI model can be updated with the obtained coupling information, guaranteeing a more accurate reconstruction of the target scene. The simulation results prove the effectiveness of our method.

Stimuli-responsive metamaterials with information-driven elastodynamics programming

<h2>Summary</h2> Biological stimuli-responsiveness has inspired material-based smart systems in various fields. The stimuli-responsive materials are generally used as building blocks to construct bioinspired systems with a programmable elastostatics property. However, the programmability of the elastodynamics property in stimuli-responsive material systems remains a major challenge. By mimicking species of the genus <i>Mimosa</i>, here we report a stimuli-responsive elastic metamaterial with shape-morphing resonators to realize the information-driven elastodynamics programming. The local resonance state of shape-morphing resonators can adaptively change from disorder to order upon external stimuli, which is programmed into the global dynamical responses of the metamaterial system. The designed metamaterials exhibit a mechanical implementation of read-write operations, leading to proprioception of stimulation information with capabilities of information interaction, perceptual coding, and physical encryption. Our work takes a significant step toward the information-driven elastodynamics programming in stimuli-responsive material systems, which may have a broad impact in design of devices with material-based proprioception for sensing, computation, and communication.

Utilizing reversible solid–liquid phase transition to tune phononic bandgaps

Phononic crystals (PCs) are periodic synthetic materials that can manipulate the propagation of elastic waves and acoustic waves. However, for traditional phononic crystals, once the structure is identified, only a certain bandgap frequency can exist. Here, a supersaturated sodium acetate solution (SSAS) is introduced to realize a reversible liquid–solid phase transition by heating/cooling, which is utilized to tune the low-frequency bandgaps of elastic waves. Based on local resonance, we designed a one-dimensional (1D) PC, which consists of a 1D assembly of a series of goblets filled with the SSAS and heater pasted on the wall of the goblet. Low-amplitude transmission testing was conducted in both the liquid and solid states of the SSAS. An analytical model was proposed to calculate the first bandgap of the PC and to verify the testing results. In addition, numerical simulations were conducted to explore more bandgap zones. The results indicate that the phase transition induces tunable bandgaps of elastic waves. The underlying mechanism is that the phase transition leads to a unit cell stiffness and local heterogeneity. The bandgap from the solid to the liquid state is broadened by 20%. The findings reported here provide a new routine for designing architected metamaterial systems with broad and wide bandgaps for a wide range of potential applications in seismic, vibration, and acoustic wave control and guiding.

Inverse Design Framework With Invertible Neural Networks for Passive Vibration Suppression in Phononic Structures

Abstract Automated inverse design methods are critical to the development of metamaterial systems that exhibit special user-demanded properties. While machine learning approaches represent an emerging paradigm in the design of metamaterial structures, the ability to retrieve inverse designs on-demand remains lacking. Such an ability can be useful in accelerating optimization-based inverse design processes. This paper develops an inverse design framework that provides this capability through the novel usage of invertible neural networks (INNs). We exploit an INN architecture that can be trained to perform forward prediction over a set of high-fidelity samples and automatically learns the reverse mapping with guaranteed invertibility. We apply this INN for modeling the frequency response of periodic and aperiodic phononic structures, with the performance demonstrated on vibration suppression of drill pipes. Training and testing samples are generated by employing a transfer matrix method. The INN models provide competitive forward and inverse prediction performance compared to typical deep neural networks (DNNs). These INN models are used to retrieve approximate inverse designs for a queried non-resonant frequency range; the inverse designs are then used to initialize a constrained gradient-based optimization process to find a more accurate inverse design that also minimizes mass. The INN-initialized optimizations are found to be generally superior in terms of the queried property and mass compared to randomly initialized and inverse DNN-initialized optimizations. Particle swarm optimization with INN-derived initial points is then found to provide even better solutions, especially for the higher-dimensional aperiodic structures.

Topological characteristic of Weyl degeneracies in a reciprocal chiral metamaterials system

Being a research hotspot in the field of topological semimetals, Weyl points (WPs) are monopoles of Berry curvature in momentum space. In this paper, we report the existence of photonic Weyl degeneracies in a reciprocal chiral metamaterials system. Due to the flat dispersion relation of the bulk plasmon modes, Weyl degeneracies here lie right on the critical transition between the type-I and type-II WPs. The photonic ‘Fermi arc’ connects the projection of pairs of WPs at the interface between the metamaterials and vacuum. Despite the bulk equi-frequency surfaces have changed dramatically, the ‘Fermi arc’ always exists. In addition, numerical simulations of topologically protected ‘Fermi arc’ surface states show that the surface waves are not scattered or reflected by the presence of sharp corners. Notably, such metamaterials host either type-I, type-II WPs or triple degenerate points (TDPs) depending on the nonlocal response. Our work provides an ideal photonic platform for studying the closely relation between WPs and other exotic states.

Multifunctional design of triangular lattice metamaterials with customizable thermal expansion and tunable bandgap properties

New approaches for realizing multifunctional research and tunable properties of materials are proposed by designing lattice metamaterials. In this study, the finite element method is used to study the bandgaps of the joint-bonded triangular cell lattice metamaterial. A new multifunctional design method is proposed for designing metamaterials with tunable bandgaps and specific thermal expansion properties. In this method, the geometric deformation characteristics of thermal expansion of materials are studied, and a design method for maximum bandgaps’ tunability under the condition of a specific coefficient of thermal expansion is proposed to realize the tuning of bandgaps’ characteristics based on external temperature change. Numerical results show that the proposed method and metamaterials can demonstrate significant tunability of bandgaps. This finding provides a common method for designing bandgaps’ tunable acoustic metamaterial systems, which have broad application potential under variable temperature loading and can be extended to other topological structures.

A Nonlocal Effective Medium Description of Topological Weyl Metamaterials

AbstractEffective medium theory lays the foundation for the development of metamaterials—artificially engineered photonic medium with subwavelength building blocks. However, the nonlocal effects, which are difficult to be incorporated into the effective medium description, are usually unavoidable in most metamaterial systems due to the finite lattice constant, preventing a precise modeling of wave propagation inside a metamaterial of large scale. On the other hand, recent development shows that nonlocal effect is essential for designing certain topological photonic systems such as ideal Weyl metamaterial. The properties of topological metamaterials rely on nonlocal effects, which, for example, determine whether a Weyl degeneracy formed by the linear crossing between a longitudinal and a transverse mode belongs to type I or type II. Here a nonlocal effective medium description of topological metamaterials is developed. For the first time, it is possible to calculate the wave propagation inside this complex medium in a large‐scale based on general numerical programs. To show the potential of the approach, wave propagation inside and at the boundary of a photonic ideal Weyl system are thoroughly studied in this work. By reformulating the electric field equation, the method paves the way toward comprehensive engineering of metamaterials with unrestricted nonlocality.

A generic theoretical approach for estimating bandgap bounds of metamaterial beams

The bandgap phenomenon in metamaterials has attracted much research interest for controlling structural vibrations. To tailor the bandgap for applications in a specific frequency range, analytical tools for bandgap bound estimations are critically important. This work presents a generic theoretical approach for fast estimation of the bandgap bounds. Starting from the lattice metamaterial systems, we develop the procedure and provide the analytical bound expressions based on a hypothesis of extreme points in the band structure of metamaterial systems. The proposed approach for the lattice system is verified by the results of transmittance analysis. Subsequently, to explore the fidelity of the proposed approach on continuous metamaterial systems, three typical metamaterial beams (metabeams) have been investigated: a metabeam with mechanical local resonators, a piezoelectric metabeam with shunt resonant circuits, and a hybrid metabeam. Finite element analysis is performed to verify the theoretical expressions of bandgap bounds derived using the proposed approach. With the verified bound expressions, bandgap tailoring and optimization are further investigated. In summary, the developed theoretical approach is generic and offers a promising technique for bandgap estimation of metamaterial systems integrated with various types of resonators.

An optimization problem for maximum vibration suppression in reconfigurable one dimensional metamaterials

Acoustic metamaterials have already been shown to be effective for vibration reduction and control. Local resonances in the metamaterial cause waves at frequencies within band gaps to become evanescent, thus preventing wave propagation through the material. Active and adaptable local resonances enables the band gaps to be shifted in frequency and increased in bandwidth. Since metamaterial local resonances are usually composite, methods to specify optimal component configurations are helpful for passive metamaterials and almost necessary for adaptable metamaterials, where the metamaterial must be reconfigured for optimal performance at various frequency ranges. To assess band gap locations and bandwidths for metamaterials, a wavenumber spectrum is commonly computed. Commonly, a parameter study of adaptable unit cell variables will be performed to assess optimal configurations of adaptable metamaterials. In this paper, the complex wavenumber is proposed as a direct optimization objective for reconfiguration of active adaptable acoustic metamaterials for maximum vibration suppression at a frequency range of choice. By directly maximizing the imaginary part of the wavenumber, associated with wave attenuation, the unit cell configuration maximum vibration suppression can be obtained for an operating frequency of choice. Additionally, since the optimization problem requires constraints for feasible solutions and the example active piezoelectric metamaterial system shown here is electrically unstable at some configurations, we also explore an experimental method for bounding the optimization problem. Numerical results of the optimization problem are presented.

Enhanced Mie resonance in a low refractive index colloidal metamaterial aided by nematic liquid crystal

We report studies on a colloidal metamaterial system in which sub-micron-sized (~530 nm) dielectric particles (SiO2) are dispersed in a nematic liquid crystal. Despite the low refractive index (~1.45) of the particles, Mie resonances are enhanced in the system due to the anisotropic nature of the host medium, as evident from the UV–Vis extinction spectra. The resultant improvement in light confinement and the interference between the dipolar Mie resonant modes give rise to forward directional scattering. Further, the Dark Field Hyperspectral Imaging and Heterodyne Near Field Scattering experiments demonstrate that the forward scattering intensity can be tuned by switching the refractive index of the liquid crystal medium between its anisotropic limits achieved by the application of an ac electric field. The extinction spectra with the deconvoluted resonance modes calculated using Mie theory and the Finite Elements Method (FEM) analysis support the experimental findings. The FEM calculations also reveal the formation of photonic nanojet from the particle. The enhanced Mie resonance and tunable forward scattering in the low index SiO2 particles induced by the nematic liquid crystal medium are highly promising for bio-compatible nano-photonic applications.

Mechanical properties of a hybrid auxetic metamaterial and metastructure system

We propose in this work an innovative hybrid auxetic metamaterial with a centersymmetric unit cell and tessellation topology similar to the one provided by the missing rib configuration. The tessellation proposed is applied to different core unit cells (star shape, cross-chiral shape with same dimensions, and reentrant). The effects of the geometric parameters of the cells on the in-plane mechanical properties of this hybrid auxetic metamaterial system are investigated via finite elements (FEMs). Representative unit cells (RUCs) with optimal mechanical behaviors are identified; those configurations exhibit the larger negative Poisson’s ratios and enhanced specific moduli. Designs related to two groups of auxetic metastructures with cylindric and cubic shapes are then developed based on the optimized RUCs along x and y directions. The equivalent mechanical performance of these metastructures under internal pressure is evaluated from a numerical standpoint. Auxetic cylindrical metastructures can be tailored by adjusting the number of the optimized RUCs along the circumferential and longitudinal directions, together with the geometric parameters of the optimized RUC itself. These hybrid auxetic metamaterials and metastructures provide the potential for multifunctional applications in biomechanics, flexible electronics, and aerospace.

Tunable Mid-Infrared Optical Properties of Monolayer Black Phosphorus Through Au Nanotriangle Arrays via Electric Field Reflection and Surface Plasmon Polaritons

A metamaterial system composed of monolayer black phosphorus and Au triangle arrays is designed. Absorptivity, transmittivity, and reflectivity are investigated in mid-infrared regime. A low transmissivity and high absorptivity can be obtained via surface plasmon polaritons at the black phosphorus and Au triangle array interface. By changing the geometrical parameters, such as angle magnitude and slit width of Au triangle, we can modulate the transmissivity, reflectivity and absorptivity properties. Different from other previous work, it is found the zigzag direction has a better photoresponse than that armchair by changing the slit width. The electric field of the external radiation field is reflected at the Au triangle edges. Thus, electric field component perpendicular to the polarization direction generates, which can also lead to surface plasmon polaritons and is not researched in others’ work. With increase in the angle of Au triangle, transmittivity (or absorptivity) for armchair direction has a blue shift and for zigzag direction has a red shift. The transmittivity decrease ( or absorptivity increase ) for special wavelength caused by the surface plasmon polaritons may be applied to design filter devices.

Ultrahigh mid-infrared electric field enhancement via nanoantenna plasmonic resonance coupled to a novel hyperbolic metalens

Concentrating optical energy to areas much smaller than the diffraction limit, with high field enhancement in mid-infrared (MIR) using noble metals such as gold (Au) or silver (Ag), is not feasible because of high losses, lack of tunability, low resolution, and low-intensity enhancement. Our methods are based on appropriate metamaterial selection, coupling system (a hyperbolic metalens and a nanoantenna), and optimizations of their geometries. Doped indium arsenide (InAs) is used as an alternative to plasmonic metals when designing a Fresnel zone plate (FZP). A hyperbolic metamaterial (HMM), placed under FZP, is formed by stacking layers of doped and undoped InAs. By combining the focal spot of the metalens with the excited plasmons of a nanoantenna, the resulting structure exhibits an ultrahigh field-intensity enhancement factor of about 1012 with a high absorption cross-section and a focusing resolution of up to 10−3 λ at the operating wavelength of 7.8 μm.

High-temperature and abrasion-resistant metal-insulator-metal metamaterials

Selective solar absorbers (SSAs) with high performance are the key to concentrated solar power systems. Optical metamaterials are emerging as a promising strategy to enhance selective photon absorption, however, the high-temperature stability (>500 °C) remains as one of the main challenges for practical applications. Here, a multilayered metamaterial system (Al2O3/W/SiO2/W) based on metal-insulator-metal Fabry-Pérot resonance effect has been demonstrated with high solar absorptance over 92%, low thermal emittance loss below 6% (100 °C blackbody), and significant high-temperature thermal stability: it has been proved that the optical performance remains 94% after 1-h thermal annealing under ambient environment up to 500 °C, and 94% after 96-h thermal cycle test at 400 °C. Outdoor tests demonstrate that a peak temperature rise (193.5 °C) can be achieved with unconcentrated solar irradiance and surface abrasion resistance test yields that SSAs have a robust resistance to abrasion attack for engineering applications.

Implications of Kleinian relativity

Inspired in metamaterials, we present a covariant mechanics for particles in Kleinian spacetime and show some of its effects, such as time contraction and length dilatation. We present the new expressions for relativistic momentum and energy for a point-like particle. To illustrate the new mechanics, we describe the particle motion under a uniform Newtonian gravitational field. We also revisit the free spin-half particle problem in Kleinian spacetime, discuss some quantum implications, like the constraint on the dispersion relation for Weyl fermions, and adapt a metamaterial analog system to Klein spacetime.

Origami-based stretchable bi-functional metamaterials: reflector and broadband absorber

Multi-functional microwave metamaterials offer a great solution for active components and modules that are potentially applicable in stealth, energy, and wireless communication systems/devices. However, it is challenging to realize a multi-functional behavior in a cost-effective and simple metamaterial system. This paper proposes and demonstrates a metamaterial inspired by origami building blocks that can be controlled by mechanical stimuli. By mechanically changing folding states, the proposed metamaterial can be switched from an ultra-broadband absorber to a reflector. In the compressed mode, the structure exhibits an absorption of more than 90% in a broad frequency range of 6–16 GHz. The absorption characteristic is insensitive to polarization angles and works with a wide range of incident angles. In the stretched mode, the absorption function is turned off and all the incident waves become reflected. Such origami-inspired metamaterials behave in multiple figures of merit involving bandwidth, frequency of operation, angle of polarization, and incidence.

Design of one-dimensional acoustic metamaterials using machine learning and cell concatenation

Metamaterial systems have opened new, unexpected, and exciting paths for the design of acoustic devices that only few years ago were considered completely out of reach. However, the development of an efficient design methodology still remains challenging due to highly intensive search in the design space required by the conventional optimization-based approaches. To address this issue, this study develops two machine learning (ML)-based approaches for the design of one-dimensional periodic and non-periodic metamaterial systems. For periodic metamaterials, a reinforcement learning (RL)-based approach is proposed to design a metamaterial that can achieve user-defined frequency band gaps. This RL-based approach surpasses conventional optimization-based methods in the reduction of computation cost when a near-optimal solution is acceptable. Leveraging the capability of exploration in RL, the proposed approach does not require any training datasets generation and therefore can be deployed for online metamaterial design. For non-periodic metamaterials, a neural network (NN)-based approach capable of learning the behavior of individual material units is presented. By assembling the NN representation of individual material units, a surrogate model of the whole metamaterial is employed to determine the properties of the resulting assembly. Interestingly, the proposed approach is capable of modeling different metamaterial assemblies satisfying user-defined properties while requiring only a one-time network training procedure. Also, the NN-based approach does not need a pre-defined number of material unit cells, and it works when the physical model of the unit cell is not well understood, or the situation where only the sensor measurements of the unit cell are available. The robustness of the proposed two approaches is validated through numerical simulations and design examples.

Tunable Fano Resonance and Enhanced Sensing in Terahertz Metamaterial

Fano resonances in metamaterial are important due to their low-loss subradiant behavior that allows excitation of high quality (Q) factor resonances extending from the microwave to the optical bands. Fano resonances have recently showed their great potential in the areas of modulation, filtering, and sensing for their extremely narrow linewidths. However, the Fano resonances in a metamaterial system arise from the interaction of all that form the structure, limiting the tunability of the resonances. Besides, sensing trace analytes using Fano resonances are still challenging. In the present work, we demonstrate the excitation of Fano resonances in metamaterial consisting of a period array of two concentric double-split-ring resonators with symmetry breaking (position asymmetry and gaps asymmetry). The tunability and sensing of Fano resonances are both studied in detail. Introducing position asymmetry in the metamaterial leads to one Fano resonance located at 0.50 THz, while introducing gaps asymmetry results in two Fano resonances located at 0.35 THz and 0.50 THz. The transmittance, position, and linewidth of the three Fano resonances can be easily tuned by varying the asymmetry deviations. The Q factor and figure of merit (FoM) of Fano resonances with different asymmetry deviations are calculated for performance optimization. The Fano resonances having the highest FoM are used for the sensing of analytes at different refractive indices, and the Fano resonance performing the best in refractive index sensing is further applied to detect the analyte thickness. The results demonstrate that the tunable Fano resonances show tremendous potential in sensing applications, offering an approach to engineering highly efficient modulators and sensors.