Homogeneous Metamaterials Research Articles

Multimaterial Metamaterials: Customize Targeted Thermal Deformation Modes Responding to Time- and Space-Variant Temperature Stimuli.

Customizing the engineering targeted thermal deformations is of vital significance for dimensional stability or shape morphing in materials and structures. However, current metamaterials are designed solely in the homogeneous form to respond only to the time-variant temperature (TVT) stimuli, far behind the practical engineering scenario and demands. Here, a new strategy is originally proposed and experimentally verified to design a series of both homogeneous and inhomogeneous multimaterial metamaterials, which uniquely output various thermal deformation modes, responding to time-variant and space-variant temperature (SVT) stimuli. Specifically, in addition to the regular isotropic thermal deformations, the metamaterials could exclusively output the entirely different positive and negative thermal deformations along the two orthotropic directions. Besides, stimulated by both TVT and SVT, the metamaterials provide more flexibility to customize the targeted thermal deformations. That is, the uniform thermal deformations could be well customized by the metamaterials stimulated by either TVT or SVT. More importantly, the customizability is remarkably broadened, as the nonuniform, specifically, mathematicized linear and nonlinear thermal deformations, are elaborately customized. Overall, these originally devised metamaterials open a new avenue for the purpose of customizing the engineering targeted thermal deformations in various modes under both TVT and SVT stimuli.

High-permittivity ceramics enabled highly homogeneous zero-index metamaterials for high-directivity antennas and beyond

Zero-index metamaterials (ZIMs) can support uniform electromagnetic field distributions at any frequency, but their applications are hampered by the ZIM’s homogenization level—only 3 unit cells per free-space wavelength, which is fundamentally limited by the low-permittivity inclusions (εr ≈ 12) and background matrix (εr ≈ 1). Here, by filling high-permittivity SrTiO3 ceramic (εr ≈ 294) pillars in BaTiO3 (εr ≈ 25) background matrix, we demonstrate a highly homogeneous microwave ZIM with an over threefold increase in the homogenization level. Leveraging such a ZIM, we achieve not only an antenna, approaching the fundamental limit in the directivity with outstanding scalability, but also a concave lens with a focal length of as short as 1λ0. Our highly homogeneous ZIM has profound implications in ceramics, ZIM-based waveguides and cavities, free-space wavefront manipulation, and microwave quantum optics, and opens up enormous possibilities in wireless communications, remote sensing, global positioning satellites, etc.

A reprogrammable mechanical metamaterial with origami functional-group transformation and ring reconfiguration

Recent advancements in reprogrammable metamaterials have enabled the development of intelligent matters with variable special properties in situ. These metamaterials employ intra-element physical reconfiguration and inter-element structural transformation. However, existing mono-characteristic homo-element mechanical metamaterials have limited reprogramming functions. Here, we introduce a reprogrammable mechanical metamaterial composed of origami elements with heterogeneous mechanical properties, which achieves various mechanical behavior patterns by functional group transformations and ring reconfigurations. Through the anisotropic assembly of two heterogeneous elements into a functional group, we enable mechanical behavior switching between positive and negative stiffness. The resulting polygonal ring exhibits rotational deformation, zero Poisson’s ratio stretching/compression deformation, and negative Poisson’s ratio auxetic deformation. Arranging these rings periodically yields homogeneous metamaterials. The reconfiguration of quadrilateral rings allows for continuous fine-tunability of the mechanical response and negative Poisson’s ratio. This mechanical metamaterial could provide a versatile material platform for reprogrammable mechanical computing, multi-purpose robots, transformable vehicles and architectures at different scales.

Long‐distance and anti‐disturbance wireless power transfer based on concentric three‐coil resonator and inhomogeneous electromagnetic metamaterials

SummaryFor wireless power transfer system (WPTS), the deflection of the resonant coil and the increase of the transmission distance will cause a sharp decline in transmission efficiency. This paper presents a novel long‐distance and anti‐deflection wireless power transfer based on concentric three‐coil resonator and inhomogeneous electromagnetic metamaterials (IEM). First, the effective receiving area of the resonator is analyzed, and the anti‐disturbance concentric three‐coil resonator is designed quantitatively. Considering the magnetic field distribution generated by the transmitting coil, this paper proposes and designs IEM with three kinds of effective permeability, which is applied in different design area, respectively. Meanwhile, the finite‐element‐method simulation proves an improvement in magnetic field modulation and flux leakage control when WPTS with IEM is used. The experimental results show that, compared with the traditional one‐coil to one‐coil, the WPTS designed in this paper achieves an increase of 43% in the efficiency at a 90° deflection. Furthermore, the efficiency of WPTS with IEM is 17% and 7% higher than that of WPTS with homogeneous electromagnetic metamaterials (HEM) and without electromagnetic metamaterials. As a result, the design method in this paper can improve the anti‐disturbance capability and increase the transmission distance and efficiency.

An FEM-based homogenization method for orthogonal lattice metamaterials within micropolar elasticity

Lattice metamaterials are widely used in weight-critical applications and gain more design freedoms with the development of additive manufacturing technology, which requires new accurate homogenization methods to model the mechanical behavior of complex microstructures. In the present work, a new homogenization method based on finite element methods was proposed within the frame of the anisotropic micropolar continuum for the metamaterial, which provided a general approach for both homogeneous and conformal metamaterials of orthogonal lattices. The mapping relation between lattice and finite element mesh as well as edge effects were clarified and modeled. The homogenization approach is verified based on detailed modal analysis and shows high numerical accuracy for engineering applications. Computational experiments confirmed that the micropolar model provides much more accurate results than the classical continuum.

Multi-resonator metamaterials as multi-band metastructures

Introducing multi-resonator microstructure into phononic metamaterials provides more flexibility in bandgap manipulation. In this work, 3D-acoustic metamaterials of the body- and face-centered cubic lattice systems encompassing nodal isotropic multivibrators are investigated. Our main results are: (1) the number of bandgaps equals the number, n, of internal masses as each bandgap is a result of the classical analog of the quantum level-repulsion mechanism between internal and external oscillations, and (2) the upper boundary frequencies, ωupper2i, i = 1, 2, ⋯, n, of the gaps formed coincide with eigen-frequencies, ωint;i2 ≠ 0, of the isolated multivibrator, ωupper2;i = ωint; i2, and the lower boundary frequencies, ωlower2,i2, are in good agreement with estimations as ωlower,i2≈ω^int;i2 (ωlower,i2<ω^int;i2), where ω^int;i2 represent the eigen-frequencies of the multivibrator when its external shell is motionless.The morphologies of the set of dispersion surfaces, ωm2(k), m = 1, 2, …, 6, in the corresponding passbands are similar to each other and to that of the set of dispersion surfaces, ωext; m2(k), obtained through the exclusion of internal masses. Thus, the problem of analyzing the acoustic properties of the complicated system is reduced to the study of two simple sets {ωint; i2} and ω^int;i2, along with {ωext; m2(k)}, the morphology of which depends only on the type of lattice symmetry. This splitting renders controlled phononic bandgaps formation in homogeneous multi-resonator metamaterials feasible.

Multi-morphology lattices lead to improved plastic energy absorption

While lattice metamaterials can achieve exceptional energy absorption by tailoring periodically distributed heterogeneous unit cells, relatively little focus has been placed on engineering heterogeneity above the unit-cell level. In this work, the energy-absorption performance of lattice metamaterials with a heterogeneous spatial layout of different unit cell architectures was studied. Such multi-morphology lattices can harness the distinct mechanical properties of different unit cells while being composed out of a single base material. A rational design approach was developed to explore the design space of these lattices, inspiring a non-intuitive design which was evaluated alongside designs based on mixture rules. Fabrication was carried out using two different base materials: 316L stainless steel and Vero White photopolymer. Results show that multi-morphology lattices can be used to achieve higher specific energy absorption than homogeneous lattice metamaterials. Additionally, it is shown that a rational design approach can inspire multi-morphology lattices which exceed rule-of-mixtures expectations.

Spin Momentum–Locked Surface States in Metamaterials without Topological Transition

AbstractThe photonic analogy of the quantum spin Hall Effect, that is, a photonic topological insulator (PTI), is of great relevance to science and technology in optics based on the promise of scattering‐free surface states. The challenges in realizing such scattering‐free surface states in PTIs and other types of symmetry‐protected topological phases are the result of the exact symmetry needed for creating a pair of time reversal pseudo‐spin states or special boundary conditions, wherein the exact symmetry imposes strict requirements on materials or boundary conditions. Here, it is experimentally demonstrated that scattering‐free edge states can be created with neither the aforementioned exact symmetry requirements for materials nor the topological transitions. This system is constructed by simply placing together regular homogeneous metamaterials, which are characterized by highly different bianisotropies. Of the particular surface states, backward reflection would be deeply suppressed, provided that the related evanescent tail into the bulk regions vanishes shortly and that the pseudo‐spin is not flipped by the scatterers. This work gives an example of constructing scattering‐free surface states in classical systems without strict symmetry protections and may potentially stimulate various novel applications in the future.

Characterization of surface-plasmon polaritons at lossy interfaces

We characterize surface-plasmon polaritons at lossy planar interfaces between one dispersive and one nondispersive linear isotropic homogeneous media, i.e. materials or metamaterials. Specifically, we solve Maxwell’s equations to obtain strict bounds for the permittivity and permeability of these media, such that satisfying these bounds implies surface-plasmon polaritons successfully propagate at the interface, and violation of the bounds impedes propagation, i.e. the field delocalizes from the surface into the bulk. Our characterization of surface-plasmon polaritons is valuable for checking the viability of a proposed application, and, as an example, we employ our method to falsify a previous prediction that surface-plasmon propagation through a surface of a double-negative refractive index medium occurs for any permittivity and permeability; instead, we show that propagation can occur only for certain medium parameters.

Analysis of magneto-optical properties for three-dimensional photonic crystals in high-symmetry arrangement doped by metamaterials and uniaxial materials

Abstract In this paper, we use a modified plane wave expansion (PWE) method to investigate the properties of photonic band gaps (PBGs) for the extraordinary mode in the three-dimensional (3D) photonic crystals (PCs) which are composed of the anisotropic dielectric (the uniaxial materials) spheres immersed in the homogeneous metamaterials (epsilon-negative materials) background with high-symmetry (body-centered-cubic) lattices, as the magneto-optical Voigt effects are considered. The equations for calculating the PBGs in the first irreducible Brillouin zone are theoretically derived. It is numerically illustrated that the anisotropic PBGs and two flattened band regions can be achieved. The influences of the ordinary-refractive index, extraordinary-refractive index, filling factor of dielectric spheres, electronic plasma frequency and cyclotron frequency on the magneto-optical properties of such 3D PCs also are studied in detail, respectively, and some corresponding physical explanations are given. The numerical results demonstrate that the anisotropy can open partial band gaps in the proposed PCs, and the complete PBGs can be obtained compared with the conventional PCs only containing the isotropic material with similar structures. The bandwidths of PBGs can be tuned by introducing the epsilon-negative materials into such PCs containing the uniaxial materials. The anisotropic PBGs can be manipulated by the parameters as mentioned above. As the proposed PCs with high-symmetry lattices, the complete PBGs can be obtained by introducing the uniaxial materials.

The effective periodic homogeneous metamaterials for infinite complementary dielectric slab characteristics

A comparison of balanced and unbalanced backward wave dispersion relations and the constitutive electromagnetic effective parameters of dielectric constant, magnetic permeability are obtained. In the balanced case, the initial transmission line lies in left handed (LH) medium and the next to cut-off frequency of higher order regions transmission line becomes in right-handed (RH) medium. In the unbalanced case, there are a tiny reject band displays due to the attenuations. It is also desirable for metamaterial parameters extraction properties should behave in antiresonance behaviour in the band gap regions. The proposed extraction technique shows the suitable results for dual negative magnetic permeability and electric permittivity that match with the theory. To demonstrate the infinite wave investigations, there are four unit cell of complementary split- ring resonators (CSRRs) microstrip implementation is obtained and the equivalent lumped element circuit is analyzed. The theory and experiment result exhibits excellent agreement that ensures minimum loss and electrical small size due to less than wavelength dimensions.

Dynamically controllable anisotropic metamaterials with simultaneous attenuation and amplification

Anisotropic homogeneous metamaterials that are neither wholly dissipative nor wholly active at a specific frequency are permitted by classical electromagnetic theory. Well-established homogenization formalisms indicate that such a metamaterial may be realized quite simply as a random mixture of electrically small (possibly nanoscale) spheroidal particles of at least two different isotropic dielectric materials, one of which must be dissipative but the other active. The dielectric properties of this metamaterial are influenced by the volume fraction, spatial distribution, particle shape and size, and the relative permittivities of the component materials. Similar metamaterials with more complicated linear as well as nonlinear constitutive properties are possible. Dynamic control of the active component material, for example via stimulated Raman scattering, affords dynamic control of the metamaterial.

Nonresonant and resonant cloaking of an electrically large dielectric spherical object by a multilayer isotropic metamaterial cover.

Mie theory and genetic algorithms are used to determine the parameters and performance of cloaks made of homogeneous isotropic metamaterials that would hide a spherical dielectric object of size comparable to the incident radiation wavelength. A single-layer (SL) cover with negative permittivity and permeability can produce a much greater reduction in the extinction efficiency than one with the permittivity and permeability of positive or opposite signs. Minimization of the extinction efficiency in the former case leads to both nonresonant and resonant solutions. Adding a second layer to the cover can lead to a significant enhancement of the bandwidth, but only to a modest reduction in the extinction efficiency at the design wavelength. In the SL case, Debye's scattering series is used to show that the nonresonant and resonant minima of the extinction efficiency correspond to scattering phase shifts approximately equal to zero and -π, respectively, and to understand the simple approximate expressions for the cloak parameters of the nonresonant solutions. The series also explains the value of the outer radius of a multilayer cloak, provides a link to a previously studied isotropic approximation to a transformation optics cloak, and indicates that a cloak consisting of an odd number of alternate double-negative and double-positive layers will probably give the best possible performance.

A unidirectional acoustic cloak for multilayered background media with homogeneous metamaterials

The acoustic cloak, which can make an object hard to detect acoustically in a homogeneous background, has attracted great attention from researchers in recent years. The inhomogeneous background media were considered in this paper. The relative constitutive parameters were derived for acoustic cloaks working in multilayered media. And a unidirectional acoustic cloak for layered background media was proposed, designed and implemented successfully in a wide frequency range. In water and NaCl aqueous solution, the acoustic cloak was designed and realized with homogeneous metamaterials which were composed of steel and porous materials. The effective parameters of the unit cells of the cloak were determined by using the effective medium theory. Numerical results demonstrated excellent cloaking performance and showed that such a device could be physically realized with natural materials which will greatly promote the real applications of an invisibility cloak in inhomogeneous backgrounds.

Design and analysis of the trapeziform and flat acoustic cloaks with controllable invisibility performance in a quasi-space

We present the design, implementation and detailed performance analysis for a class of trapeziform and flat acoustic cloaks. An effective large invisible area is obtained compared with the traditional carpet cloak. The cloaks are realized with homogeneous metamaterials which are made of periodic arrangements of subwavelength unit cells composed of steel embedded in air. The microstructures and its effective parameters of the cloaks are determined quickly and precisely in a broadband frequency range by using the effective medium theory and the proposed parameters optimization method. The invisibility capability of the cloaks can be controlled by the variation of the key design parameters and scale factor which are proved to have more influence on the performance in the near field than that in the far field. Different designs are suitable for different application situations. Good cloaking performance demonstrates that such a device can be physically realized with natural materials which will greatly promote the real applications of invisibility cloak.

Experimental realization of broadband acoustic omnidirectional absorber by homogeneous anisotropic metamaterials

We design a two-dimensional broadband acoustic omnidirectional absorber (AOA) simply comprising homogeneous anisotropic metamaterials to enlarge the absorption cross-section of a smaller core with matched acoustic impedance as the previous AOAs do, which generally involve complicated design of gradient-index or negative-index materials. Furthermore, AOAs with asymmetric/large geometrics can be realized conveniently, which will otherwise require a complex redesign of parameters. The proposed scheme is also extendable to three-dimensional cases. An implementation using angularly distributed fins was demonstrated experimentally, showing the broadband functionality of the designed absorber. Such metamaterial-based acoustic absorbers may have potential applications in various fields such as acoustic energy concentration, noise control, etc.

Design of a broadband ultra-large area acoustic cloak based on a fluid medium

A broadband ultra-large area acoustic cloak based on fluid medium was designed and numerically implemented with homogeneous metamaterials according to the transformation acoustics. In the present work, fluid medium as the body of the inclusion could be tuned by changing the fluid to satisfy the variant acoustic parameters instead of redesign the whole cloak. The effective density and bulk modulus of the composite materials were designed to agree with the parameters calculated from the coordinate transformation methodology by using the effective medium theory. Numerical simulation results showed that the sound propagation and scattering signature could be controlled in the broadband ultra-large area acoustic invisibility cloak, and good cloaking performance has been achieved and physically realized with homogeneous materials. The broadband ultra-large area acoustic cloaking properties have demonstrated great potentials in the promotion of the practical applications of acoustic cloak.

Extreme coupling: A route towards local magnetic metamaterials

Genuinely homogeneous metamaterials, which may be characterized by local effective constitutive relations, are required for many spectacular metamaterial applications. Such metamaterials have to be made of meta-atoms, i.e., subwavelength resonators, which exhibit only electric and or magnetic dipole and negligible higher-order multipolar polarizabilities in the spectral range of interest. Here, we show that these desired meta-atoms can be designed by exploiting the extreme coupling regime. Appropriate meta-atoms are identified by performing a multipole analysis of the field scattered from the respective meta-atom. To design those particular meta-atoms it is important to disclose the frequency and angular-dependent polarizability of both dipole moments. We demonstrate the applicability of a purely analytical model to accurately calculate for a normally incident plane wave reflection and transmission from meta-surfaces made of periodically arranged meta-atoms. With our work we identify a possible route towards the engineering of artificial materials while only considering the response from its constituents. Our approach is generally applicable to all spectral domains and can be used to evaluate and design metamaterials made from different constituting materials, e.g., metals, dielectrics, or semiconductors.

Towards negative index self-assembled metamaterials

We investigate the magnetic response of meta-atoms that can be fabricated by a bottom-up technique. Usually such meta-atoms consist of a dielectric core surrounded by a large number of solid metallic nanoparticles. In contrast to those meta-atoms considered thus far, we study here for the first time hollow metallic nanoparticles (shells). In doing so we solve one of the most pertinent problems of current self-assembled metamaterials, namely implementing meta-atoms with sufficiently large resonance strength and small absorption. Both conditions have to be met for deep sub-wavelength meta-atoms to obtain effectively homogeneous metamaterials which may be meaningfully described by negative material parameters. Eventually we show that by using these findings self-assembled negative index materials come in reach.

The anisotropic photonic band gaps in three-dimensional photonic crystals with high-symmetry lattices composed of metamaterials and uniaxial materials

In this paper, the properties of anisotropic photonic band gaps (PBGs) for three-dimensional (3D) photonic crystals (PCs) composed of tellurium (Te) spheres (the uniaxial materials) in homogeneous single-negative metamaterials (epsilon-negative materials) background with high-symmetry (simple-cubic) lattices are theoretically investigated based on the plane wave expansion method. The equations for calculating the anisotropic PBGs in the first irreducible Brillouin zone are theoretically deduced. The influences of the ordinary-refractive index, extraordinary-refractive index, filling factor of dielectric, the electronic plasma frequency, the dielectric constant of epsilon-negative materials on the anisotropic PBGs are also studied in detail, respectively, and some corresponding physical explanations are also given. The numerical results show that the anisotropy can open partial band gaps in simple-cubic lattices and the complete PBGs also can be achieved compared to such 3D PCs doped by the conventional isotropic materials. It also is shown that the anisotropic PBGs can be manipulated by the parameters as mentioned above. Introducing the uniaxial materials into 3D dielectric-epsilon-negative materials PCs can enlarge the PBGs, and also provide a way to obtain the complete PBGs as such kind of 3D PCs with high-symmetry lattices.