Anisotropic Metamaterials Research Articles

Dual-Band and High-Efficiency Circular Polarization Convertor Based on Anisotropic Metamaterial

In this paper, a dual-band and high-efficiency circular polarization (CP) convertor based on anisotropic metamaterial (AMM) was proposed and investigated in microwave region. The proposed AMM based CP convertor is composed of a sub-wavelength metal grating sandwiched with bi-layered disk-split-ring (DSR) structure array, which can convert the normal incident CP wave to its orthogonal one around two adjacent frequency ranges. Based on the intrinsic anisotropic and Fabry-Perot-like cavity-enhanced effect, a high CP conversion efficiency can be achieved by applying the proposed AMM. Numerical simulation results indicate that the cross-polarization transmission coefficients can achieve maximum values of 0.84 at 4.5 GHz, and 0.92 at 7.9 GHz, respectively, which is in well agreement with experiment. In addition, the measured CP conversion efficiency is beyond 99% at resonance frequencies. The mechanism of the CP conversion properties can be explained by the electromagnetic (EM) interference model and the simulated electrical field distribution. Due to its excellent polarization conversion properties, the proposed CP convertor based on AMM structure shows potential application in such as radar, remote sensing, and satellite communication.

Research progress of information metamaterials

<sec>Metamaterials are artificial structures composed of subwavelength unit cells in periodic or non-periodic arrays, which are regarded as one of the most important tops in today’s physics and information engineering. Traditional metamaterials are characterized by effective medium theory, in which the array of differently-shaped subwavelength particles can be described as an effective medium with effective permittivity and permeability. The metamaterials allow us to engineer the medium parameters with unusual values, such as negative permittivity and permeability, zero index of refraction, etc. In 2014, Cui et al. (Cui T J, Qi M Q, Wan X, Zhao J, Cheng Q 2014 <i>Light-Sci. Appl.</i> <b>3</b> e218) proposed the concept of information metamaterial, which is a digital version of the metamaterial with each unit cell described by digital codes representing different reflection/refraction phases. The direct connection between information metamaterials and digital logic devices allows the dynamic controlling of the electromagnetic (EM) waves by real-time programming the digital states of each unit cell in the information metamaterials with preloaded digital coding sequences. As information metamaterials build up a digital world (digital coding information) directly in the physical world, digital information can be processed on the information metamaterials directly without any intermediate conversion process, thus realizing the unification of microwave engineering and digital processing.</sec><sec>In this paper, we review the recent developments of digital coding metamaterials, programmable metamaterials, and information metamaterials, mainly focusing on their basic concepts, working mechanisms, experimental realizations, and system-level applications. Firstly, we introduce the concepts of digital coding and programmable metamaterials and present their advantages to realize the dynamic controlling of EM waves at low cost. The working mechanisms of isotropic, anisotropic, and tensor digital coding metamaterials are described, following the first prototype of the programmable metamaterial. Then we introduce the concept of information entropy for the information metamaterial and reveal the connection between the amount of information carried by the coding pattern and the radiated field of the information metamaterial. Convolution operation and addition theorem are further presented to show their powerful manipulations of EM wave in generating arbitrary beam patterns pointing to arbitrary directions. Finally, we introduce three representative system-level applications of information metamaterials, including a reprogrammable hologram imaging system which can dynamically project different microwave images at the imaging plane through the preloaded coding sequences stored in field programmable gate array (FPGA), a machine-learning reprogrammable metasurface imager that can be trained <i>in-situ</i> to produce high-quality images and high-accuracy object recognition in the real case at low cost, and directly digital wireless communication systems, in which the digital information is directly processed and radiated to free space by using the information metamaterial and FPGA. The information metamaterials are currently advancing towards higher frequencies (millimeter waves, terahertz, and infrared) to have higher capacity of information, and are becoming more “intelligent” with the combination of many advanced algorithms in computer science. We believe that the future information metamaterials possess signatures of self-sensing, self-learning, self-adaptive, and self-decision.</sec>

Phononic band structure and figure of merit of three-dimensional anisotropic asymmetric double-cone pentamode metamaterials

<sec> Pentamode metamaterial (PM) is a kind of artificial microstructure extremum material with solid morphology and fluid properties proposed by Milton and Cherkaey. By decoupling the compression and the shear waves, the periodic structure is difficult to be compressed, but the shear deformation occurs easily. Theoretically, acoustic metamaterials consisting of such periodic arrangement of structural units can achieve complete matching with water. Therefore, the characteristics of adjustable modulus anisotropy, small stuffing rate and broadband endow the PMs with excellent acoustic control ability, which has attracted more attention of researchers.</sec><sec> In this paper, the narrow-diameter intersection point <i>P</i> (0.25<i>a</i>, 0.25<i>a</i>, 0.25<i>a</i>) of an isotropic three-dimensional PM selected as the reference point in four different directions (X-axis, Y-axis, Z-axis and body diagonal). When the <i>P</i>-point moves, the farther the <i>P</i>-point is, the greater the degree of anisotropy is. The introduction of anisotropy will cause the structural bifurcation of the three-dimensional PM to change structural parameters, and the structural parameters are important factors affecting the band characteristics of the three-dimensional PM of Bragg scattering. In order to study the influence of anisotropy on the band structure and pentamode properties of three-dimensional asymmetric double-cone PMs, we use the finite element simulation software COMSOL to calculate the primitive-cell of three-dimensional anisotropic PMs under Bloch boundary conditions. </sec><sec> By adjusting the position of <i>P</i> point, four different types of three-dimensional anisotropic asymmetric double-cone PMs are constructed. Since the anisotropy changes in different directions have different effects on the parameters of the asymmetric double-cone structure, the band characteristics and the pentamode characteristics will also receive different degrees of influence. In this paper, the relationship between the degree of anisotropy and the band gap characteristics, single-mode region and figure of merit (FOM) are given, and the result can provide guidance for the design of asymmetric double-cone PM acoustic device. Compared with the isotropic double-cone PMs, the relative bandwidth of the first band gap of the anisotropic double-cone PMs can be broadened to 123%, and the FOM can be increased to 6.9 times. Due to the introduction of anisotropy, Due to the introduction of anisotropy, the structure of three-dimensional asymmetric double-cone PMs are more complex, the demand for sample fabrication is further improved, and the stability of PMs also reduced. Therefore, PMs with high stability and easy to be fabricated still needs further research and exploration.</sec>

Multi-scale topology optimization method of anisotropic acoustic metamaterials based on high-frequency homogenization method

Multi-scale topology optimization method of anisotropic acoustic metamaterials based on high-frequency homogenization method

Multi-scale topology optimization method of anisotropic acoustic metamaterials using propagating direction determination based on high-frequency homogenization method

Multi-scale topology optimization method of anisotropic acoustic metamaterials using propagating direction determination based on high-frequency homogenization method

Berreman effect in bimetallic nanolayered metamaterials

The visible and ultraviolet responses of a binary regular stack, composed of alternating Al and Ag nano-thin layers, are both experimentally and theoretically studied. It is found that the s- and p-polarization reflectivity spectra (Rs and Rp, respectively) of the inherently anisotropic bimetallic system are noticeably different due to the appearance of a prominent dip near the zero of the Ag permittivity. Such an effect is similar to the Berreman one, which is observed in thin polar-crystals films near the longitudinal-optical phonon frequency, as well as in thin metal films near plasma frequency. Using both the effective medium approach and a nonlocal homogenization formalism, being valid even beyond the long wavelength limit, it is demonstrated that the bimetallic multi-layer stack behaves as a uniaxial anisotropic metamaterial. As it is shown, the appearance of the dip in the p-polarization spectrum is associated to the zero of the longitudinal component of the effective permittivity (ϵz). The calculation of 45-degree reflectometry spectrum, namely the difference Rp−Rs2, from experimental and theoretical reflectivity data confirmed that the observed dip in Rp is connected to a plasma resonance of the energy-loss function ℑ(−1∕ϵz).

Envelope Dyadic Green\u2019s Function for Uniaxial Metamaterials

We introduce the concept of the envelope dyadic Green’s function (EDGF) and present a formalism to study the propagation of electromagnetic fields with slowly varying amplitude (EMFSVA) in dispersive anisotropic media with two dyadic constitutive parameters: the dielectric permittivity and the magnetic permeability. We find the matrix elements of the EDGFs by applying the formalism for uniaxial anisotropic metamaterials. We present the relations for the velocity of the EMFSVA envelopes which agree with the known definition of the group velocity in dispersive media. We consider examples of propagation of the EMFSVA passing through active and passive media with the Lorentz and the Drude type dispersions, demonstrating beam focusing in hyperbolic media and superluminal propagation in media with inverted population. The results of this paper are applicable to the propagation of modulated electromagnetic fields and slowly varying amplitude fluctuations of such fields through frequency dispersive and dissipative (or active) anisotropic metamaterials. The developed approach can be also used for the analysis of metamaterial-based waveguides, filters, and delay lines.

Isogeometric topology optimization of anisotropic metamaterials for controlling high‐frequency electromagnetic wave

SummaryThis study presents a level set–based topology optimization with isogeometric analysis (IGA) for controlling high‐frequency electromagnetic wave propagation in a domain with periodic microstructures (unit cells). The high‐frequency homogenization method is applied to characterize the macroscopic high‐frequency waves in periodic heterogeneous media whose wavelength is comparative to or smaller than the representative length of a unit cell. B‐spline basis functions are employed for the IGA discretization procedure to improve the performance of electromagnetic wave analysis in a unit cell and topology optimization. Also, to keep the same order of continuity on the periodic boundaries as on other element edges in the domain, we propose the extended domain approach, while incorporating Floquet periodic boundary condition (FPBC). Two types of optimization problems are taken as examples to demonstrate the effectiveness of the proposed method in comparison with the standard finite element analysis (FEA). The optimization results provide optimized topologies of unit cells qualified as anisotropic metamaterials with hyperbolic and bidirectional dispersion properties at the macroscale.

Valley anisotropy in elastic metamaterials

Valley, as a new degree of freedom, raises the valleytronics in fundamental and applied science. The elastic analogs of valley states have been proposed by mimicking the symmetrical structure of either two-dimensional materials or photonic valley crystals. However, the asymmetrical valley construction remains unfulfilled. Here, we present the valley anisotropy by introducing asymmetrical design into elastic metamaterials. The elastic valley metamaterials are composed of bio-inspired hard spirals and soft materials. The anisotropic topological nature of valley is verified by asymmetrical distribution of the Berry curvature. We show the high tunability of the Berry curvature both in magnitude and sign enabled by our anisotropic valley metamaterials. Finally, we demonstrate the creation of valley topological insulators and show topologically protected propagation of transverse elastic waves relying on operating frequency. The proposed topological properties of elastic valley metamaterials pave the way to better understanding the valley topology and to creating a new type of topological insulators enabled by an additional valley degree of freedom.

A single-detector acoustic camera based on space-coiling anisotropic metamaterial

Acoustic imaging is important in diverse applications but is limited by hardware complexity, as an acoustic camera relies on a group of detectors to image sound sources. This study uses spatially encoded structures to solve the problem of single-detector planar acoustic imaging at audio frequencies: a space-coiling metamaterial with high anisotropy is proposed to realize a single-detector acoustic camera. This design outperforms the conventional acoustic camera in terms of dimensions, bandwidth, and cost, and is expected to have real impact on engineering in single-detector acoustic imaging.

Ultrabroadband absorber based on a funnel-shaped anisotropic metamaterial

A design of a funnel-shaped anisotropic metamaterial absorber is reported with a periodic array of nickel–germanium (Ni/Ge). The optical properties of the suggested anisotropic metamaterial absorber are numerically calculated using the 3D finite-difference time-domain technique. Composed of both cylindrical and conical units, the reported anisotropic metamaterial attains high absorption over a wavelength range from 200 nm to 9000 nm with an average absorption of 96%. A wide wavelength absorption band is achieved from the ultraviolet to near infrared including the visible range. The cylindrical unit could harvest low-order modes, while the longer wavelengths are trapped through the conical part. The enhanced broadband absorption is due to the excitation of multiple orders of slow-light modes. In addition, the effect of the incident angle on the absorption of transverse magnetic and transverse electric-polarized waves is also investigated. Further, the structural geometrical parameters are studied in order to maximize the absorption through the proposed design.

Acoustic cloaking: Geometric transform, homogenization and a genetic algorithm

A general process is proposed to experimentally design anisotropic inhomogeneous metamaterials obtained through a change of coordinates in the Helmholtz equation. The method is applied to the case of a cylindrical transformation that allows cloaking to be performed. To approximate such complex metamaterials we apply results of the theory of homogenization and combine them with a genetic algorithm. To illustrate the power of our approach, we design three types of cloaks composed of isotropic concentric layers structured with three types of perforations: curved rectangles, split rings and crosses. These cloaks have parameters compatible with existing technology and they mimic the behavior of the transformed material. Numerical simulations have been performed to qualitatively and quantitatively study the cloaking efficiency of these metamaterials.

Control of underwater acoustics using anisotropic solid metamaterials with continuously tuned material axes

Inertial transformation acoustics is a method for deriving a spatial pattern of material parameters for use in the arbitrary regulation of acoustic trajectories. This work highlights the discussion on a general metamaterial model with material properties that meet all the needs of the inertial transformation parameters. The proposed solid metamaterials are described to possess broadband anisotropic density and continuously tuned material axis while exhibiting fluid-like elasticity, namely, isotropic bulk stiffness and sufficiently small shear resistance. In particular, the metamaterial structure embodies a regular cell shape with an unaltered profile against the rotation of material axes, which facilitates the assembly of functional devices. An underwater acoustic rotator is designed and numerically verified to elucidate the wave dispersion and effective-medium characteristics of the metamaterial. A guideline for constructing the transformation device from the basic cells is also provided herein. The presented model can serve as a new material platform on the exploration of arbitrary acoustic-control via the inertial transformation method.

Tunable optical filter based on one-dimensional periodic structure composed of SiO2 and anisotropic metamaterial (AMM) with a liquid crystal defect layer sandwiched by two SiO2

This paper reports the tunable transmission properties of asymmetric one-dimensional periodic structure (1DPS) composed of SiO2 and anisotropic metamaterial (AMM) layers with defect of liquid crystal (LC) sandwiched by two SiO2 layers, i.e., (SiO[Formula: see text]AMM)[Formula: see text]SiO[Formula: see text]LC[Formula: see text]SiO[Formula: see text](SiO[Formula: see text]AMM)3 using the transfer matrix method (TMM). We have studied the optical properties of the periodic structure (SiO[Formula: see text]AMM)3 [Formula: see text]SiO[Formula: see text]LC[Formula: see text]SiO[Formula: see text](SiO[Formula: see text]AMM)3 with a different incident angle of the electromagnetic wave for particular director angle of LC molecules. The tunability of the transmission property of the considered 1DPS shows that the transmittance depends upon the orientation of LC molecules. Such an asymmetric periodic structure (1DPS) composed of SiO2 and AMM with a defect of LC sandwiched by two SiO2 layers, (SiO2 [Formula: see text]AMM)[Formula: see text]SiO[Formula: see text]LC[Formula: see text]SiO[Formula: see text](SiO[Formula: see text]AMM)3, may be used as a tunable optical filter and bi-stable device.

Broadband Anisotropy in Terahertz Metamaterial With Single-Layer Gap Ring Array

To convert the polarization of terahertz waves pumped by a femtosecond laser in a terahertz time domain system, a broadband anisotropic metamaterial is proposed. The metamaterial is constructed with a single-layer gapped metallic ring array, which supports different resonant modes in orthogonal directions. With the aid of simulations and measurements, the anisotropy of the terahertz transmission is demonstrated and discussed. The experimental results of THz transmission in the metamaterial indicate that the anisotropic band is as wide as 0.56 THz, which accords well with our theoretical prediction.

Ultrathin and broadband highly efficient terahertz reflective polarization converter based on four L-shaped metamaterials

We propose a simple ultrathin design of reflective linear polarization converter with broadband and high efficiency based on four L-shaped anisotropic metamaterials. Simulated results demonstrate our design can convert a linearly polarized wave to its cross-polarized wave and the polarization conversion ratio over 98% is achieved from 4.2 to 5.2 THz. The further simulated results indicate that the deflection angles relative to y axis have greater impact on the cross-polarization reflection coefficient. Our design will provide an important reference for the practical applications of the metamaterials in polarization manipulation.

Extremely anisotropic epsilon-near-zero media in waveguide metamaterials.

Extremely anisotropic media such as anisotropic zero-index media allow wave propagation in extraordinary ways that are absent in isotropic systems. Here, we propose an approach to realize a type of extremely anisotropic effective epsilon-near-zero media by exploiting anisotropic waveguide metamaterials. The metamaterial is composed of two kinds of dielectrics with metal wires in waveguides. Extreme anisotropy is realized by alternating layers of the two kinds of dielectrics, which lead to different cut-off frequencies for transverse electric modes in different directions. Anisotropic effective epsilon-near-zero media with low loss can be obtained around the cutoff frequencies. Based on the extreme anisotropy, the unique phenomena of directive emission, nearly perfect bending waveguides and arbitrary control of energy flux are demonstrated. Interestingly, subwavelength focusing of energy flux with a spotsize <0.2λ is observed (λ is the wavelength in free space). Our work provides a convenient approach for the realization of extremely anisotropic epsilon-near-zero media and unique applications in low-loss waveguide metamaterials.

Realization of an Acoustic Third-Order Topological Insulator.

The recent discovery of higher-order topological insulators (TIs) has opened new possibilities in the search for novel topological materials and metamaterials. Second-order TIs have been implemented in two-dimensional (2D) systems exhibiting topological "corner states," as well as three-dimensional (3D) systems having one-dimensional (1D) topological "hinge states." Third-order TIs, which have topological states three dimensions lower than the bulk (which must thus be 3D or higher), have not yet been reported. Here, we describe the realization of a third-order TI in an anisotropic diamond-lattice acoustic metamaterial. The bulk acoustic band structure has nontrivial topology characterized by quantized Wannier centers. By direct acoustic measurement, we observe corner states at two corners of a rhombohedronlike structure, as predicted by the quantized Wannier centers. This work extends topological corner states from 2D to 3D, and may find applications in novel acoustic devices.

Photonic analogues of the Haldane and Kane-Mele models

Abstract The condensed matter Haldane and Kane-Mele models revolutionized the understanding of what is an “insulator,” as they unveiled novel classes of media that behave as metals near the surface, but are insulating in the bulk. Here, we propose exact electromagnetic analogues of these two influential models relying on a photonic crystal implementation of “artificial graphene” subject to an effective magnetic field. For the Haldane model, the required effective magnetic field for photons can be emulated with a spatially variable pseudo-Tellegen response. For the Kane-Mele model, the spin-orbit coupling can be mimicked using matched anisotropic dielectrics with identical permittivity and permeability, without requiring any form of bianisotropic couplings. Using full-wave numerical simulations and duality theory we verify that the nontrivial topology of the two proposed platforms results in the emergence of topologically protected gapless edge states at the interface with a trivial photonic insulator. Our theory paves the way for the emulation of the two condensed matter models in a photonic platform and determines another paradigm to observe topologically protected edge states in a fully reciprocal all-dielectric and non-uniform anisotropic metamaterial.

Improved wireless power transfer efficiency with non-perfect lenses

Negative refractive index metamaterials (MMs) have found widespread interest in shaping electromagnetic waves. One attractive area is energy transfer using low frequency magnetic waves or wireless power transfer (WPT). Previous reports have presented an isotropic metamaterial with μr = −1 as a “perfect lens” to focus energy and enhance WPT efficiency. In this work, we show that, while the perfect lens condition does enhance efficiency, anisotropic MMs with μr ≠ −1 can provide a larger enhancement in efficiency. These “non-perfect” lenses offer higher efficiency by enhancing the coupled field while incurring lower losses in the magnetostatic waves excited in or on the materials.