Electromagnetic Metamaterials Research Articles

Research progress of imaging technologies based on electromagnetic metasurfaces

Electromagnetic metamaterials are artificial structures engineered on a subwavelength scale to have optical properties that are not observed in their constituent materials and may not be found in nature either, such as negative refractive index. They have enabled unprecedented flexibility in manipulating light waves and producing various novel optical functionalities. Since the beginning of this century, with the development of nanofabrication and characterization technologies, there has been aroused a tremendous growing interest in the study of electromagnetic metamaterials and their potential applications in different fields including super-resolution imaging, optical biosensing, electromagnetic cloaking, photonic circuits and data storage. Electromagnetic metasurfaces are two-dimensional metamaterials composed of subwavelength planar building blocks. Although metasurfaces sacrifice some functionalities compared with their bulk counterparts, they provide us with distinct possibility to fully control light wave with ultrathin planar structures. Based on Huygens principle, the metasurfaces are able to arbitrarily manipulate the phases, amplitudes or polarizations of optical waves. For example, metasurfaces made of gold nanoantenna-arrays are able to create phase discontinuities for light propagating through the interfaces and drastically change the flows of reflected and refracted light at infrared frequencies. Comparing traditional dielectric optic elements, the thickness values of metasurface-based optical devices are much smaller. In addition to the control of free-space incident light, metasurfaces can also be used to precisely control and manipulate surface electromagnetic waves. In this review, we introduce the generalized Snell's law and the fundamental principles to modulate phase by using metasurfaces. Research progress of a variety of imaging technologies based on metasurfaces is then presented, including plasmonic metasurface, all-dielectric metasurface and metal/insulator hybrid metasurface. Finally, we summarize several frontier problems associated with metasurface, which maybe provide some references for the future researches and applications.

Inverse Doppler effect of acoustic metamaterial with negative mass density

It is always an issue for researchers to control the propagation of sound wave at will. A kind of acoustic metamaterial built with artificial microunits attracts the attention of researchers, because it possesses many unique properties that cannot be realized by natural materials, such as negative refractive index, slab focusing, and cloak. The Doppler effect leads to the frequency change of a wave because of the relative motion between the observer and the source. In 1968, Veselago[Veselago V G 1968 Soviet Physics Uspekhi 10 509] theoretically proposed that a metamaterial with a negative refraction can result in an inverse Doppler effect. The investigation of inverse Doppler effect has been developed with the improvement of metamaterials. However, the design methods of these metamaterials generally need ideal material parameters, which are difficult to obtain experimentally. Besides, although the inverse Doppler effects are realized by some electromagnetic metamaterials in optical and microwave frequencies, the relevant researches in acoustic metamaterials make slow progress. In this work, a 2D acoustic metamaterial with negative mass density is fabricated. Our previous work has demonstrated that the air in the internal cavity of the unit cell will vibrate back and forth to generate the vibration velocity when the air is driven by a sound source. As the source frequency reaches the resonant frequency, large amounts of energy will be stored in the internal cavity. This accumulation of energy will cause the acceleration of the air in opposite direction to the sound pressure, thus this metamaterial will exhibit negative mass density. In this case, the direction of the phase velocity is exactly opposite to that of the group velocity of the sound wave. Therefore, the inverse Doppler effect of sound wave can be realized by this metamaterial. Since the unit cells with different lengths have different resonant frequencies and there is only weak interaction among the adjacent unit cells, the frequency band of the metamaterial with negative mass density can be broaden by combining several different unit cells. Our previous experiments have demonstrated that the mass density and refractive index of this metamaterial are negative over a broad frequency range from 1560 Hz to 5580 Hz and 1500 Hz to 5480 Hz, respectively. A testing equipment is constructed to measure the Doppler effect of this metamaterial from 1200 Hz to 6500 Hz. The experimental results show that when the sound source witha frequency of 2000 Hz approaches to the detector, the detected frequency is 1999.27 Hz, which is 0.73 Hz smaller than the source frequency; when the sound source recedes from the detector, the detected frequency is 2000.68 Hz, which is 0.68 Hz larger than the source frequency. Therefore, the inverse Doppler effect appears at 2000 Hz. The experimental results within the whole frequency range of negative refractive index show broadband inverse Doppler phenomena.

Research on Communication Resource Management System based on Metamaterials

Electromagnetic Metamaterial (EMM), which cannot be found in nature and possesses the novel electromagnetic properties of negative refraction, reversals of both Doppler shift and Cherenkov radiation, is a kind of artificial composed material. In order to solve the problem of long delay and high packet loss rate, the paper proposed a communication resource management system based on Metamaterials. By using the electromagnetic simulation method, the triple band and tunable transmission/reflection characteristics are investigated. Experiment result shows that the proposed method can improve the performance in the communication resource management.

A topological optimization method for planar electromagnetic metamaterials

A topological optimization method for planar electromagnetic metamaterials

Ferrite Film Loaded Frequency Selective Metamaterials for Sub-GHz Applications.

Electromagnetic metamaterials are constructed with sub-wavelength structures that exhibit particular electromagnetic properties under a certain frequency range. Because the form-factor of the substructures has to be comparable to the wavelength of the operating frequency, few papers have discussed the metamaterials under GHz frequency. In this paper, we developed an innovative method to reduce the resonant frequency of metamaterals. By integrating the meta-structures with ferrite materials of higher permeability, the cell size of the meta-structure can be scaled down. This paper describes the methodology, design, and development of low-profile GHz ferrite loaded metamaterials. A ferrite film with a permeability of 20 could reduce the resonant frequency of metamaterials by up to 50%. A prototype has been fabricated and the measurement data align well with the simulation results. Because of the lowered operational frequency, the proposed ferrite loaded metamaterials offer more flexibility for various sub-GHz microwave applications, such as cloaks, absorbers, and frequency selective surfaces.

Nonlinear effective properties of heterogeneous materials with ellipsoidal microstructure

The analysis and the synthesis of the nonlinear effective response of particulate composite materials are of great importance for developing new systems such as nonlinear elastic and electromagnetic metamaterials, nonlinear waveguides, nonlinear magnetoelectric devices and photonic or phononic crystals. Typically, classical homogenization schemes take into account the shape of the inhomogenieties but neglect the spatial correlation among them, a crucial feature for the above applications. In this paper we develop a nonlinear homogenization technique for dispersions of nonlinear particles in a linear matrix, which is able to take account of spatial correlation by means of the so-called ellipsoidal microstructure. While the linear result corresponds to the well known Ponte Castañeda–Willis estimate, we propose new formulae for the second and third order nonlinear behavior. We finally show applications to the nonlinear elastic Landau coefficients and to the nonlinear hypersusceptibility of transport processes.

Finite element analysis of electromagnetic waves in two-dimensional transformed bianisotropic media.

We analyze the wave propagation in two-dimensional bianisotropic media with the Finite Element Method (FEM). Starting from the Maxwell-Tellegen's equations in bianisotropic media, we derive some system of coupled Partial Differential Equations (PDEs) for longitudinal electric and magnetic field components. These PDEs are implemented in FEM using a solid mechanics formulation. Perfectly Matched Layers (PMLs) are also discussed to model unbounded bianisotropic media. The PDEs and PMLs are then implemented in a finite element software, and transformation optics is further introduced to design some bianisotropic media with interesting functionalities, such as cloaks, concentrators and rotators. In addition, we propose a design of metamaterial with concentric layers made of homogeneous media with isotropic permittivity, permeability and magnetoelectric parameters that mimic the required effective anisotropic tensors of a bianisotropic cloak in the long wavelength limit (homogenization approach). Our numerical results show that transformation based electromagnetic metamaterials can be extended to bianisotropic media.

Resonance-based metamaterial in the shallow sub-wavelength regime: negative refractive index and nearly perfect absorption

The research on magnetic resonances in typical meta-atoms has led to the discovery of electromagnetic metamaterials (MMs). These new materials played a crucial role in achieving extraordinary phenomena as well as promised potential applications. In this paper, we numerically and experimentally investigated two different MM effects: the absorption and the negative refraction, which induced by magnetic resonances in a symmetric structure. The meta-atom sandwich model that includes two parallel flat rings separated by an insulating slab was designed. Firstly, three resonances in sub-wavelength range were demonstrated, revealing the negative permittivity and permeability effects. Notably, negative refractive index (NRI) was gained at the third-gap resonance, resulting from superposition of the rest of the electric resonance and the magnetic one accompanied by multi-plasmon. Moreover, the manipulation of the structural parameters could control the NRI behavior and, interestingly, a nearly perfect absorption peak arises in shallow sub-wavelength regime.

Hybrid semiconductor–dielectric metamaterial modulation for switchable bi-directional THz absorbers

There is an increasing interest for electromagnetic metamaterials that show mutable absorption properties with real-time and dynamic control. In this paper, we investigate a modulation of bi-directional metamaterial absorbers that is thermally switchable at terahertz frequencies. The metamaterial absorber is composed of symmetric hybrid semiconductor-dielectric cut-wire-pair structures, whose electromagnetic responses can be actively manipulated by utilizing an external heat source. As increasing the temperature of metamaterials from 300 to 350K, we demonstrate that the magnetic resonance can be systematically blue-shifted and overlapped with the electric resonance, which is unaffectedly settled at about 0.8THz. This superposition provides an effective mechanism to control the absorption intensity from 43% to nearly 95%. Finite integration simulation technique, standard retrieval method, and equivalent circuit model are employed to elaborate our idea.

Effective material parameter retrieval of anisotropic elastic metamaterials with inherent nonlocality

In this paper, the scattering (S-) parameter retrieval method is presented specifically for anisotropic elastic metamaterials; so far, no retrieval has been accomplished when elastic metamaterials exhibit fully anisotropic behavior. Complex constitutive property and intrinsic scattering behavior of elastic metamaterials make their characterization far more complicated than that for acoustic and electromagnetic metamaterials. In particular, elastic metamaterials generally exhibit anisotropic scattering behavior due to higher scattering modes associated with shear deformation. They also exhibit nonlocal responses to some degrees, which originate from strong multiple scattering interactions even in the long wavelength limit. Accordingly, the conventional S-parameter retrieval methods cannot be directly used for elastic metamaterials, because they determine only the diagonal components in effective tensor property. Also, the conventional methods simply use the analytic inversion formulae for the material characterization so that inherent nonlocality cannot be taken into account. To establish a retrieval method applicable to anisotropic elastic metamaterials, we propose an alternative S-parameter method to deal with full anisotropy of elastic metamaterials. To retrieve the whole effective anisotropic parameter, we utilize not only normal but also oblique wave incidences. For the retrieval, we first retrieve the ratio of the effective stiffness tensor to effective density and then determine the effective density. The proposed retrieval method is validated by characterizing the effective material parameters of various types of non-resonant anisotropic metamaterials. It is found that the whole effective parameters are retrieved consistently regardless of used retrieval conditions in spite of inherent nonlocality.

Microwave memristive behavior in split-ring resonator metamaterials

Photonic memristors, which behave as memristors operating with electromagnetic fields, present an effective means to achieve all-optical networking, and can promote the development of optical communications and computer technology. In this paper, we report a microwave memristive phenomenon at room temperature in metamaterials consisting of negative temperature coefficient thermistor ceramic disk and split-ring resonator (SRR). Hysteretic transmission-incident field power loops, the area of which varies with the scan rate of power, (similar to the fingerprint of memristors) were observed in the metamaterials. These effects are attributed to the increasing conductivity of the ceramic disk with increasing temperature generated by the interaction between electromagnetic waves and metamaterials. This work offers new opportunities for the development of photonic memristors.

Near-field thermal radiation between homogeneous dual uniaxial electromagnetic metamaterials

Recently, near-field thermal radiation has attracted much attention in several fields since it can exceed the Planck blackbody limit through the coupling of evanescent waves. In this work, near-field radiative heat transfer between two semi-infinite dual uniaxial electromagnetic metamaterials with two different material property sets is theoretically analyzed. The near-field radiative heat transfer is calculated using fluctuational electrodynamics incorporated with anisotropic wave optics. The underlying mechanisms, namely, magnetic hyperbolic mode, magnetic surface polariton, electrical hyperbolic mode, and electrical surface polariton, between two homogeneous dual uniaxial electromagnetic metamaterials are investigated by examining the transmission coefficient and the spectral heat flux. The effect of vacuum gap distance is also studied, which shows that the enhancement at smaller vacuum gap is mainly due to hyperbolic mode and surface plasmon polariton modes. In addition, the results show that the contribution of s-polarized waves is significant and should not be excluded due to the strong magnetic response regardless of vacuum gap distances. The fundamental understanding and insights obtained here will facilitate the finding and application of novel materials for near-field thermal radiation.

Simulation of Isotropic Acoustic Metamaterials

The model use to study electromagnetic metamaterials in transverse electric mode was modified to study the pressure distribution in an acoustic metamaterial in a two dimensional geometry. An electromagnetic wave of 30 GHz in transverse magnetic mode at normal incidence propagating through a two dimensional isotropic semi infinite double negative metamaterial slab of 640×830 cells embedded in free space with low loss damping frequency 108s-1 was studied by finite difference time dependent method with Yee’s algorithm with an explicit leapfrog scheme. The multiple cycle m-n-m pulses generating beams were used as sources. The simulations for refractive indices n=-1 and n=-16 at different time steps are presented. For n=-1, the sub-wavelength imaging is apparent, diverging the waves from source into two sources, one inside and the other outside the slab. The reverse propagation is much more significant for showing that the group velocity is much larger inside the metamaterial by the closely packed wave front, making the continuation of the wave pattern much more significant through the metamaterial into the normal space.

Simulation of Isotropic Acoustic Metamaterials

The model use to study electromagnetic metamaterials in transverse electric mode was modified to study the pressure distribution in an acoustic metamaterial in a two dimensional geometry. An electromagnetic wave of 30 GHz in transverse magnetic mode at normal incidence propagating through a two dimensional isotropic semi infinite double negative metamaterial slab of 640×830 cells embedded in free space with low loss damping frequency 108s-1was studied by finite difference time dependent method with Yee’s algorithm with an explicit leapfrog scheme. The multiple cyclem-n-mpulses generating beams were used as sources. The simulations for refractive indices n=-1 and n=-16 at different time steps are presented. For n=-1, the sub-wavelength imaging is apparent, diverging the waves from source into two sources, one inside and the other outside the slab. The reverse propagation is much more significant for showing that the group velocity is much larger inside the metamaterial by the closely packed wave front, making the continuation of the wave pattern much more significant through the metamaterial into the normal space.

Elastic metamaterials for independent realization of negativity in density and stiffness.

In this paper, we present the first realization of an elastic metamaterial allowing independent tuning of negative density and stiffness for elastic waves propagating along a designated direction. In electromagnetic (or acoustic) metamaterials, it is now possible to tune permittivity (bulk modulus) and permeability (density) independently. Apparently, the tuning methods seem to be directly applicable for elastic case, but no realization has yet been made due to the unique tensorial physics of elasticity that makes wave motions coupled in a peculiar way. To realize independent tunability, we developed a single-phased elastic metamaterial supported by theoretical analysis and numerical/experimental validations.

Functionalized Metamaterials Enable Frequency and Polarization Agility in a Miniaturized Lightweight Antenna Package

Electromagnetic metamaterials share a perceived disadvantage with miniature radio communication antennas: limited operating bandwidths. In conventional radio systems, tuning has been confined to the radio, requiring broadband antennas and materials. With the advent of software defined and digital radios, adding the antenna into the plethora of tunable radio subsystems can become a reasonable proposition, allowing miniaturized antennas with narrow instantaneous (channel) bandwidths to be tuned across entire communications bands, depending on the channel in use. Moreover, the antenna will provide an effective filtering stage before the signal reaches the radio. The tunable metamaterial presented herein enables an antenna showcasing this functionality and more. Dramatic size reductions are made possible by a tunable, lightweight, miniaturized metamaterial. Tuning the metamaterial and antenna in tandem provides a dynamic operating channel, with a tunable, nearly arbitrary polarization response as an added benefit. Finally, this antenna provides one of the first examples in the literature of a practical device improved by functionalized metamaterials, which has been tested on a real‐world platform.

Theoretical observation of optical Fano resonance at Gratting nano structure

Recently, Fano resonances have been studied in nano particles, plasmonics, photonic crystals and electromagnetic metamaterials .In this study, we studied the Fano resonance in individual semiconductor and investigated the effects on gratting. In our studies, we introduced a simple semiconductor nano-structure and described the simulations and analysis of optical Fano resonance in scattering and absorption spectra of structure and investigated experimentally it as a function of different thickness, change the width and radius of semiconductor and at different wavelengths at periodic conditions and angle of incidence equal 0°. According to the results, Fano resonances are observed in the mentioned structures and they are depend on angle of incidence, width and radius of structure, wavelengths.

Non-local Optical Topological Transitions and Critical States in Electromagnetic Metamaterials

Just as the topology of the Fermi surface defines the properties of the free electrons in metals and semiconductors, the geometry of the iso-frequency surface in the phase space of the propagating electromagnetic waves, determines the optical properties of the corresponding optical materials. Furthermore, in the direct analog to the Lifshitz transition in condensed matter physics, a change in the topology of iso-frequency surface has a dramatic effect on the emission, propagation and scattering of the electromagnetic waves. Here, we uncover a new class of such optical topological transitions in metamaterials, induced by the non-locality of the electromagnetic response inherent to these composites.

Metamaterials for Rapidly Forming Large-Area Distributed Plasma Discharges for High-Power Microwave Applications

Electromagnetic metamaterials have broad application potential including new high-power microwave (HPM) sources and anti-HPM devices. In the previous work, we demonstrated that an initial breakdown at one location within a multiresonator unit cell of the single-layer metamaterial emitted vacuum ultra-violet (VUV) radiation that induced breakdowns at the neighboring locations even though the electric field intensities were below the breakdown thresholds. In this paper, we report the results of experimental investigations of single-layer metamaterials deliberately designed to exploit this effect. When illuminated by 26-kW, 9.382-GHz, 0.8- $\mu \text{s}$ intense microwave pulses, breakdown was initially induced only in one small location where the radio frequency (RF) electric fields exceeded the breakdown threshold. In experiments with the metamaterials, the initial breakdown locally occurred in 5–10 ns, after which the VUV preionization effect facilitated the rapid spread of the breakdown across the entire surface within 10–20 ns. In contrast, without a metamaterial, the breakdown remained localized and was delayed by 25–30 ns compared with the metamaterial cases. The experimental results are expected to provide a useful guideline for designing metamaterials in HPM systems.

Optimum topological design of negative permeability dielectric metamaterial using a new binary particle swarm algorithm

This paper presents a Particle Swarm Optimization-based topology optimization method for the design of negative permeability dielectric metamaterials.As the electromagnetic metamaterials have some physical properties not available in nature, they have attracted a huge scientific research interest for decades. In fact, electromagnetic metamaterials can exhibit simultaneously negative permeability and negative permittivity. The aim of this work is to find an optimal topology of a dielectric metamaterial that achieves negative permeability at a given frequency. A binary Particle Swarm Optimization is developed and applied to a negative permeability dielectric metamaterial topology design problem. The optimization process is achieved using a developed numerical model of the studied metamaterial, which is solved by the Finite Element Method.First, the governing equations and the weak formulation of the electromagnetic problem are presented. Then, the optimization problem to be solved is formulated. The developed binary Particle Swarm Optimization method, and the developed interfacing method are explained. Some numerical examples are presented to demonstrate that the binary Particle Swarm Optimization is adapted to the topology optimization of negative permeability dielectric metamaterials, at given frequencies, to demonstrate the utility and validity of the presented method.