Spatial dispersion in metamaterials with negative dielectric permittivity and its effect on surface waves

The effect of spatial dispersion on the electromagnetic properties of a metamaterial consisting of a three-dimensional mesh of crossing metallic wires is reported. The effective dielectric permittivity tensor epsilon(ij)(omega, k) of the wire mesh is calculated in the limit of small wavenumbers. The procedure for extracting the spatial dispersion from the omega versus k dependence for electromagnetic waves propagating in the bulk of the metamaterial is developed. These propagating modes are identified as similar to the longitudinal (plasmon) and transverse (photon) waves in a plasma. Spatial dispersion is found to have the most dramatic effect on the surface waves that exist at the wire mesh-vacuum interface.     

Resonant excitation of evanescent spatial harmonics in medium formed by parallel metallic nanorods

The results of analytical modeling of the resonant excitation of evanescent harmonics in a medium formed by parallel metallic nanorods taking into account the spatial dispersion are presented. Analytical expressions are derived for the reflection and transmission coefficients, as well as for the amplitudes of electromagnetic waves inside the medium. These expressions are compared to similar expressions that were previously obtained using a local model of an ultimately anisotropic material without taking into account the spatial dispersion. The obtained expressions are simplified for various partial cases, including the superresolution imaging of a source that is located at a considerable distance from the metamaterial layer. A layer of a medium composed from finitesized wires is numerically simulated and it is demonstrated that, due to the effect of resonant excitation of evanescent spatial harmonics in the layer, subwavelength details of an object that is considerably distant from the layer can be distinguished inside of the layer.     

Equivalent electromagnetic parameters for microwave metamaterial absorber using a new symmetry model

Transmission line theory uses the complex nature of permeability and permittivity of a conventional magnetic absorber to evaluate its absorption properties and mechanism. However, because there is no method to obtain the electromagnetic parameters of a metamaterial-absorber integrated layer(composed of a medium layer and a periodic metal array), this theory is seldom used to study the absorption properties of the metamaterial absorber. We propose a symmetry model to achieve an equivalent complex permittivity and permeability model for the integrated layer, which can be combined with the transmission line theory to calculate metamaterial absorption properties. The calculation results derived from both the transmission line theory and the high-frequency structure simulator are in good agreement. This method will be beneficial in practical investigations of the absorption mechanism of a metamaterial absorber.     

Genuine effectively biaxial left-handed metamaterials due to extreme coupling

Most left-handed metamaterials cannot be described by local effective permittivity or permeability tensors in the visible or near-infrared due to the mesoscopic size of the respective unit cells and the related strong spatial dispersion. We lift this problem and propose a metamaterial exhibiting artificial magnetism that does not suffer from this restriction. The artificial magnetism arises from the extreme coupling between both metallic films forming the unit cell. We show that its electromagnetic response can be properly described by biaxial local constitutive relations. A genuine biaxial left-handed fishnet metamaterial is suggested, which can be realized by atomic layer deposition to fabricate the nanoscaled spacing layers required for extreme coupling.     

可听声频段的声学超材料

制作了一维周期排列的亥姆霍兹共振器超材料,在空气环境下测试了其在可听声频段声学透射行为.实验结果表明,在2.1—3.5 kHz附近该材料具有透射衰减的吸收峰,利用声传输线理论（ATLM）计算的透射率和实验结果一致,同时由计算的等效阻抗分析可知,实验中出现的吸收峰是由HRs共振的回波反射引起的.另外,实验测试的样品中透射信号分布进一步验证了材料的共振效应,也就是会出现与外加激励反相响应.基于前述的共振模型计算出该材料的等效弹性模量为负. 关键词： 亥姆霍兹共振器 声传输线 吸收峰 等效弹性模量     

1.7—2.7GHz宽频带小单元异向介质设计及其介质参数提取

提出一种单元电尺寸小、工作频带宽、损耗小、结构简单的异向介质设计方案，在1.7—2.7 GHz上所设计的异向介质结构单元电尺寸小于0.035，相对带宽达到45.5%，在整个工作频带上单个结构单元传输损耗小于0.75 dB.对由上述异向介质单元构成的半无限大异向介质平板的电磁波反射、透射特性进行了数值仿真分析，并提取出了电磁波在该异向介质平板中传播时的波数、相速、折射率以及该异向介质平板的有效介电常数和有效磁导率等一系列电磁特性参数，仿真与计算结果表明复波数的实部、相速以及折射率的实部在1.7—2.7 GHz的范围上为负值，并且在相同频带上，有效介电常数和有效磁导率的实部同时为负值，从而有效地验证了“后向行波效应”、“负折射效应”、“双负效应”等异向介质特有的电磁特性，对上述异向介质的存在性给予有力证明. 关键词： 异向介质 宽频带 小单元 介质参数     

Bright and dark spatial solitons in metallic nanowire arrays

We investigate the formation and propagation of bright and dark three-dimensional unstaggered spatial solitons with cylindrical symmetry in a nonlinear nanowire metamaterial. The metamaterial is formed by metallic nanowires embedded in a Kerr-type dielectric host and is modeled using an effective medium approach. Unlike conventional Kerr media, the metamaterial supports bright solitons when the host is a self-defocusing material and dark solitons when the host is a self-focusing material. Our numerical calculations show that the confinement of the spatial-solitons results from the interplay of the host nonlinear response strength and the hyperbolic dispersion of the photonic states in the nanowire array. Subwavelength solitary beams may be observed for sufficiently strong nonlinearities.     

Metamaterials and their application in microwaves: A review

A metamaterial is a composite material that has attracted the attention of researchers since the late 1990s-early 2000s. This material contains an artificial periodic structure, which modifies its permittivity and permeability and, thereby, makes it possible to control the dispersion, refraction, and reflection of electromagnetic waves in the metamaterial. Analytical and experimental studies of the properties of metamaterials, as well as their applications, cover a wide frequency range from radio waves to the visible range. In recent years, considerable progress has been made toward the application of these materials in the microwave range (1–100 GHz). Works on development and application of metamaterials in the microwave range published over the last 8–10 years are reviewed. Artificial transmission lines as 1D metamaterials are discussed. Resonators, filters, and phase shifters based on the “metamaterial philosophy” are considered. Special attention is given to the application of metamaterials in the antenna technology.     

Optical properties of a three-dimensional chiral metamaterial

A three-dimensional chiral metamaterial with four-fold rotational symmetry is designed, and its optical properties are investigated by numerical simulations. The results show that this chiral metamaterial has the following features: high polarization conversion, perfect circular dichroism, and asymmetric transmission of circularly polarized light. A comparison of the results of chiral metamaterials without and with weak coupling between the constituent nanostructures enables us to confirm that the optical properties of our proposed nanostructure are closely related to the coupling between the single nanoparticles. This means that the coupling between nanoparticles can enhance the polarization conversion, circular dichroism, and asymmetric transmission. Due to the excellent optical properties, our metamaterial might have potential applications in the development of future multi-functional optical devices.     

Multilayer graphene waveguides

We study dispersion properties of TM-polarized electromagnetic waves guided by a multilayer graphene metamaterial. We demonstrate that both dispersion and localization of the guided modes can be efficiently controlled by changing the number of layers in the structure. Remarkably, we find that in the long wavelength limit, the dispersion of the fundamental mode of the N-layer graphene structure coincides with the dispersion of a plasmon mode supported by a single graphene layer, but with N times larger conductivity. We also compare our exact dispersion relations with the results provided by the effective media model.     

Membrane-based acoustic metamaterial with near-zero refractive index

We investigate a one-dimensional acoustic metamaterial with a refractive index of near zero(RINZ) using an array of very thin elastic membranes located along a narrow waveguide pipe. The characteristics of the effective density, refractive index, and phase velocity of the metamaterial indicate that, at the resonant frequency fm, the metamaterial has zero mass density and a phase transmission that is nearly uniform. We present a mechanism for dramatic acoustic energy squeezing and anomalous acoustic transmission by connecting the metamaterial to a normal waveguide with a larger cross-section. It is shown that at a specific frequency f_1, transmission enhancement and energy squeezing are achieved despite the strong geometrical mismatch between the metamaterial and the normal waveguide. Moreover, to confirm the energy transfer properties, the acoustic pressure distribution, acoustic wave reflection coefficient, and energy transmission coefficient are also calculated. These results prove that the RINZ metamaterial provides a new design method for acoustic energy squeezing,super coupling, wave front transformation, and acoustic wave filtering.     

Investigation of optical properties of Tamm states in higher bands of multilayer stacks including right and left handed materials

We study the electromagnetic surface wave localizing, the so-called surface Tamm states, at an interface separating a left-handed metamaterial (LHM) and a semi-infinite one-dimensional photonic crystal made of alternative left-handed metamaterial and right-handed materials. We show that the existence of metamaterial causes the Tamm states with backward energy flow and allows flexible control of dispersion properties of the surface modes. We study the effect of the physical parameters of the photonic crystal on the dispersion properties and the group velocity of the Tamm states. We also study dispersion properties of the Tamm states in higher order photonic band gaps of the photonic crystal and compare our results with the case when the LHM medium is replaced by a right-handed material (RHM).     

Experimental verification of negative refraction in a double cross metamaterial

We report on microwave experiments with a metamaterial composed of pairs of metallic crosses. The transmission properties of the structure show a left-handed transmission band at frequencies around 10.2 GHz. The validity of the negative effective index of refraction is verified by a Snell’s law refraction experiment performed on a wedge-shaped sample of the metamaterial. A second measurement of a similar wedge made from blank FR4 boards is done for reference. The results of the measurements show positive refraction over the whole measured frequency band for the FR4 wedge as well as the refraction of the incident radiation to negative angles within the designated left-handed frequency band for the metamaterial sample.     

Metamaterial composed of wire pairs exhibiting dual band negative refraction

The design of the metamaterial that can exhibit negative refraction at two frequency bands is presented. The components of this metamaterial are cut wire pairs and continuous wires. The cut wire pairs structure in our sample can achieve the magnetic resonance at two frequency bands by appropriately designing the cut wire dimension. Through numerical simulation, the transmission property of the proposed dual band negative index metamaterial is investigated and its result shows that with the introduction of continuous wires, the stop bands for cut wire pairs (permeability μ<0) and the frequency band for continuous wires (permittivity ε<0) components would overlap and lead to the appearance of pass bands near the two magnetic resonance frequency bands. Its electromagnetic properties are then retrieved to demonstrate that the dual band left-hand behavior can be obtained in our sample structure. It is believed that our approach will be effective to make this kind of dual band negative refractive metamaterial based on the multiple magnetic resonances work at optical frequency.     

Layer homogenization of a 2D periodic array of scatterers

The homogenization of a metamaterial made of a collection of scatterers periodically disposed is studied from an asymptotic theory and an optimization algorithm. Detailed numerical results are given for resonant scatterers and the spatial dispersion is investigated.     

Numerical simulations of terahertz double-negative metamaterial with isotropic-like fishnet structure

The isotropic-like fishnet metamaterial with an array of cross-shaped holes penetrating through the multiple layers has been numerically studied in the terahertz regime. Its left-handed properties are described by the retrieved effect media parameters and validated by the simulation of a wedge-shaped model. The influences of the various geometrical parameters on the electromagnetic response and the left-handed performance are investigated. This isotropic-like fishnet metamaterial has the advantage of lower losses and higher transmission and holds potential for further applications at higher frequencies.     

Band structure of collective modes and effective properties of binary magnonic crystals

In this paper a theoretical study of the band structure of collective modes of binary ferromagnetic systems formed by a submicrometric periodic array of cylindrical cobalt nanodots partially or completely embedded into a permalloy ferromagnetic film is performed. The binary ferromagnetic systems studied are two-dimensional periodic, but they can be regarded as three-dimensional, since the magnetization is non uniform also along the z direction due to the contrast between the saturation magnetizations of the two ferromagnetic materials along the thickness. The dynamical matrix method, a finite-difference micromagnetic approach, formulated for studying the dynamics in one-component periodic ferromagnetic systems is generalized to ferromagnetic systems composed by F ferromagnetic materials. It is then applied to investigate the spin dynamics in four periodic binary ferromagnetic systems differing each other for the volume of cobalt dots and for the relative position of cobalt dots within the primitive cell. The dispersion curves of the most representative frequency modes are calculated for each system for an in-plane applied magnetic field perpendicular to the Bloch wave vector. The dependence of the dispersion curves on the cobalt quantity and position is discussed in terms of distribution of effective “surface magnetic charges” at the interface between the two ferromagnetic materials. The metamaterial properties in the propagative regime are also studied (1) by introducing an effective magnetization and effective “surface magnetic charges” (2) by describing the metamaterial wave dispersion of the most representative mode in each system within an effective medium approximation and in the dipole-exchange regime. It is also shown that the interchange between cobalt and permalloy does not necessarily lead to an interchange of the corresponding mode dispersion. Analogously to the case of electromagnetic waves in two-dimensional photonic crystals, the degree of localization of the localized collective modes is expressed in terms of an energy concentration factor.     

Optically active Babinet planar metamaterial film for terahertz polarization manipulation

A planar Babinet‐inverted dimer metamaterial possessing strong optical activity is proposed and characterized. An original fabrication method to produce large area (up to several cm²) freely suspended flexible metallic membranes is implemented to fabricate the metamaterial. Its optical properties are characterized by terahertz time‐domain spectroscopy, revealing anisotropic transmission with high optical activity. A simple coupled resonator model is applied to explain the principal optical features of the dimers, with predictive power of positions and number of resonances through a parametrical model. The model is validated for correct polarization‐dependent quantitative results on the optical activity in transmission spectra. The fabrication method presented in this work as well as the slit dimer design has great potential for exploitation in terahertz optics.     

Temperature-controlled tunable acoustic metamaterial with active band gap and negative bulk modulus

The local resonant band gap and the negative bulk modulus of the acoustic metamaterial with Helmholtz resonators are strongly affected by the temperature of water. In this paper, the acoustic transmission line method (ATLM) is introduced to investigate the influences of the temperature of water on the local resonant band gap and the negative bulk modulus of the acoustic metamaterial. Results show that the relative variations of the local resonant band gap and the negative effective bulk modulus suffering from the change of the temperature of water are approximately equivalent and are up to about 11%. The reason is that the local resonant frequency is proportional to the sound speed of fluid which is strongly effects by the temperature of water. By achieving the unambiguous relationships between these unusual properties of the acoustic metamaterial and the temperature of water, we find that the temperature-controlled acoustic metamaterial with the active band gap and the active negative bulk modulus can be realized in theory. This idea opens a new avenue for the design of the tunable acoustic metamaterial that can modulate the acoustic wave propagation.