磁谐振和电谐振结构构成的左手材料设计

用磁谐振器和电谐振器组合构成左手材料是设计左手材料的一个重要方法.基于这一设计方法,提出了能够抑制双各向异性的磁谐振器和电谐振器设计原理,即单回路镜像对称设计原理和双(四)回路镜像对称设计原理.根据这两个设计原理,设计了具有双负特性(负介电常数和负磁导率)的新结构,并通过实验仿真验证了结构的双负特性,从而验证了两种设计原理的正确性.所提出的两个设计原理以及新结构对于设计新型左手材料具有重要的实际意义和指导意义. 关键词： 左手材料 单回路镜像对称 双回路镜像对称     

A candidate three-dimensional GHz left-handed metamaterial composed of coplanar magnetic and electric resonators

GHz left-handed metamaterials (LHMs) composed of coplanar magnetic and electric resonators were proposed in this paper. On each of the unit cells, the electric resonator is placed in the center space of the magnetic resonator. By adjusting the geometrical dimensions of the resonators, negative magnetic response of the magnetic resonator and negative electric response of the electric resonator can be tuned to be coexistent at the same frequencies. The effective constitutive parameters were retrieved. The results verified simultaneous negative permeability and permittivity as well as negative index of the proposed LHMs. Based on the above work, two- and three-dimensional LHMs that make use of coplanar magnetic and electric resonators were proposed. The work done in this paper is of great reference values in fabricating three-dimensional LHMs.     

Hybridization effect in coupled metamaterials

Although the invention of the metamaterials has stimulated the interest of many researchers and possesses many important applications, the basic design idea is very simple: composing effective media from many small structured elements and controlling its artificial EM properties. According to the effective-media model, the coupling interactions between the elements in metamaterials are somewhat ignored; therefore, the effective properties of metamaterials can be viewed as the “averaged effect” of the resonance property of the individual elements. However, the coupling interaction between elements should always exist when they are arranged into metamaterials. Sometimes, especially when the elements are very close, this coupling effect is not negligible and will have a substantial effect on the metamaterials’ properties. In recent years, it has been shown that the interaction between resonance elements in metamaterials could lead to some novel phenomena and interesting applications that do not exist in conventional uncoupled metamaterials. In this paper, we will give a review of these recent developments in coupled metamaterials. For the “metamolecule” composed of several identical resonators, the coupling between these units produces multiple discrete resonance modes due to hybridization. In the case of a “metacrystal” comprising an infinite number of resonators, these multiple discrete resonances can be extended to form a continuous frequency band by strong coupling. This kind of broadband and tunable coupled metamaterial may have interesting applications. Many novel metamaterials and nanophotonic devices could be developed from coupled resonator systems in the future.     

Electromagnetic energy density in a single-resonance chiral metamaterial

We derive the electromagnetic energy density in a chiral metamaterial consisting of uncoupled single-resonance helical resonators. Both the lossless and absorptive cases are studied, and the energy density is shown to be positively definite. The key relation making the derivation successful is the proportionality between the magnetization and the rate of change of the electric polarization of the medium. The same time-domain formulation of energy density also applies to the bianisotropic medium proposed by Zhang et al. [Phys. Rev. Lett.102, 023901 (2009)PRLTAO0031-900710.1103/PhysRevLett.102.023901]. This work may provide insights for studying time-dependent phenomena in metamaterials.     

Optimal helix shape: Equality of dielectric,magnetic, and chiral susceptibilities

The possibility of linearly polarized electromagnetic microwave conversion into a circularly polarized wave using single-turn helices with preliminary calculated optimal parameters is demonstrated. The helices are characterized by equal dielectric, magnetic, and chiral susceptibilities. Such optimal helices can be further used, for example, to develop reflectionless coatings and metamaterials with negative electromagnetic wave refraction. It is demonstrated that the examined helices activated by both electric and magnetic fields, that is, for any arbitrary orientation of the incident wave polarization plane, have optimal characteristics. This is the advantage of the optimal helices over other possible metamaterial elements, for example, rectilinear vibrators or circular resonators.     

Magnetic monopole-like response in metamaterials

In this paper, we explored magnetic monopole-like responses in metamaterials. We designed a sub-wavelength metamolecule that is composed of two dielectric-spaced split-ring resonators. In response to incident waves, the induced magnetic field in the metamolecule resembles that of a two-dimensional magnetic monopole. The magnetic monopole-like response is resulted from electric resonance of the metamolecule, so an electric dipole is always attached. By combining two mirror-symmetric metamolecules with inward and outward radial magnetic fields, magnetic dipole-like responses can be produced just as an electric dipole is formed by separating two opposite-signed electric charges.     

电谐振器和磁谐振器构成的左手材料的实验验证

加工、制作和测试了由电谐振器和磁谐振器构成的左手材料.首先分别测试了电谐振器、磁谐振器各自的传输频谱,得到各自的负参数区域,然后测试了由电谐振器和磁谐振器构成的左手材料的传输频谱,并提取了其等效介电常数和磁导率.实验结果表明,在电谐振器和磁谐振器各自传输禁带的重合区域,出现了一个通带,在该通带内,等效介电常数和磁导率同时为负,验证了材料的左手特性.     

Tunable dual-band infrared chiral metamaterials based on double-layered asymmetric U-shape split ring resonators

We have numerically demonstrated chiral metamaterials based on double-layered asymmetric U-shape split ring resonators. Dual-band circular dichroism is found in the whole frequency regime from 50 to 250 THz by studying the transmission properties. Huge optical activity and the induced negative refractive index are obtained at resonance by calculating the optical activity and ellipticity of the transmitted E-fields. Chirality parameter and effective refractive index are retrieved to illustrate the tunable optical properties of the metamaterials. The underlying mechanisms for the observed circular dichroism are analyzed. These metamaterials would offer flexible electromagnetic applications in the infrared regime.     

Magnetoelectric-Field Electrodynamics: Search for Magnetoelectric Point Scatterers

Resonant scattering of electromagnetic (EM) waves by small particles is considered as one of the basic problems in metamaterial science. At present, special subwavelength resonators are considered as structural elements in chiral and bianisotropic metamaterials. There is a general consensus that these small scatterers behave like “artificial atoms” with strong electrical and magnetic responses and an interconnection between these responses. However, the observed effect of magnetoelectric (ME) coupling in these meta-atoms is not associated with the near-field manipulation properties caused by intrinsic magnetoelectricity. This arises the question whether ME point scatterers of EM radiation really exist. In this paper, we show that there are mesoscopic structures with electric and magnetic dipole-carrying excitations that behave like point scatterers with their inherent magnetoelectricity. In such subwavelength resonators, coherent oscillations of the electric polarization and magnetization can be considered as quasistatic oscillations described by electrostatic (ES) and magnetostatic (MS) scalar wave functions. The ME resonance effect arises from the coupling of two, ES and MS, oscillations. The near fields of these resonators, called the ME near fields, are characterized by simultaneous violation of time reversal and inversion symmetry. In study of ME fields and EM problems associated with these fields, we put forward the concept of ME-field electrodynamics.     

Giant Asymmetric Transmission and Circular Dichroism with Angular Tunability in Chiral Terahertz Metamaterials

Inspired by the chiral structure from mode interaction, a chiral terahertz metamaterial and its complementary structure with the coexistence of asymmetric transmission and circular dichroism response are designed. The asymmetric transmission happens in the presence of electric and magnetic modes perpendicular to each other, resulting from intrinsic chirality under normal incidence. Circular dichroism exists in the presence of electric and magnetic modes parallel to each other under oblique incidence because of extrinsic chirality. Both asymmetric transmission and circular dichroism are enhanced by complementary structures, benefitting from the coupling between adjacent units. The maximum of chiral parameter κ can achieve 450, which is one hundred times higher than previously reported. The chiral response can be tuned by the incident angle and is sensitive to the environmental refractive index. The results highlight the potential applications of these metamaterials in chiral sensing and polarization transformation devices.     

Discrete breathers in nonlinear magnetic metamaterials

Magnetic metamaterials composed of split-ring resonators or U-type elements may exhibit discreteness effects in THz and optical frequencies due to weak coupling. We consider a model one-dimensional metamaterial formed by a discrete array of nonlinear split-ring resonators where each ring interacts with its nearest neighbors. On-site nonlinearity and weak coupling among the individual array elements result in the appearance of discrete breather excitations or intrinsic localized modes, both in the energy-conserved and the dissipative system. We analyze discrete single and multibreather excitations, as well as a special breather configuration forming a magnetization domain wall and investigate their mobility and the magnetic properties their presence induces in the system.     

A discrete model of damped acoustic metamaterials

For damped acoustic metamaterials, a discrete model is proposed in the form of a periodic structure with cells of the simplest type. The effective parameters of the model are determined by criteria based on the equality of dispersion of waves. The general properties of the model are studied. An example of one negative type of metamaterials is presented. The model is useful for analyzing wave properties and creating metamaterials with given acoustic properties.     

局域共振型声学超材料薄板带隙特性的能量解法

研究局域共振型声学超材料的带隙特性时,面临的首要问题是寻求一种带隙特性的高效解法。为此,本文以局域共振型声学超材料薄板为研究对象,提出了一种基于能量泛函变分原理及正交多项式级数展开的带隙计算方法。该方法通过合并基体板及共振子的动、势能及外力功建立元胞的能量泛函,经过变分运算得到元胞的振动控制方程,进而引入第一类Chebyshev正交多项式作为基函数将控制方程进行离散。为克服无法直接对离散控制方程中的广义坐标施加周期边界条件的困难,该方法将连续边界条件弱化到若干选定的配点上以离散方式实现,并将对应的一组线性约束条件通过拉格朗日乘子法施加到系统中。仿真结果表明,该方法具有较高的计算精度和效率,并可应用于"局域共振子-夹层板"等复合结构的带隙特性研究。本文方法为局域共振型薄壁超材料的带隙求解提供了新的思路,丰富了声学超材料的振-声学理论,并为薄壁超材料的工程设计和优化提供了技术支撑。      

Resonances of split-ring resonator metamaterials in the near infrared

We study the spectral response of optical metamaterials consisting of gold split-ring resonators. We utilize reflection spectroscopy in the near infrared region at normal incidence in the experiments. Our theoretical modeling is based on rigorous diffraction theory. We perform a comprehensive analysis of the dependence of the features of both the electric and the oscillating-circuit resonance on the geometry of the metamaterial. We show that theory and experiment are in good agreement. PACS 73.20.Mf; 78.67.-n; 78.66.Sq; 78.67.Bf     

Lowering plasma frequency by enhancing the effective mass of electrons： A route to deep sub-wavelength metamaterials

Deep sub-wavelength metamaterials are the key to the further development of practical metamaterials with small volumes and broadband properties. We propose to reduce the electrical sizes of metamaterials down to more sub-wavelength scales by lowering the plasma frequencies of metallic wires. The theoretical model is firstly established by analyzing the plasma frequency of continuous thin wires. By introducing more inductance elements, the effective electron mass can be enhanced drastically, leading to significantly lowered plasma frequencies. Based on this theory, we demonstrate that both the electric and the magnetic plasma frequencies of metamaterials can be lowered significantly and thus the electrical sizes of metamaterials can be reduced to more sub-wavelength scales. This provides an efficient route to deep sub-wavelength metamaterials and will give rigorous impetus for the further development of practical metamaterials.     

Double-T metamaterial for parallel and normal transverse electric incident waves

Electric and magnetic hybridized plasmonic modes are obtained by stacking two T-shaped resonators. We show that head-to-toe configuration leads to inverse the hybridization. The frequency shift between the resonances is finely controlled by adjusting the gap between the two resonators. A negative refractive index close to -1 is numerically and experimentally demonstrated at 4.3 GHz for TE waves. This left-handed behavior is similar for parallel and normal TE incident wave vectors. The proposed double-T unit cell is well adapted for developing terahertz and IR metamaterials.     

基于遗传算法的五模材料分层优化*

由于超材料的微结构尺度不能忽略,利用超材料微结构所集成的器件等效参数不是连续分布的,而是呈离散分布的。因此在利用超材料设计声学器件的过程中,需要对器件进行分层离散化,而分层方案的选取往往会影响器件的工作效果。该文基于五模材料水下隐身衣的设计理论,提出了一种基于遗传算法的五模材料隐身衣分层优化策略,并分别对优化前和优化后的分层隐身衣做了声学仿真。仿真结果表明采用优化策略后,隐身衣背景声场的散射能量和反射能量均有大幅度降低,并且对窄带探测信号和宽带探测信号均有较好的隐身效果,因此该分层优化策略能够显著提升隐身衣的隐身效果。该文所提出的分层优化策略为提升五模材料隐身衣性能提供了理论支持,并可以进一步推广至其他超材料器件的微结构设计中。     

超材料谐振子间的电耦合谐振理论与实验研究

通过将两个金属开口环谐振器口对口地放置, 实现了超材料谐振子间的电耦合谐振. 对电耦合谐振的微波等效电路进行了理论分析和数值计算, 结果表明耦合后的超材料谐振子能产生两个谐振频率, 其中一个随耦合强度的增加逐渐向低频方向移动, 而另一个固定在单谐振子的谐振频率处不变. 微波透射谱的实验测试和电磁仿真结果表明, 两个谐振峰随耦合强度的增加分别向低频和高频方向移动. 分析表明: 低频谐振峰的位置主要是由超材料谐振子间的电耦合强度决定的; 高频谐振偏离单谐振子的谐振频率主要是由不可避免的磁耦合引起的, 而且在耦合间距越小时磁耦合影响越大. 提出的基于超材料谐振子间的电磁耦合实现的双频谐振及其可调性极大地增加了超材料的设计与应用空间.     

一种新颖的变异开口谐振环双频带磁谐振特异材料

根据磁谐振等效电路原理,设计出了工作在X波段的基于变异开口谐振环(split ring resonators)结构的新型双频带磁谐振特异材料.由于内外环间具有弱耦合的优越特性,所提出的磁谐振特异材料的两个工作频点可以较为方便地调节.该研究成果为特异材料的多频带、宽带化提供有益的的研究思路和设计方法. 关键词： 磁谐振特异材料 双频带 弱耦合     

Dielectric-Based Electromagnetically Induced Transparency Metamaterial in the Microwave Band

An electromagnetically induced transparency metamaterial (EITM) based on dielectric resonators is presented in this paper, which consists of a cut wire (CW) and ten dielectric columns. Destructive interference between the CW, which is a branch of the microstrip line, and the dielectric resonators causes the EIT phenomenon. The simulation results show that the dielectric-based EITM designed can obtain a transmission peak of 0.938 at 2.17 GHz, and the two-oscillator model is utilized to verify the effectiveness of the structure. In addition, samples of the dielectric-based EITM are also fabricated and the effectiveness of the metamaterial is experimentally verified. The proposed metamaterial has several advantages, including high efficiency, low loss, a simple structure, and ease of manufacture, which has good prospects for application in the fields of electromagnetic switches, sensors, and nonlinear metamaterials.