Tunable electromagnetically induced transparency at terahertz frequencies in coupled graphene metamaterial

A graphene-based metamaterial with tunable electromagnetically induced transparency(EIT)-like transmission is numerically studied in this paper. The proposed structure consists of a graphene layer composed of coupled cut-wire pairs printed on a substrate. The simulation confirms that an EIT-like transparency window can be observed due to indirect coupling in a terahertz frequency range. More importantly, the peak frequency of the transmission window can be dynamically controlled over a broad frequency range by varying the Fermi energy levels of the graphene layer through controlling the electrostatic gating. The proposed metamaterial structure offers an additional opportunity to design novel applications such as switches or modulators.     

Cut-wire pairs and plate pairs as magnetic atoms for optical metamaterials

We study the optical properties of metamaterials made from cut-wire pairs or plate pairs. We obtain a more pronounced optical response for arrays of plate pairs, a geometry that also eliminates the undesirable polarization anisotropy of the cut-wire pairs. The measured optical spectra agree with simulations, revealing negative magnetic permeability in the range of telecommunications wavelengths. Thus nanoscopic plate pairs might serve as an alternative to the established split-ring resonator design.     

具有极化选择特性的超材料频率选择表面的设计

基于超材料结构的设计理论,通过长金属线周期阵列和具有Lorentz谐振形式等效介电常数的金属短线的组合可频率选通特性.由不同周期排布和结构尺寸的长金属线和金属短线组合而成的频率选择表面具有不同频段的选通特性.文章提出了双层及单面两种具有极化选择特性的频率选择表面,两种结构均可实现对特定频段TE及TM波选通.具有极化选择特性的超材料频率选择表面的设计为极化滤波器以及极化波产生器的设计提供了借鉴. 关键词： 频率选择表面 超材料 极化选择     

Effective material parameter retrieval for thin sheets: Theory and application to graphene,thin silver films,and single-layer metamaterials

An important tool in the field of metamaterials is the extraction of effective material parameters from simulated or measured scattering parameters of a sample. Here we discuss a retrieval method for thin-film structures that can be approximated by a two-dimensional scattering sheet. We determine the effective sheet conductivity from the scattering parameters and we point out the importance of the magnetic sheet current to avoid an overdetermined inversion problem. Subsequently, we present two applications of the sheet retrieval method. First, we determine the effective sheet conductivity of thin silver films and we compare the resulting conductivities with the sheet conductivity of graphene. Second, we apply the method to a cut-wire metamaterial with an electric dipole resonance. The method is valid for thin-film structures such as two-dimensional metamaterials and frequency-selective surfaces and can be easily generalized for anisotropic or chiral media.     

Single-/dual-band metamaterial absorber based on cross-circular-loop resonator with shorted stubs

A single-/dual-band metamaterial absorber (MMA) based on cross-circular-loop resonator (CCLR) with shorted stubs is discussed at microwave frequencies in this paper. The single-/dual-band characteristics are realized by adjusting the positions of the shorted stubs. We briefly analyze the equivalent circuit model of the MMA unit cell and then numerically and experimentally investigate the near-perfect absorptions in such two conditions (single- and dual-band). The results indicate that the proposed MMA exhibits near-perfect impedance matching with free space and high absorptivity of 99.74?% at 8.65?GHz for single-band condition, and absorptivities of 99.75?% and 97.35?% at 8.525 and 9.1?GHz, respectively, for dual-band condition. It also exhibits a wide range of angles of incidence for both transverse electric (TE) and transverse magnetic (TM) radiation. The two operating frequency bands, in dual-band condition, can be further controlled by adjusting the positions of shorted stubs or adding other shorted stubs. So it opens the way to fabricate controllable MMAs, and so controllable perfectly matched layers and bolometers.     

基于石墨烯振幅可调的宽带类电磁诱导透明超材料设计

基于石墨烯的电控特性提出了一种由金属线谐振器和"H"型谐振器组成的宽带可调的类电磁诱导透明(类EIT)超材料结构.首先,利用CST Microwave Studio软件对该超材料结构的透射特性进行了仿真.该结构在1.05—1.46 THz内的透射窗由金属线谐振器的等离子谐振和"H"型谐振器的电感-电容谐振干涉相消引起,且暗模式谐振器的数量增多导致了透射窗带宽的增加.其次,仿真模拟了该结构在不同石墨烯费米能级下的透射特性.当石墨烯费米能级由0 eV逐渐增加到1.5 eV时,该结构透射窗在1.05—1.46 THz内的平均透射振幅由87%逐渐减少到25%,实现了宽带可调.同时,通过仿真模拟该结构在1.26 THz下的电场分布对其透射机理进行了分析,并实验制备了类EIT超材料结构样品,且利用太赫兹时域光谱对样品进行了透射性能测试,测试结果与仿真分析的趋势基本一致.     

微穿孔板复合板型声学超材料的低频吸声

针对单层微穿孔板的低频吸声问题提出了微穿孔板复合板型声学超材料结构。将板型声学超材料置入微穿孔板结构的背腔内部实现结构复合。实验结果表明:在相同背腔厚度下,复合结构的吸声性能整体优于单层微穿孔板结构,其中复合结构的吸声曲线从396~892 Hz均大于0.6,在453 Hz处吸声系数达到0.972。利用有限元方法对复合结构进行了仿真,仿真计算的吸声曲线与实验吸声曲线的趋势基本相同,同时发现低频吸声主要由板型声学超材料与声波相互作用贡献。板型声学超材料的吸声峰值的对应频率处,其等效动态质量密度从正变负。在复合结构内部的微穿孔板和板型声学超材料存在相互耦合作用,使得复合结构的第一峰值发生微小偏移。增加板型声学超材料的质量块重量可以使第一吸声峰值向低频移动;保持总背腔厚度不变,增加板型声学超材料的子腔厚度,也可以使第一吸声峰向低频移动。      

Composite metamaterial enabled excellent performance of microstrip antenna array

This paper reports that the split ring resonators and complementary split ring resonators are compounded to construct a novel compact composite metamaterial.The composite metamaterial exhibits a unique property of blocking electromagnetic wave propagating in two directions near the resonant frequency.An example of two-element microstrip antenna array demonstrates that the developed metamaterial enables array performance that is an improvement in comparison with the traditional one,including mutual coupling suppression of 9.07 dB,remarkable side lobe suppression and gain improvement of 2.14 dB.The mechanism of performance enhancement is analysed based on the electric field and Poynting vector distributions in array.The present work not only is a meaningful exploration of new type composite metamaterial design,but also opens up possibilities for extensive metamaterial applications to antenna engineer.     

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.     

石墨烯和对称开口谐振环超材料中可调谐的双等离激元诱导透明现象

提出了由石墨烯和两对对称开口谐振环构成的等离激元诱导透明(PIT)超材料结构,该PIT超材料结构之间的耦合形式为暗-亮-暗模式.通过有限元方法模拟,观察到双PIT透明窗口,通过改变石墨烯的费米能级或者改变开口谐振环的几何参数来动态地调制PIT窗口.理论结果表明,当石墨烯与两对对称的开口谐振环之间的相互作用距离为0.5 μm、石墨烯费米能级为1.5 eV时,可得到最优的双透明窗口.双PIT效应在非线性器件、可调谐传感器、开关和慢光器件等方面有潜在的应用价值.     

Analysis of metamaterials using transmission line models

We use simple transmission line models with lumped elements of inductance and capacitance to interpret optical transmission and reflection spectra of cut wires and cut-wire pairs in the near infrared region. The numerical values of the elements are obtained by fitting experimental or numerical simulated reflectance and transmittance spectra. The scattering parameters and the retrieved effective material parameters calculated from the transmission line models show good agreements with those obtained from experiments or numerical simulations. This indicates that transmission line theory is a powerful tool for designing and analyzing metamaterials at optical frequencies. PACS 41.20.Jb; 78.67.-n; 78.66.Sg     

Transmission properties of electromagnetic metamaterials: From split-ring resonator to fishnet structure

We experimentally presented the electromagnetic behavior of transformative magnetic metamaterials: from the early invented split-ring resonator to its improvement, the cut-wire pair, for providing a negative magnetic permeability. By adding the continuous wire to cut-wire pair structure, the left-handed (LH) transmission made by doubly negative permittivity and permeability was demonstrated in combined structure. Interestingly, until the width of cut-wire pair increases to be physically merged with the adjacent continuous wire, in other words, to form the so-called fishnet structure, the LH behavior is still observed. This result indicates that in a broad sense, the essence of the electromagnetic response in the fishnet structure is similar to that in a combined structure. Our experimental results show a good agreement with previous theoretical study.     

基于电磁谐振的极化无关透射吸收超材料吸波体

仿真并实验验证了基于电磁谐振的极化无关透射吸收超材料吸波体, 该吸波体可以实现低频透射和高频吸收.实验测试结果表明, 该吸波体在6.77 GHz 吸收率峰值为83.6%, 半功率带宽为4.3%, 实现窄带强吸收.为进一步拓展该谐振型超材料吸波体的吸收带宽, 利用其低频透射特性, 将两个工作于不同频段的吸波体叠加在一起, 测试结果表明, 复合后超材料吸波体的半功率带宽可以增大到10.9%, 吸收率也略有增强. 该超材料吸波体设计简单, 具有较强的实用性和应用前景. 关键词： 极化无关 透射吸收 超材料吸波体     

基于石墨烯互补超表面的可调谐太赫兹吸波体

通过在石墨烯超表面设计周期性切条,实现了基于石墨烯互补超表面的可调谐太赫兹吸波体.通过改变外加电压来改变石墨烯的费米能级,吸波体实现频率可调谐特性.研究了石墨烯费米能级、结构尺寸对超材料吸波体吸收特性的影响,并利用多重反射理论研究了其物理机理并且证明了模拟方法的可行性.研究结果表明:当石墨烯费米能级取0.6 eV,基底厚度13μm,石墨烯上切条长宽分别为2.9μm,0.1μm时,吸波体在1.865 THz可以实现99.9%的完美吸收;石墨烯费米能级从0.4 eV增大到0.9 eV,吸波体共振频率从1.596 THz蓝移到2.168 THz,且伴随共振吸收率的改变,吸收率在0.6 eV时达到最大;通过改变费米能级实现的最大吸收率调制度达84.55%.     

Acoustic wave propagation in fluid metamaterial with solid inclusions

Acoustic wave propagation in a composite of water with embedded double-layered silicone resin/silver rods is considered. Approximate values of effective dynamical constitutive parameters are obtained. Frequency ranges of simultaneous negative constitutive parameters are found. Localized surface states on the interface between metamaterial and “normal” material are found. The Doppler effect in metamaterial is considered. The presence of anomalous modes is shown.     

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.     

Transformation optics and infrared metamaterials for optical devices

The coordinate transformation approach is applied for the design of an optical mode adaptor between two different width waveguides in the near IR telecom optical domain. The control of the mode profile in the adaptor is achieved by considering a composite waveguide consisting of a thin metamaterial (MM) layer intercalated between 2???m thick SU8 photoresist slab and a glass substrate. We demonstrate that intercalated metafilm enables the realization of a space coordinate transformation and allows a precise control of the light propagation in the composite waveguide. Numerical simulations and experimental realizations of the metamaterial layer are performed in order to validate the device concept and realization feasibility.     

基于电阻型频率选择表面的低频宽带超材料吸波体的设计

基于电阻型频率选择表面(Resistance Frequency Selective Surface, RFSS) 设计了一种低频宽带、 极化不敏感和宽入射角特性的超材料吸波体. 该吸波体的基本单元由开槽十字型平面超材料(Cave Cross Planar Metamaterial, CCPM)、 RFSS、 电介质基板和金属背板组成. 采用FDTD方法数值模拟得到的结果表明: 相比于单纯的CCPM吸波体、 RFSS吸波体, CCPM和RFSS复合结构吸波体低频吸收特性得到极大改善, 在整个1-5 GHz频率范围内, 吸收率大于80%, 吸收峰值达到98%以上. 数值模拟得到的不同极化角和不同入射角下的吸收率表明: 该复合结构吸波体具有极化不敏感和宽角度吸收特性.     

Tunable dual-band ferrite-based metamaterials with dual negative refractions

We report on three types of tunable dual-band metamaterial with dual negative refraction in this paper. The three types of metamaterial are composed of ferrite slabs and three different metallic resonators, including split-ring resonators (SRR), Ω-like resonators, and short wire pairs. The ferrite slabs under an applied magnetic bias provide one magnetic resonance frequency band and the three metallic resonators provide another magnetic resonance frequency band, respectively. The continuous wires within the metamaterials provide the negative permittivity in a wide frequency band covering the two magnetic resonance bands. We give the design, analysis and numerical demonstrations of three such types of metamaterial in detail. The effective electromagnetic parameters obtained from the simulated S-parameters indicate that the three types of metamaterial indeed exhibit two negative refraction passbands and the two passbands can also be shifted by changing the magnetic bias. Our results open the way to fabricate tunable dual-band metamaterial cloaks, absorbers, and antennas.