Characterization of nanodiamonds for metamaterial applications

Several different types of nanodiamonds were characterized in order to find the best sample to be used in further experiments with metamaterials. In this work we present the results of optical analysis of aqueous suspensions containing nanodiamonds, SEM analysis of diamond particles dispersed on silicon substrates and measurements of photoluminescence from defects in nanodiamonds.     

一种超材料梁对机械波振动吸收的模拟研究

本文设计了一种能够对机械波进行吸收的超材料梁,超材料梁由若干质量-弹簧微结构系统和一根各向同性梁构成.通过对两种不同结构的质量-弹簧系统的分析,从理论上解释了"负有效质量"和"负有效刚度".根据Hamilton原理,导出了超材料梁的代表性胞元的控制方程.通过数值模拟分析了两种不同结构的有限超材料梁对机械波的吸收性能,一种是弹性系数线性变化的质量-弹簧系统在有限梁内均匀分布,另一种是由固有频率线性变化的弹簧-阻尼器子系统构成四个相同吸收器子群在有限梁内均匀分布.模拟结果表明,进入梁中的机械波与质量-弹簧振子 关键词： 超材料梁 负有效质量 负有效刚度 机械波     

基于声学超材料的低频可调吸收器

在当今社会,噪声污染已经成为人类健康的一大威胁,如何有效地控制和消除噪声污染一直是科研领域的一个重要话题.本文以开口环嵌套结构为模型,设计并制备了一种声学超材料.通过理论分析、数值模拟和实验测试,发现由于模型内部空腔的强烈耦合共振效应,该超材料可以在低频区域实现接近完美的吸声效应.此外,通过简单地绕轴旋转其内腔开口方向,即可改变该超材料的相对阻抗值,进而在较宽的频带范围内实现对吸收峰位置的可调控制.由于该超材料具有深亚波长的尺寸,因此非常有利于低频吸声器件的小型化和集成化,同时该模型也为宽带吸收器的设计奠定了基础.     

Engineering the dispersion of metamaterial surface for broadband infrared absorption

We propose a broadband infrared absorber by engineering the frequency dispersion of metamaterial surface (metasurface) to mimic an ideal absorbing sheet. With a thin layer of structured nichrome, a polarization-independent absorber with absorption larger than 97% is numerically demonstrated over a larger than one octave bandwidth. It is shown that the bandwidth enhancement is related with the transformation of the Drude model of free electron gas in metal film to the Lorentz oscillator model of a bound electron in the structured metallic surface. We believe that the concept of dispersion engineering may provide helpful guidance for the design of a broadband absorber.     

基于超材料的吸波体设计及其波导缝隙天线应用

设计了一种基于超材料电磁特性的吸波体, 并将其应用于波导缝隙天线. 该吸波体是由两层金属及其中间的有耗介质组成, 上层金属是由刻蚀交叉缝隙的贴片形成的电谐振器, 下层金属不刻蚀, 作为整个金属地板. 通过优化结构参数, 得到了一种极化不敏感、宽入射角的超薄吸波体, 吸波率达到99.1%, 厚度只有约0.01λ. 将该吸波体应用与波导缝隙天线, 在5.48-5.7 GHz工作频段内, 天线雷达散射截面减缩都在3 dB以上, 在鼻锥方向的-25°-+25°范围的角度上, 天线雷达散射截面减缩均在5 dB以上, 雷达散射截面减缩最大超过12 dB, 而天线前向增益仅降低了0.53 dB. 实验结果与仿真结果符合得较好, 证实了该吸波体具有好的天线雷达散射截面减缩效果, 可以应用于天线目标的隐身.     

一种新型声学超材料平板对机械波吸收性能的模拟与实验研究

基于质量-弹簧微结构通过光学振动模式对机械波进行选择性吸收的原理, 本文设计了一个由集中质量与弹性薄膜构成的二维声学超材料等效平板结构. 利用有限元法对等效平板代表性胞元的振动特性模拟结果表明, 二维超材料平板结构表现出与质量-弹簧微结构相同的振动模式. 基于模拟结果, 制作加工了一个正方形超材料平板, 并通过Polytec扫描式激光测振仪(PSV)对其振动特性进行了试验测试, 结果表明, 本文设计制作的超材料平板可以对频率157.5 Hz的机械波进行选择吸收.     

Near field enhancement and absorption properties of the double cylindrical microcavities based on triple-band metamaterial absorber

We numerically study the near field enhancement and absorption properties inside the double cylindrical microcavities based on triple-band metamaterial absorber. The compact single unit cell consists of concentric gold rings each with a gold disk in the center, and a metallic ground plane separated by a dielectric layer. At the normal incidence of electromagnetic radiation, the obtained reflection spectra show that the resonance frequencies of the double microcavities are 16.65 THz,20.65 THz, and 25.65THz, respectively. We also calculate the values of contrast C(C = 1- Rmin), which can reach 95%,97%, and 95% at the corresponding frequencies by optimizing the geometry parameters of structure. Moreover, we demonstrate that the multilayer structure with subwavelength electromagnetic confinement allows 104～105-fold enhancement of the electromagnetic energy density inside the double cavities, which contains the most energy of the incoming electromagnetic radiation. Moreover, the proposed structure will be insensitive to the polarization of the incident wave due to the symmetry of the double cylindrical microcavities. The proposed optical metamaterial is a promising candidate as an absorbing element in scientific and technical applications because of its extreme confinement, multiband absorptions, and polarization insensitivity.     

基于电谐振单元的超介质吸波材料及矩形波导匹配终端应用研究

针对在自由空间中测试超介质吸波材料的高复杂性及高成本特点, 实验研究了4种由电谐振单元构成的超介质吸波材料在X波段(8–12 GHz)矩形波导里的吸波性能. 实验结果表明, 此4种吸波材料在终端短路的矩形波导里显示出与其在自由空间中相似的吸波性能及吸波机理.据此, 进一步研究了基于超介质吸波材料的矩形波导匹配终端应用. 分析结果显示, 此种新型匹配终端具有结构紧凑、工作频段可简单控制、成本低等优点. 通过展宽超介质吸波材料的吸波频段可设计出宽频带的矩形波导匹配终端. 关键词： 电谐振单元 超介质 吸波材料 匹配终端     

极化无关双向吸收超材料吸波体的仿真与实验验证

设计并制作出极化无关双向吸收超材料吸波体.仿真结果表明,该吸波体对斜入射横电极化和横磁极化电磁波具有较好的吸收效果,其强损耗主要来自介质基板的介电损耗.实验测试结果表明,该吸波体在11.1 GHz对垂直入射电磁波实现双向强吸收,吸收率分别为95.9％和90.8％.该吸波体厚度较薄,工作波长约为1/34.该超材料吸波体具有吸收率高、厚度薄、设计简单、易于加工等优点.     

Left-handed metamaterial based superlens for subwavelength imaging of electromagnetic waves

Lenses made of negative index materials have the ability to focus the propagating and evanescent components of electromagnetic waves. Such a possibility enables super resolution, in turn resulting in sharper, subwavelength size images. In this present work, we present subwavelength imaging that was obtained from a one-dimensional left-handed metamaterial (LHM) composed of alternating layers of split-ring resonators and thin wires. We investigated the effect of the thickness of LHM lenses on image size. The left-handed pass band within the negative permittivity and permeability region is shown experimentally and theoretically for different thicknesses of LHM slabs. We also studied the transmission-phase of LHMs with a different number of unit cells along the propagation direction. The phase decreases with the increasing thicknesses of LHM slabs, proving that the phase velocity is negative in the left-handed transmission band. PACS 42.25.-p; 41.20.Jb; 81.05.-t     

Simulated and experimental studies of a multi-band symmetric metamaterial absorber with polarization independence for radar applications

We develop a simple new design for a multi-band metamaterial absorber (MTMA) for radar applications. Computer Simulation Technology (CST) Studio Suite 2018 was used for the numerical analysis and absorption study. The simulated results show four high peaks at 5.6 GHz, 7.6 GHz, 10.98 GHz, and 11.29 GHz corresponding to absorption characteristics of 100%, 100%, 99%, and 99%, respectively. Furthermore, two different structures were designed and compared with the proposed MTMA. The proposed structure remained insensitive for any incident angle and polarization angle from 0° to 60°. Moreover, negative constitutive parameters were retrieved numerically. To support the simulated results, the proposed design was fabricated by using a computer numerical control-based printed circuit board prototyping machine and tested experimentally in a microwave laboratory. The absorption mechanism of the proposed MTMA is presented through the surface current and electric field distributions. The novelties of the proposed structure are a simple and new design, ease of fabrication, low cost, durability, suitability for real-time applications and long-term stability given the fabrication technique and non-destructive measurement method and very high absorption. The proposed structure has potential applications in C and X band frequency ranges.     

共享孔径人工电磁媒质设计及其在高增益低雷达散射截面天线中的应用

提出了一种具有部分反射特性和吸波特性的共享孔径人工电磁媒质(shared aperture metamaterial, SA-MTM).该媒质由上层斜十字金属图案加载集总电阻的吸波表面、下层开条带缝隙金属面的部分反射表面以及中间介质层构成, 吸波表面和部分反射表面在垂直维度上共享了一个物理孔径使该媒质同时实现了吸波特性和部分反射特性.将SA-MTM与天线一体化设计, 利用SA-MTM的部分反射表面和天线表面构成的法布里-珀罗(Fabry-Perot, F-P)谐振腔提升天线的增益, 利用SA-MTM的吸波表面吸收入射电磁波实现低雷达散射截面(radar section cross, RCS)天线的设计.仿真和实验结果表明, SA-MTM 的加载使天线的前向增益在5.57–5.94 GHz 的工作带宽范围内都提升了3 dB以上, 且天线的后向RCS在2–9 GHz范围内都有明显的减缩.该研究成果克服了天线辐射性能和散射性能无法兼顾的矛盾, 对高增益低RCS天线的设计具有重要的指导意义.     

Electromagnetic wave absorption of polymeric nanocomposites based on ferrite with a spinel and hexagonal crystal structure

We have investigated composites designed for microwave absorption based on magnetic filler, composed of phases within the SrO-Fe₂O₃ system, embedded in a polyphenylene sulfide matrix with a concentration ratio of 80:20 by weight. The formation of the nanosized particles of SrFe₁₂O₁₉ and Fe₃O₄, as the principal magnetic phases was achieved via the co-precipitation of Sr²⁺/Fe³⁺ ions using different molar ratios. The various precursors obtained were calcined between 600 °C and 900 °C in air. The electromagnetic parameters of the composites were measured with a vector network analyzer at 400 MHz to 32 GHz. The results show that with a composite composed of a complex magnetic filler comprising the nanoparticles of two magnetically diverse phases, i.e., a spinel phase as the electromagnetic wave absorber in the lower GHz range and a hexagonal phase operating at a higher GHz range, above 32 GHz, a microwave absorber with an broad absorption range can be prepared.     

Experimental demonstration of terahertz metamaterial absorbers with a broad and flat high absorption band

We present the design, numerical simulations and experimental measurements of terahertz metamaterial absorbers with a broad and flat absorption top over a wide incidence angle range for either transverse electric or transverse magnetic polarization depending on the incident direction. The metamaterial absorber unit cell consists of two sets of structures resonating at different but close frequencies. The overall absorption spectrum is the superposition of individual components and becomes flat at the top over a significant bandwidth. The experimental results are in excellent agreement with numerical simulations.     

Electromagnetic and microwave absorption properties of Alnico powder composites

A novel application of Alnico powder as a thinner microwave absorbing material was investigated. The flake-like Alnico fine powder was found to have excellent complex permeability μ=μ^′-jμ^″$\mu ={\mu }^{\prime }-j{\mu }^{″}$ and permittivity ε=ε^′-jε^″$\epsilon ={\epsilon }^{\prime }-j{\epsilon }^{″}$ in the frequency range of 2–18 GHz. The powder was made from extremely brittle ribbons which were produced using a single roller melt spinning technique. The dependences of the absorption characteristics on the frequency, thickness, and both the dielectric permittivity and magnetic permeability were obtained based on a model in which an electromagnetic wave is incident normal to the surface of the absorbing material backed by a perfect conductor. The samples have the largest values of μ^′${\mu }^{\prime }$ and μ^″${\mu }^{″}$ at 2 GHz. A minimum reflection loss of −11.4 dB is obtained at 2 GHz for composite with volume fraction 60% of Alnico powder and absorber thickness of 1.8 mm. The results show that flake-like Alnico powder composites can be applied as thinner microwave absorbers in S-band (2–4 GHz).     

一种基于超材料的吸波材料的设计与制备

设计了一种基于超材料的吸波材料,吸波材料由周期性排列的电阻片,基体以及金属背底构成. 采用时域有限差分法(FDTD)计算了吸波材料的反射率,并用遗传算法优化了吸波材料的吸波带宽,得到一种反射率在8-18 GHz小于-10 dB的吸波材料,材料厚度仅为3 mm. 采用手糊工艺制备了空心石英纤维增强环氧树脂基体,丝网印刷制备了电阻片,实现了所设计的吸波材料,测试表明,实验结果和设计结果一致. 关键词： 雷达吸波材料 超材料 频率选择表面 遗传算法     

Broadband asymmetric transmission for linearly and circularly polarization based on sand-clock structured metamaterial

We proposed a sandwich structure to realize broadband asymmetric transmission(AT) for both linearly and circularly polarized waves in the near infrared spectral region. The structure composes of a silica substrate and two sand-clock-like gold layers on the opposite sides of the substrate. Due to the surface plasmons of gold, the structure shows that the AT parameters of linearly and circularly polarized waves can reach 0.436 and 0.403, respectively. Meanwhile, a broadband property is presented for the AT parameter is over 0.3 between 320 THz and 340 THz. The structure realizes a diode-like AT for linearly wave in forward and circularly wave in backward, respectively. The magnetic dipoles excited by current in the two gold layers contribute to the broadband AT. The current density in top and bottom metallic layers illustrates the mechanism of the polarization conversion for broadband AT in detail.     

Waveform contenders for 5G: Description,analysis and comparison

5G will have to cope with a high degree of heterogeneity in terms of services and requirements. Among these latter, flexible and efficient use of all available non-contiguous spectra for different network deployment scenarios is one challenge for the future 5G. To maximize spectrum efficiency, a flexible 5G air interface technology capable of mapping various services to the best suitable combinations of frequency and radio resources will also be required. In this work, a fair comparison of several 5G waveform candidates (UFMC, FBMC-OQAM, and FBMC-QAM) is proposed under a common framework. Spectral efficiency, power spectral density, peak to average power ratio and performance in terms of bit error rate under various realistic channel conditions are assessed. The waveforms are then compared in an asynchronous multi-user uplink transmission. Based on these results, in order to increase the spectral efficiency, a bit loading algorithm is proposed to cope with the non-uniform distribution of the interference across the carriers. The benefits of these new waveforms for the foreseen 5G use cases are clearly highlighted. It is also stressed that some concepts still need to be improved to achieve the full range of expected benefits of 5G.     

Low-frequency and broadband metamaterial absorber based on lumped elements: design,characterization and experiment

In this paper, we propose and experimentally validate a low-frequency metamaterial absorber (MMA) based on lumped elements with broadband stronger absorptivity in the microwave regime. Compared with the electric resonator structure MMA, the composite MMA (CMMA) loaded with lumped elements has stronger absorptivity and nearly impedance-matched to the free space in a broadband frequency range. The simulated voltage in lumped elements and the absorbance under different substrate loss conditions indicate that incident electromagnetic wave energy is mainly transformed to electric energy in the absorption band with high efficiency and subsequently consumed by lumped resistors. Simulated surface current and power loss density distributions further clarify the mechanism underlying observed absorption. The CMMA also shows a polarization-insensitive and wide-angle strong absorption. Finally, we fabricate and measure the MMA and CMMA samples. The CMMA yields below ?10 dB reflectance from 2.85 to 5.31 GHz in the experiment, and the relative bandwidth is about 60.3 %. This low-frequency microwave absorber has potential applications in many martial fields.