基于超材料设计实现C/Co复合材料的超宽带吸收

本文通过简单的一步热解法合成了C/Co复合材料,在材料质量填充浓度为35%时观察到其具有优异的介电频散性能。当材料厚度2.5 mm时,该复合材料在11.2 GHz的最小反射率为-23.7 dB,小于-10 dB的带宽为3.8 GHz。为了进一步提高吸收体的吸波性能,我们利用全介质超材料的设计对吸收体的宏观结构进行优化,此方法成功地将小于-10 dB的吸收带宽扩展到15 GHz （7.0~22.0 GHz）。该研究结果可以为高性能微波吸收材料的制备工艺和结构设计提供有价值的参考。     

“工”字型径向弹性超材料零频起始带隙特性研究

为了得到零频起始带隙,提出"工"字型径向弹性超材料,并研究了其不同约束状态下弹性波的传播特性。通过对色散关系,频响函数和本征模位移场计算研究,发现在双面约束状态下可产生0~16 849 Hz的零频起始宽频带隙,主要由约束作用引起的模态转变产生。进一步,通过研究有限周期结构的方向振动位移场,发现零频起始带隙产生的机制为局域共振机制。最后,分析了结构参数对零频带隙位置及带宽的影响。研究结论可以应用于超低频减振等工程领域。     

3D Printed Meta‐Helmet for Wide‐Angle Thermal Camouflages

Thermal camouflage utilizes the manipulation of heat fluxes to conceal an arbitrary object in various environments from being detected via thermography. In the past decade, the field of thermal metamaterials and the technique of 3D printing have been rapidly developed, which makes nonintuitive heat flux manipulation feasible. However, when thermal metamaterials are applied to the thermal camouflaging, their conductivities are dependent on the properties of background, leading to the damage of background integrality. Moreover, previous thermal camouflaging schemes have mostly worked in the 2D regime, largely restricting their functional angles and application scenarios, especially in complex environments. Here, wide‐angle radiative thermal camouflaging is realized by using a 3D‐printed meta‐helmet of extremely anisotropic thermal conductivities. Based on 3D coordinate transformation, this meta‐helmet directly maps temperature distributions from the background to the metamaterial surface without damaging background integrity. The non‐invasive device is efficient in wide‐angle thermal camouflage by rendering the same emissivity to the background medium and can self‐adjust to various even unknown background thermal fields, which is demonstrated in numerical simulations and experiments. This work opens a door to the 3D transformation‐thermotics‐based devices for versatile practical applications in thermal infrared stealth of macro‐sized objects and others.     

Electromagnetic performance analysis of omega‐type metamaterial radomes

Radomes usually used for protection of antennas from environment should have minimum degrading effects on antenna performances. Recently, metamaterials are used in radome designs to improve their characteristics. In this work, various combinations of omega type metamaterial are investigated in planar radome designs. The simulations show that some of these structures have good performances in the designed frequency range 13–17 GHz up to incident angle 70°. Moreover, retrieval method is used to determine the effective parameters of the structure. The radome is also investigated in the presence of a microstrip antenna. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE , 2011.     

Luminescence of a Transition Metal Complex Inside a Metamaterial Nanocavity

Modification of the local density of optical states using metallic nanostructures leads to enhancement in the number of emitted quanta and photocatalytic turnover of luminescent materials. In this work, the fabrication of a metamaterial is presented that consists of a nanowire separated from a metallic mirror by a polymer thin film doped with a luminescent organometallic iridium(III) complex. The large spin–orbit coupling of the heavy metal atom results in an excited state with significant magnetic‐dipole character. The nanostructured architecture supports two distinct optical modes and their assignment achieved with the assistance of numerical simulations. The simulations show that one mode is characterized by strong confinement of the electric field and the other by strong confinement of the magnetic field. These modes elicit drastic changes in the emitter's photophysical properties, including dominant nanocavity‐derived modes observable in the emission spectra along with significant increases in emission intensity and the total decay rate. A combination of simulations and momentum‐resolved spectroscopy helps explain the mechanism of the different interactions of each optical mode supported by the metamaterial with the excited state of the emitter.     

Isogeometric topology optimization of anisotropic metamaterials for controlling high-frequency electromagnetic wave

This study presents a level set–based topology optimization with isogeometric analysis (IGA) for controlling high-frequency electromagnetic wave propagation in a domain with periodic microstructures (unit cells). The high-frequency homogenization method is applied to characterize the macroscopic high-frequency waves in periodic heterogeneous media whose wavelength is comparative to or smaller than the representative length of a unit cell. B-spline basis functions are employed for the IGA discretization procedure to improve the performance of electromagnetic wave analysis in a unit cell and topology optimization. Also, to keep the same order of continuity on the periodic boundaries as on other element edges in the domain, we propose the extended domain approach, while incorporating Floquet periodic boundary condition (FPBC). Two types of optimization problems are taken as examples to demonstrate the effectiveness of the proposed method in comparison with the standard finite element analysis (FEA). The optimization results provide optimized topologies of unit cells qualified as anisotropic metamaterials with hyperbolic and bidirectional dispersion properties at the macroscale.     

Enhanced THz absorption of graphene cavity-based electromagnetic metamaterial structures

ABSTRACT

We investigate THz absorption characteristics of graphene cavity-based electromagnetic metamaterial structures by using the conductivity characteristic matrix method. We demonstrate that the proposed structure can obtain ideal terahertz absorption due to the strong localization of photons in the defect layer of the electromagnetic metamaterial structure. The THz absorption can be continuously adjusted from 0% to 100% by controlling the chemical potential of graphene through a gate voltage. The maximum THz absorption value can be tailored by adjusting the incident angle or the period number of the two PCs with respect to the graphene layer. The position of the THz absorption peak can be adjusted by changing the thickness ratio of the layers constituting the electromagnetic metamaterial structure. Our proposal may have potentially important applications in photodetectors, saturable absorbers, and photovoltaics.     

隐身雷达罩技术研究进展综述

雷达罩作为飞行器的结构/透波功能一体化部件,其隐身要求具有很大的技术挑战性。雷达罩的隐身设计与实现必须保证雷达天线工作频带内的透波功能,实现雷达罩透波/隐身的一体化功能。隐身雷达罩是航空武器装备实现隐身性能的关键部件,对于降低飞行器头向RCS值、提高飞行器的综合性能和作战效能具有举足轻重的作用。文中分析了雷达罩的散射特点,综述了各种雷达罩隐身措施的具体途径及优缺点,重点阐述了频率选择表面技术、电磁带隙、等离子体隐身技术及超材料等技术的特点、实现隐身的作用机理及在隐身雷达罩中的应用,指出了未来发展的趋势。     

TE振荡模在含各向异性左手材料劈形平面波导中的传输

研究了含各向异性左手材料的劈形平面波导TE振荡模的传输特性.首先,从麦克斯韦方程组出发,得到该模的色散方程、反射系数方程、传输系数方程以及功率损耗方程.然后,根据这些方程画出了相关特性曲线并对这些曲线进行了仔细分析,研究发现:(1)当模阶数m=0,1,2时,TE模的有效折射率具有较大的值,且随着波导长度的增加而快速减小;(2)TEo模具有超大的反射系数,最大值接近0.967;(3)TEo模传输系数总是小于等于1.2×10-3;(4)当劈形波导斜率k=0.01时,其功率损耗大于等于0.998,而且,当频率为5.0 GHz时,功率损耗仍大于等于0.98.     

基于有源超材料的可调超薄雷达吸波体研究

在超材料结构中引入电阻和有源变容二极管,通过合理设计微结构型式以及微结构之间的连线方式,实现吸波频带的动态可调,研究电阻、电容和入射波极化方向对吸波特性的影响。结果表明:通过改变外加电压调整超材料的吸收频段,在3.7倍频带范围内实现吸波频段的主动自调节;吸波体的总厚度仅为波长的1/181,相比于传统吸波材料,在同等吸波性能条件下,表现出了优异的超薄特性;TE和TM极化电磁波表现出相同的吸波效果,即吸波特性对入射波的极化方向不敏感。