An ultrathin wide-band planar metamaterial absorber based on a fractal frequency selective surface and resistive film

We propose an ultrathin wide-band metamaterial absorber （MA） based on a Minkowski （MIK） fractal frequency selective surface and resistive film. This absorber consists of a periodic arrangement of dielectric substrates sandwiched with an MIK fractal loop structure electric resonator and a resistive film. The finite element method is used to simulate and analyze the absorption of the MA. Compared with the MA-backed copper film, the designed MA-backed resistive film exhibits an absorption of 90% at a frequency region of 2 GHz-20 GHz. The power loss density distribution of the MA is further illustrated to explain the mechanism of the proposed MA. Simulated absorptions at different incidence cases indicate that this absorber is polarization-insensitive and wide-angled. Finally, further simulated results indicate that the surface resistance of the resistive film and the dielectric constant of the substrate can affect the absorbing property of the MA. This absorber may be used in many military fields.     

Tunable wideband absorber based on resistively loaded lossy high-impedance surface

A lossy high-impedance surface comprised of two layers of resistive frequency selective surfaces is employed to design a tunable electromagnetic absorber. The tunability is realized through changing the composite unit cell by moving the top layer mechanically. To explain the absorbing mechanism, an equivalent circuit model with an interacting coefficient is proposed. Then, simulations and measurements are carried out and agree well with each other. Results show that the complex structure with a thickness less than λ0/4 is able to achieve a wideband absorption in a frequency range from5.90 GHz to 19.73 GHz. Moreover, it is tunable in the operation frequency band.     

Rainbow trapping based on long-range plasmonic Bragg gratings at telecom frequencies

The group velocity of long-range surface plasmon polaritons (LRSPPs) in a wide frequency bandwidth at infrared frequencies is significantly reduced by dielectric gratings of graded thickness on both sides of a thin metal film. This structure can reduce the propagation loss of slow surface plasmons in "rainbow trapping" systems based on plasmonic Bragg gratings. Compared with dielectric gratings of graded thickness on a single side of a metal film, the proposed structure is able to guide slow light with a much longer propagation distance for the same group index factor. Finite- difference time-domain simulation results show that slow LRSPPs with the group velocity of c/14.5 and the propagation distance of 10.4 μm are achieved in dielectric gratings of uniform thickness on both sides of a thin metal film at 1.62 μm.     

A novel polarization converter based on the band-stop frequency selective surface

A dual-passband single-polarized converter based on the band-stop frequency selective surface(FSS)with a low radar cross-section(RCS)is designed in this article.The unit cell of the proposed converter is formed by a polarization layer attached to the band-stop frequency selective surface.The simulation results reveal that the co-polarization reflection coefficients below-10 d B are achieved in 3.82–13.64 GHz with a 112.4%fractional bandwidth(the ratio of the signal bandwidth to the central frequency).Meanwhile,a polarization conversion band is realized from 8.14 GHz to 9.27 GHz with a polarization conversion ratio which is over 80%.Moreover,the 1 d B transmission window is obtained in two nonadjacent bands of 3.42–7.02 GHz and 10.04–13.91 GHz corresponding to the relative bandwidths of 68.9%and 32.3%,respectively.Furthermore,the radar cross-section of the designed structure can be reduced in the wideband from 2.28 GHz to 14 GHz,and the 10 d B RCS reduction in the range of 4.10–13.35 GHz is achieved.In addition,the equivalent circuit model of this converter is established,and the simulation results of the Advanced Design System(ADS)match well with those of CST Microwave Studio(CST).The archetype of the designed converter is manufactured and measured.The experiment results match the simulation results well,which proves the reliability of the simulation results.     

Research on asymmetric ＂Jerusalem＂ unit

An asymmetric Jerusalem unit and the frequency selective surface （FSS） structure composed of such units are designed. The transmittance of the designed FSS structure is calculated by mode-matching method and compared with the test results. The comparison results show that the FSS center frequency of the asymmetric structure unit drifts little with the variation of the incident angles of the electromagnetic waves and keeps relatively stable. The research offers a new choice for the application of FSS under the large scanning angle of electromagnetic waves.     

Modeling of conducting bridge evolution in bipolar vanadium oxide-based resistive switching memory

We investigate the resistive switching characteristics of a Cu/VOx/W structure. The VOx film is deposited by radio- frequency magnetron sputtering on the Cu electrode as a dielectric layer. The prepared VOx sample structure shows reproducible bipolar resistive switching characteristics with ultra-low switching voltage and good cycling endurance. A modified physical model is proposed to elucidate the typical switching behavior of the vanadium oxide-based resistive switching memory with a sudden resistance transition, and the self-saturation of reset current as a function of compliance current is observed in the test, which is attributed to the conducting mechanism is discussed in detail. growth pattern of the conducting filaments. Additionally, the related     

Self-Organized Micro-Columns and Nano-Spheres Generated by Pulsed Laser Ablation of Ti/A1 Alloy in Water

Dense arrays of micro-columns are formed on the surface of Ti-AI alloy by cumulative nanosecond pulsed laser ablation in water. The fabric-like structure characterized by Ti-A1 nano-spheres absorbed on micro-duster in liquid is most likely responsible for the occurrence of laser micro-etching and localized melting, resulting in continuous deepening of micro-holes and the formation of micro-columns. Laser induced plasma spectroscopy is carried out to reveal the effect of micro-columns on subsequent pulse laser ablation. The intensity of spectral lines from Ti ions by additional laser ablation of the modified spot is higher than that created over a smooth surface. These results suggest that the micro-columns lead to an enhanced absorption of the following laser energy. The proposed results and relevant discussions are of importance for the development of light-trapping coatings on a metal surface.     

Design and realization of a magnetic-type absorber with a broadened operating frequency band

In this paper, we present an efficient method to obtain absorbers with broadened operating frequency bands. They are accomplished by using conventional magnetic absorbing materials (MAMs) in the forms of array and mesh structures, which are similar to those in the case of a frequency selective surface. The proposed approach is verified not only by simulations but also by experimental results under the normal incidence at microwave frequencies. Moreover, the wideband absorber is lighter than the conventional magnetic absorber. These results indicate that our proposed absorbing structures can be used for designing good electromagnetic absorbers.     

New 4H silicon carbide metal semiconductor field-effect transistor with a buffer layer between the gate and the channel layer

A new 4H silicon carbide metal semiconductor field-effect transistor (4H-SiC MESFET) structure with a buffer layer between the gate and the channel layer is proposed in this paper for high power microwave applications.The physics-based analytical models for calculating the performance of the proposed device are obtained by solving one-and two-dimensional Poisson’s equations.In the models,we take into account not only two regions under the gate but also a third high field region between the gate and the drain which is usually omitted.The direct-current and the alternatingcurrent performances for the proposed 4H-SiC MESFET with a buffer layer of 0.2 μm are calculated.The calculated results are in good agreement with the experimental data.The current is larger than that of the conventional structure.The cutoff frequency (fT) and the maximum oscillation frequency (f max) are 20.4 GHz and 101.6 GHz,respectively,which are higher than 7.8 GHz and 45.3 GHz of the conventional structure.Therefore,the proposed 4H-SiC MESFET structure has better power and microwave performances than the conventional structure.     

Electromagnetic Bloch oscillation in one-dimensional multiple microcavities composed of metamaterials

We propose a photonic structure stacked sequentially by one-dimensional photonic crystals and cavities. The whole structure is composed of single-negative and double-negative materials. The optical Wannier-Stark ladder （WSL） can be obtained in a low frequency region by modulating the widths of the cavities in order. We simulate the dynamical behavior of the electromagnetic wave passing through the proposed photonic structure. Due to the dispersive characteristics of the metamaterials, a very narrow WSL can be obtained. The long-period electromagnetic Bloch oscillation is demonstrated theoretically to have a period on a microsecond time scale.     

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.     

基于超材料与电阻型频率选择表面的薄型宽频带吸波体的设计

设计了三种类型吸波体, 分别为基于正方形金属贴片(square metal patch, SMP) 结构超材料吸波体、 电阻型频率选择表面(Resistance Frequency Selective Surface, RFSS) 吸波体和SMP与RFSS的复合结构吸波体. 采用FDTD算法分别对这三种类型吸波体的电磁波吸收特性进行数值模拟分析. 模拟得到的结果表明: 在整个2-30 GHz频率范围内, SMP吸波体, 通过几何参数的设计可以实现多频窄带强吸收; RFSS吸波体, 通过方块电阻的设计可以实现高频宽带强吸收, 但强吸收的带宽有限; SMP与RFSS的复合结构吸波体, 在3-25 GHz之间吸收率大于90%以上, 且宽频范围内与自由空间具有较好的阻抗匹配特性.     

超光谱红外卫星资料同步反演不同地表类型的大气廓线、地表温度和地表发射率

大气温度、水汽、地表温度和地表发射率是大气和地表的本征信息量。利用卫星红外资料精确反演大气温湿廓线有利于准确预报天气和研究气候变化,同时地表温度和地表发射率光谱的反演为研究植物生长与作物产量、地表水分蒸发与循环、能量平衡、地表成分及物理性质、气候变迁与全球环境提供重要参数指标。把大气和地面作为一个整体系统来考虑,建立一种能同步反演大气温度廓线、大气水汽廓线、地表温度和地表发射率的反演方法,利用超光谱红外卫星资料(atmospheric infrared sounder, AIRS),针对我国新疆地区沙漠和雪地两种典型发射率地表同步反演大气温度廓线、水汽廓线、地表温度和地表发射率。反演方法首先线性化地球-大气系统红外辐射传输方程, 提出通过经验正交函数构建大气廓线和地表发射率光谱,有效减少反演变量数,建立同步物理反演模式,然后以美国国家环境预报中心(National Centers for Environmental Prediction,NCEP)的预报结果(初始大气温度、水汽廓线以及地表参数)作为初始值,最后通过牛顿迭代得到最优化解。反演观测区域覆盖我国新疆塔克拉玛干沙漠和准噶尔盆地,分别选择位于塔克拉玛干沙漠腹地的塔中探测站(纬度38.98°, 经度83.64°)和准噶尔盆地的阜康荒漠生态系统国家野外科学观测研究站(纬度44.2°, 经度87.9° )为反演地面验证点。反演结果表明,塔克拉玛干沙漠地表温度明显高于准噶尔盆地地表温度,与实际情况相一致;根据反演的8.6和13.4 μm处的地表发射率分布情况,可以看出在8.6 μm处沙漠地表发射率明显低于雪地发射率,在6～15 μm范围内,反演的沙漠地区(塔中站)地表发射率和雪地地区(阜康站)地表发射率与美国喷气推进实验室测量的沙漠发射率光谱和雪地发射率光谱相一致。研究表明,把大气和地面作为一个整体系统来考虑,把地表发射率加入到反演中,通过比较和分析沙漠地区(塔中)和雪地地区(阜康)的大气廓线反演结果与当地气象探空值和传统反演方法反演值,改进了大气温度廓线和水汽廓线反演精度,特别是边界层温度和水汽改进尤为明显;同时分析表明在发射率光谱变化较大的沙漠地区, 大气廓线反演精度的改进比雪地要高,这是由于地表发射率光谱在沙漠、戈壁地区变化较大,而雪地的发射率光谱变化不大。用该方法针对地表发射率光谱变化较大的地区(沙漠)同步反演大气廓线、地表温度和地表发射率,可以更有效的提高大气温度廓线、水汽廓线的反演精度。该研究结果可以为数值天气预报和我国未来超光谱红外卫星应用提供服务和有力支持, 具有十分重要的意义。     

中红外波段地表发射率特性分析

陆表温度(LST)在地-气相互作用过程中扮演着重要的角色,是全球变化研究的关键参数。陆表发射率是陆表温度反演的关键输入参数之一。中红外谱区(3~5 μm)介于可见光-近红外谱区(0.38~2.5 μm)与热红外谱区(8~14 μm)之间,地物的发射率在该谱区表现出独特的光谱特性,可用于霜冻监测、矿物成分分析等研究。由于传感器在中红外谱区探测到的能量既有来自于地物自身发射的热辐射能量,又有反射的太阳辐射能量,这两部分的能量分离机理比较复杂,因此中红外谱区发射率特性分析的相关文献较少。本文针对单一均匀地表和具有混合像元的复杂地表计算了MODIS红外通道的有效发射率,发现通道有效发射率在单一均匀地表下与温度的耦合效应不强烈;但在复杂地表下,通道有效发射率与混合像元内的成分比例以及成分的地表温度具有耦合效应。在误差允许的范围内,混合像元的有效发射率可以忽略成分地表温度的影响。发射率误差对陆表温度反演精度的敏感性随着波长的变化而变化。在热红外波段,敏感性是其在中红外波段的2倍左右,说明利用中红外波段进行陆表温度反演具有一定的优势。     

颜料含量对复合涂层红外发射率的作用机制研究

以聚氨酯(PU)为粘合剂,某片状金属粉为颜料,采用刮涂法制备了PU/片状金属复合涂层。对不同颜料含量条件下涂层的红外发射率进行了测试,并采用扫描电镜对涂层的微结构进行了观察。发射率测试结果表明,涂层发射率随金属粉含量增加呈“∪”型变化。微结构观察结果表明,PU/片状金属复合涂层由PU和片状金属粉交错堆叠而成,具有类似一维光子结构特征。基于涂层的微结构特征,对具有转折变化特征的4个金属粉含量点状态下涂层中一维光子结构的反射光谱进行了模拟计算,结果表明：PU/片状金属复合涂层的发射率随金属粉含量变化呈“∪”型变化关系特征主要是由涂层中一维光子结构的主反射峰中心波长随颜料含量增加所产生的蓝移现象引起的。     

Preparation of a novel infrared low-emissive coating from the Cu powder modified by the polyethylene wax

Cu powder was coated with polyethylene wax via the flux-capping method in hope to avoid the oxidation of it, so the increment of the infrared emissivity of the coating can be greatly reduced. The prepared product was characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The infrared emissivity of the prepared material was measured by Infrared Emissometer. The influence parameters that will affect the emissivity of the coating were systematically investigated, such as the content of coated Cu powder, coating fineness, coating thickness and aging process. The results indicated that the infrared emissivity value of the coating was reduced after Cu powder was coated with polyethylene wax. The polyethylene wax/Cu composites presented a homogenous sheet structure when the content of Cu powder increased to 30 wt.%, and it has a lower emissivity. The infrared emissivity of the coating increases rapidly as thickness increases and becomes steady above thickness of 70 μm. The composite coating exhibits lower emissivity value and excellent physical properties at coated Cu content of 20 wt.%. The emissivity of the coating that was prepared from the modification of the Cu powder was decreased with the decrement of the grinding fineness and increased with the aging time. The emissivity of the coating that was prepared from the modification of the Cu powder is always lower than that of the coating that only composed of the Cu powder with the increment of the aging time. Therefore, it can be concluded that the anti-oxidation of Cu powder is greatly improved after it was modified by polyethylene wax, which results in a novel coating with long-run low emissivity.     

An Infrared Metamaterial Broadband Absorber Based on a Simple Titanium Disk with High Absorption and a Tunable Spectral Absorption Band

A metamaterial absorber is proposed that functions in the medium- (3–5 µm) and long-wavelength (8–12 µm) infrared (medium-wavelength infrared, MWIR, and long-wavelength infrared, LWIR, respectively) regions. The proposed design, which consists of periodic cells, can be tuned to achieve single-band or dual-band light absorption by changing the periodicity of the structure. Each cell forming the metamaterial absorber consists of a bottom metal plate (Al), a top metal disk (Ti), and an intermediate dielectric medium (Si or ZnS) in which a metal disk (Ti) is embedded. For a period of 0.85 µm, the absorber achieves broadband absorption in the LWIR region, with an average absorption of 92.1%. Further, the absorber shows acceptable tolerance to irradiation at oblique incidence. For a period of 2 µm, a peak absorption of 99.05% is achieved in the MWIR region, thereby providing dual-band absorption. Tuning the periodicity of the structure enhances the localized surface plasmon resonance, with the absorption mechanism explained by establishing an equivalent parallel LC circuit. The absorption properties demonstrated by the proposed metamaterial absorber are promising for thermal imaging and infrared spectroscopy.     

ITO导电膜红外发射率理论研究

根据红外辐射理论和薄膜光学原理计算了高品质ITO(indium tin oxide)导电膜的红外发射 率,其理论曲线与实测曲线基本符合. 并得出方块电阻小于30Ω时,ITO膜在红外波段8—14μ m的平均红外发射率理论值小于0.1.实际制备方块电阻小于10Ω的ITO膜具有优良的红外隐身 性能. 讨论了高品质ITO膜具有低红外发射率的物理机理,并提出了低红外发射率临界方块电 阻值,这有利於理论研究和工艺制备红外隐身ITO膜. 关键词： 红外发射率 ITO薄膜 理论计算 方块电阻     

Optically active helical polyurethane@attapulgite composites: Effect of optical purity of S-1,1′-binaphthyl-2,2′-diol on infrared emissivity

Helical polyurethane@attapulgite (HPU@ATT) composites were prepared after the surface modification of the rod-like attapulgite (ATT). HPU@ATT composites based on S-1,1′-binaphthyl-2,2′-diol (S-BINOL) with different optical purity (O.P.) were characterized by Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The results indicate that the helical polyurethane has been successfully grafted onto the surfaces of the modified ATT without destroying the original crystalline structure of ATT. The rod-like nanoparticles were confirmed by transmission electron microscopy (TEM). Infrared emissivity values of HPU@ATT composites have been investigated, and the results indicate that the optical purity of monomer plays a very important role in the infrared emissivity for HPU@ATT owing to the effect of helical conformation and interchain hydrogen bonds. Along with the increased optical purity of S-BINOL, the infrared emissivity of HPU@ATT is reduced evidently. Infrared emissivity value of HPU@ATT based on S-BINOL with 100% optical purity is the lowest one (0.431).