基于电阻型频率选择表面的宽带雷达超材料吸波体设计

将多边开缝式电阻型频率选择表面 (frequency selective surface, FSS)与传统的磁性吸波材料(radar absorption materials, RAM)复合, 提出了一种新型吸波体模型. 在分析该模型材料结构和拓扑结构的基础上, 得到等效电路模型; 基于传输线理论获得该模型的反射率及输入阻抗. 采用CST仿真软件对电阻型FSS与无电阻型FSS进行对比仿真, 通过分析吸波效率, 结果表明电阻型FSS在8.7 GHz附近具有更加优良的吸波性能, 实物测试结果与仿真结果一致. 同时FSS复合传统的磁性吸波材料RAM 产生了拓频效果, 在8–15 GHz范围内起到全频段吸收.     

一种介质-金属加载圆孔单元厚屏频率选择表面

厚屏频率选择表面能较好地改善FSS带宽性能,是实现隐身雷达天线罩的有效手段.为了全面掌握厚屏FSS的传输特性,给FSS的工程应用提供可靠依据,本文设计了一种介质-金属加载圆孔单元的厚屏频率选择表面.采用矩量法对此结构进行分析计算,主要研究了加载金属直径、加载介质厚度、电磁波入射角等几个参数对厚屏FSS传输特性的影响规律.仿真实验结果表明:带宽、中心频率及其透过率都对填充介质直径、加载金属直径、加载介质厚度、电磁波入射角和不同排列方式有不同程度的敏感性,可以通过合理调整加载金属直径及加载介质厚度来获得较宽的通带和较高的透过率.这为实现厚屏FSS在曲面隐身雷达天线罩上的应用提供了一种有价值的借鉴.     

Photoacoustic investigation of resonance absorption on corrugated surfaces

The PhotoAcoustic (PA) method has been used to study the optical resonance absorptions on corrugated surfaces. The resonance absorptions due to the excitations of surface plasmons and guide-waves of corrugated surfaces have been measured experimentally, and the effects of the groove depth and period of the corrugated surface on the absorption curve have been observed. Moreover, the differential method based on the rigorous use of electromagnetic theory is used to calculate the parameters of the grooves and the optical absorptivities of the samples. The theoretical results on the resonance absorption peaks are in good agreement with those of the PA experiments. Then, the parameters of the grooves can be estimated by the PA investigations.     

Frequency selective surface structure optimized by genetic algorithm

Frequency selective surface (FSS) is a two-dimensional periodic structure which has prominent characteristics of bandpass or bandblock when interacting with electromagnetic waves. In this paper, the thickness, the dielectric constant, the element graph and the arrangement periodicity of an FSS medium are investigated by Genetic Algorithm (GA) when an electromagnetic wave is incident on the FSS at a wide angle, and an optimized FSS structure and transmission characteristics are obtained. The results show that the optimized structure has better stability in relation to incident angle of electromagnetic wave and preserves the stability of centre frequency even at an incident angle as large as 80°, thereby laying the foundation for the application of FSS to curved surfaces at wide angles.     

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.     

双层螺旋环超表面复合吸波体等效电路模型及微波损耗机制

针对超材料吸波频带窄的问题,采用金属螺旋环超表面与碳纤维吸波材料相复合的方式,设计了宽频高性能复合吸波体.研究发现,在碳纤维吸波材料中引入双层螺旋环超表面能显著增强吸收峰值和吸波带宽,且适当增加螺旋环初始线长和吸收层厚度有利于提高复合吸波体的吸波性能, 9.2—18.0 GHz频段的反射损耗均优于–10 dB (带宽达8.8 GHz),吸收峰值达–14.4 dB.利用S参数计算得到螺旋环-碳纤维复合吸波体的等效电磁参数和特征阻抗呈现多频点谐振特性,通过构建双层螺旋环超表面等效电路模型,定量计算了复合吸波体的电磁谐振频点,发现由等效电路模型获得的谐振频点计算值与仿真值基本相符,说明该复合吸波体多频点电磁谐振是宽频电磁损耗的主要机制.     

Drag current in a quantum superlattice during propagation of a high-intensity bichromatic electromagnetic wave

We study the effect of electron drag in a semiconductor superlattice during intraband absorption of a biharmonic electromagnetic wave in a process accompanied by the emission (absorption) of a phonon. The problem has been solved in the second order of perturbation theory. The effective interaction Hamiltonian method makes it possible to take into account the multiphoton nature of the electromagnetic wave absorption. With increasing field, the current increases and attains a peak value, after which it decreases in an oscillatory manner due to the ionization stabilization effect.     

射电天文用太赫兹三通带频率选择表面设计

本文设计一种基于单屏改进型开口谐振环(SRR)的太赫兹频率选择表面(FSS). 改进型 SRR 谐振单元由具有开口缝的金属贴片组成, 开口缝的物理尺寸会影响其阶跃特征阻抗特性. 本文通过对改进型 SRR 单元结构建立LC等效电路模型, 提取等效电路模型参数, 并结合传输线理论, 得到 FSS 的基频计算公式和谐波关系式. 相比于传统均匀 SRR, 本文所提出的改进型 SRR 多频带传输的控制更为灵活. 基于此特点, 设计了一款中心频率依次为 0.46 THz, 0.86 THz和 1.03 THz, 可应用于射电天文的三通带太赫兹 FSS. 采用电磁仿真软件对影响该 FSS 传输特性的关键参数、周期间隔、小型化程度以及入射角敏感性等重要指标进行分析研究. 结果表明, 改进型 SRR 三通带 FSS 在三个通带内的反射系数分别为 -37.6 dB, -13 dB和 -19.6 dB, 在0°–60° 范围内均具有稳定的频率响应特性, 且具有小型化程度高、损耗低等特点. 这种三通带 FSS 在太赫兹频段射电天文方面有潜在的应用价值.     

双屏频率选择表面结构的遗传算法优化

双屏频率选择表面能够更好地实现平顶效应和快速下降的边缘,是一种实现雷达舱对探测带外雷达波隐身的有效手段.本文针对双屏频率选择表面(FSS)在电磁波不同角度入射下的宽带通应用,采用遗传算法对FSS介质厚度、介电常数、单元图形、排列周期进行了整体优化,给出优化后的FSS结构和传输特性曲线;同时采用镀膜和光刻技术制备出相应的实验件,在微波暗室进行了测试.结果表明,优化后的结构在保证2 GHz的工作带宽下,对电磁波的入射角度有良好的传输稳定性,这为实现双屏FSS在曲面大角度下的应用奠定了基础. 关键词： 频率选择表面 遗传算法 优化     

Mössbauer Studies and the Microwave Properties of Al<Superscript>3+</Superscript>- and In<Superscript>3+</Superscript>-Substituted Barium Hexaferrites

The correlation of the chemical composition, the structure, and the microwave characteristic of solid solutions of the BaFe_{12 – x}D_xO₁₉ (0.1 ≤ x ≤ 1.2) barium hexaferrite substituted with diamagnetic Al³⁺ and In³⁺ ions has been studied. The precise data on the crystal structure have been obtained by powder neutron diffraction using a high-resolution Fourier diffractometer (Dubna, JINR). The data on the distribution of the diamagnetic substituting ions in the hexaferrite structure have been obtained by Mössbauer spectroscopy. The microwave properties (the transmittance and the reflectance) have been studied in the frequency range 20–65 GHz and in external magnetic fields to 8 kOe. It is found that the transmission spectra are characterized by a peak that corresponds to the resonant frequency of the electromagnetic energy absorption, which is due to the ferromagnetic resonance phenomenon. The correlation of the chemical composition, the features of the ion distribution in the structure, and the electromagnetic properties has been revealed. It is shown that external magnetic fields shift the absorption peak of electromagnetic radiation to higher frequencies due to an increase in the magnetocrystal anisotropy. The results enable the conclusion that the features of the intrasublattice interactions and the electromagnetic properties should be explained using the phenomenological Goodenough–Kanamori model.     

频率选择表面结构的电子系统K/Ka波段电磁屏蔽分析

电子设备和无线技术不断向K/Ka波段发展以及电子系统集成度的不断提高给电子系统的电磁屏蔽设计带来了严峻挑战。提出一种将频率选择表面（FSS）用于电子系统屏蔽的新方法,可以替代传统散热孔阵,在满足通风散热性能的同时确保电子系统在5G毫米波段的电磁屏蔽性能。基于金属腔中心点屏蔽效能和全局屏蔽效能,分析了FSS孔阵排布方式、电磁波极化与入射角度对金属外壳电磁屏蔽效能（SE）的影响。结果表明:FSS孔阵排布方式对金属腔屏蔽性能的影响较小,并且SE不受入射电磁波极化方式影响;含FSS通风孔阵的金属外壳在23.0～25.5 GHz范围内屏蔽效能约为30 dB,比含传统散热孔阵金属腔屏蔽效能提高15 dB。     

双带频率选择表面设计

为了实现频率选择表面(FSS)的双带特性,设计了由矩形栅格和三圆环组合单元FSS。对FSS的谱域求解方法进行了详细的描述。采用谱域法分析了不同角度和极化入射波下FSS的频率响应性能。结果表明,所设计的FSS对于不同入射角度和极化电磁波具有稳定的双带、平顶传输及陡峭下降边缘特性。双带特性大致表现为1.8～5.4GHz的阻带和5.4～20.0GHz的通带。阻带谐振频率稳定在3.1GHz左右,而通带在-4dB的平顶传输带宽达14.3GHz以上。其陡峭下降边缘特性表现为S波段信号强烈反射,而其他波段信号通过,从而实现多波段通讯。该结构FSS可应用于卫星通信、雷达罩及其他相关领域。     

Numerical and experimental investigations of dependence of photoacoustic signals from gold nanoparticles on the optical properties

Gold nanoparticles (AuNPs) are used as a contrast agent of the photoacoustic (PA) imaging. The efficiency of AuNPs has been discussed with the absorption cross section. However, the effects of the scattering of the light by AuNPs and surrounding medium on the PA signal from AuNPs have not been discussed. The PA signals from the aqueous solution of AuNPs were examined in the numerical simulation and the experiment. In the numerical simulation, the absorption and scattering cross sections of spherical and polyhedral AuNPs were calculated by Mie theory and discrete dipole approximation. Monte Carlo simulation calculated the absorbed light energy in the aqueous solution of AuNPs. Based on the PA wave equation, the PA signals were simulated. In the experiment, the PA signal from the aqueous solution of AuNP was measured by use of a piezoelectric film and a Q-switched Nd:YAG laser operated at 532 nm. The results of the numerical simulation and the experiment agreed well. In the numerical simulation and the experiment, a single Au nanocube with 50-nm edge generated the peak value of the PA signal significantly. It was approximately 350 times and twice as large as the peak values of the spherical AuNPs with 10- and 50-nm diameters, respectively. The peak value of the PA signal depended on both the absorption and scattering coefficients of the AuNPs and the surrounding medium. The peak value increased with the scattering coefficient in a quadratic manner. The character of the temporal profile of the PA signal such as full width at half maximum depended on the scattering coefficient of the AuNPs.     

利用窄刻槽金属光栅实现石墨烯双通道吸收增强

构建基底/窄刻槽金属光栅/覆盖层/石墨烯结构,利用金属光栅激发的表面等离子体激元共振和窄光栅刻槽支持的法布里-珀罗共振,在可见光波段实现单层石墨烯的双通道吸收增强,并结合简化模型估算出双吸收通道所在位置.在波长462和768 nm处,石墨烯的光吸收效率分别为35.6%和40.1%,相比石墨烯本征光吸收率的增强均超过15.5倍.进一步研究发现由于短波处吸收增强源于金属光栅的表面等离子体激元共振,其吸收特性受覆盖层厚度、刻槽深度和宽度变化的影响较小;而由于长波处吸收增强源于窄刻槽中的法布里-珀罗共振,因此呈现出良好的角度不敏感吸收特性.     

基于超材料的偏振不敏感太赫兹宽带吸波体设计

本文设计了一种基于超材料的偏振不敏感太赫兹宽带吸波体.吸波体包含两层金属和一层中间介质,表面金属层每一个周期单元由五种尺寸接近的金属块按照相邻不同的规律排列成5×5的方形阵列.各种尺寸金属块分别产生单峰谐振吸收,五个谐振吸收峰相互靠近从而产生宽带吸收.通过研究吸波体表面电流和电场z分量分布情况可知,入射太赫兹能量的吸收主要是由y方向上电场引起的电偶极子振荡和z方向上磁场引起的磁极化产生,而且金属层的欧姆损耗起主要作用.仿真结果表明,吸波体吸收率在80%以上的带宽约为1.2 THz,最高吸收率可达98.7%,半峰全宽(FWHM)为1.6 THz,该宽带吸波体的厚度约为中心波长的二十分之一,对偏振方向不敏感,且能实现大角度吸收,在太赫兹频段的电磁隐身、测辐射热探测器以及宽带通信等领域有潜在的应用价值.     

Effect of thermal annealing on r.f. sputtering-deposited nanocrystalline GaN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> thin films

GaNAs thin films were deposited on Corning glass substrates by radio frequency (r.f.) sputtering in molecular nitrogen ambient. The stoichiometry in the GaNAs alloy was controlled by changing the nitrogen incorporation in the film during the growth process, through the variation of the r.f. power in the range 30–80 watts which produced films with N concentrations in the range: x = 0.85–0.90. The structural and optical properties of the GaNAs thin films were studied by X-ray diffraction (XRD), photoacoustic (PA) and photoluminescence (PL) spectroscopies. XRD measurements show a broad diffraction band with a peak close to the (002) diffraction line of the GaN hexagonal phase, and a slight shoulder at the position corresponding to the (111) GaAs cubic phase. The PA absorption spectra showed a remarkable shift to higher energies of the absorption edge as the r.f. power decreases corresponding to the films with higher N concentrations. Thermal annealing of the GaNAs films at temperatures of 450 °C produced a GaAs nanocrystalline phase with grain sizes in the range 10–13 nm, as confirmed by the XRD measurements that showed a well-defined peak in the (111) GaAs direction, and also by the PA spectra which showed an absorption band at energies around 1.45 eV due to the quantum confinement effects. PL spectra of thermal-annealed GaNAs films showed a very intense emission at 1.5 eV which we have associated to transitions between the first electron excited level and acceptor states in the GaAs nanocrystallites.     

Effect of surface contamination on the heat of adsorption of gases on metals

In the infrared surface electromagnetic waves (SEW) can propagate several centimeters over a metal and some ferroelectrics. Therefore SEW can be used to integrate overlayer surface absorption over very much larger path lengths than in the usual transmission and reflection surface studies. Using a continuum model, we have calculated the surface electromagnetic wave modes for a dielectric slab on a metal against a third (weakly absorbing) medium. Unlike previous treatments, absorption has been included and the absorption expected using the two prism SEW technique has been calculated. For a thick slab (1 mm or more), we show that the absorption coefficient of the slab material can be obtained very simply. For very thin slabs both surface-plasmon-like modes and surface-phonon-like modes (including Fuchs-Kliewer modes) are found. A new parameter Δ₁ + iΔ₂ is introduced to describe the thin film absorption. It has the physical significance of an effective susceptibility for the metal-coating-overlayer system. Further, its experimental significance is that for the normalized transmission of SEW between two prisms, Δ₂ is a dominating parameter which can be measured. For the two prism experiments, numerical examples show 50 percent absorption at peak for a monolayer of CO on Pt and a 4 percent absorption for CuO on copper. Applications to the study of thin films, catalysis, corrosion and other surface problems are anticipated.     

Near edge X-ray absorption fine structure (NEXAFS) of model compounds for the humic acid/actinide ion interaction

To understand spectral features of humic acid (HA) C 1s-near edge X-ray absorption fine structure (NEXAFS) with and without metal ion complexation, a set of model compounds is investigated. Halogenated benzoic acids and anthranilic acid are examined to demonstrate the effect of electron withdrawing groups on the C 1s-NEXAFS spectra of complex organic acids, including HA. The peak positions for aromatic and carboxylic groups in these spectra are in agreement with common assignments. The spectral position of the peak for substituted aromatic carbon shifts with increasing electronegativity of the substituent to higher photon energies. Polyacrylic acid (PAA) and different PAA metal ion complexes are investigated as model substances for metal cation complexation by HA. Tb(III)-, Zr(IV)- and U(VI)–PAA exhibit general spectral signatures previously observed for the PAA/Eu(III)–PAA system. For these different metal cations, similar spectral changes and distinct variations in peak intensities are observed going from the uncomplexed to the metal-loaded macromolecules. These spectral changes are comparable to those for U(VI)- and Th(IV)–HA systems.     

电磁屏蔽用低频比小型化双频带频率选择表面

为满足敏感电子设备对频段密集相邻干扰信号的屏蔽需求,提出了一种低频比双频段带阻频率选择表面（FSS）结构。该结构由介质层和印刷在其两侧并谐振在不同频率的金属导带层构成。通过对两侧金属导带的互补型设计,削弱了两个谐振点间的耦合影响,使该FSS结构具有两个可以独立调节且紧密相邻的阻带,呈现出低频比特点。仿真结果表明,此结构可以实现低至1.16的谐振频率比。基于弯折结构的小型化设计使该FSS的单元尺寸仅为0.071λ,确保所提结构在TE和TM两种极化电磁波照射下,电磁屏蔽效能大于24 dB的入射角度稳定性高达60°。制作了实物并进行测试,实测结果与仿真结果吻合良好,验证了FSS结构设计的可靠性。     

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.