膨胀石墨 3 mm波消光数值计算

为探讨膨胀石墨作为3 mm波干扰材料的消光、散射特性及其影响因素, 基于有限长度、有限电导率圆柱状导体的电磁散射, 利用矩量法建立了膨胀石墨的消光、散射、吸收及后向散射截面(雷达散射截面RCS)的计算式. 运用Mathematica编程计算并分析了膨胀石墨长度、半径、电导率、磁导率等因素与膨胀石墨消光、散射、吸收截面及RCS的关系. 结果表明: 当膨胀石墨的长度为1.5 mm、半径为0.05 mm时, 具有较好的消光、散射效果; 适当增大膨胀石墨的电导率、磁导率, 有利于提高其消光、散射能力. 本研究为探索增强膨胀石墨干扰3 mm波效果的技术途径提供了有价值的参考. 关键词： 膨胀石墨 石墨层间化合物 消光截面 矩量法     

Scattering and propagation of UV pulses in soot aerosols

Scattering and propagation of a UV pulse in soot aerosols are studied using generalized multi-sphere Mie theory (GMM) and a two-frequency mutual coherence function. Soot aerosols are obtained by the diffusion-limited aggregation (DLA) model. Scattering characteristics of aggregate structures in soot aerosols are analyzed by GMM theory in detail. Scattering intensities versus scattering angles are given and discussed. The effects of different-positions of the aggregate on the scattering intensities, scattering cross section, extinction cross section, absorption cross section and asymmetry factor are computed and compared. The two-frequency mutual coherence functions of UV pulses in soot aerosols are simulated, and the effects of optical distance, frequency difference are analyzed.     

二氧化钒纳米点阵红外光学特性研究

针对二氧化钒纳米点阵从半导体到金属的可逆相变,考虑到点阵中各个点之间散射光的交互作用,基于VO2在不同温度和波长下的折射率和消光系数,以及小颗粒的吸收和散射特性,建立了VO2纳米颗粒的数学模型,研究了VO2纳米颗粒的相变光学特性.结果表明,随着波长变化,吸收截面相对散射截面占主导,金属相在980 nm附近出现吸收峰值|随着温度变化,可见光区域的消光系数变化较小,而红外区域较大,其中在近红外区域的消光系数变化最大.在纳米点阵中,消光截面随着颗粒间距变化,当颗粒间距增大时,消光峰值出现红移,且峰值大小也会随之增大|当间距超过一定数值后,峰值反而会逐渐减小.采用多孔氧化铝掩模的方法,通过磁控反应溅射制备VO2纳米点阵,测试结果表明其透过率比薄膜的透过率高.     

纳米碳纤维导电性对红外消光性能的影响

纳米碳纤维是一种新型吸波材料。为探讨烟幕粒子的红外消光特性,利用矩量法建立了纳米碳纤维感应电流和散射场的计算模型,在此基础上得出了纳米碳纤维吸收、散射和消光截面的计算表达式。通过Matlab编程计算分析了纳米碳纤维红外消光截面与电导率的关系。结果表明,纳米碳纤维红外消光特性与电导率密切相关,随着电导率的增加,消光截面迅速增加,当波长大于纤维本身长度时,消光截面随电导率的变化出现峰值,峰值电导率与波长有关。     

背景散射对LRCS测量精度的影响分析

针对激光雷达散射截面的测量精度,指出测量中存在的背景散射问题。给出了激光雷达散射截面的测量方程,分析了地面、支撑架等背景散射对激光雷达散射截面测量精度影响的机理。根据测量方程和影响机理提出了敷设消光材料、多次比对测量、尺寸匹配等方法,并对敷设消光材料法进行了实验验证。实验表明：所用方法能够有效地减小背景散射,提高激光雷达散射截面的测量精度。     

高分子介质中Al_2O_3微粒子的光学截面及散射强度分布的计算

本文利用 A.L.Aden和 M.Kerker微粒子 Mie散射理论计算了处于高分子介质中的Al2 O3微粒子的消光截面、散射截面以及散射强度分布函数 ,并和它们在空气介质中的结果进行了比较。     

超冷钠原子弹性散射特性的精确计算

基于精确的原子之间相互作用势,系统研究了钠原子在超冷温度下的弹性散射特性,精确计算了钠原子间碰撞时的s波散射长度、有效力程、p波散射长度以及束缚态数目等散射参数.超冷温度下单重态和三重态原子间的弹性散射截面主要为s波贡献,随着碰撞能量的增加散射截面有丰富的形状共振出现,计算发现单重态和三重态散射截面分别存在显著的f波和i波形状共振.应用简并内态近似方法获得了超精细态相互作用时的s波散射长度,所得结果与精确值比较符合.     

随机取向烟尘团簇粒子的光学截面的数值计算

采用蒙特卡罗方法根据团簇—团簇凝聚（CCA）模型对由球形原始微粒凝聚而成的烟尘团簇粒子进行了模拟,利用离散偶极子近似（DDA）方法数值计算了不同原始微粒粒径和数目组成的随机取向的烟尘团簇粒子的总消光截面、吸收截面及散射截面等光学特性参数,研究了原始微粒粒径及数目对随机取向烟尘团簇粒子光学特性的影响。结果表明：当入射波长一定时,随机取向烟尘団簇粒子的光学特性主要取决于原始微粒的粒径和数目;烟尘团簇粒子对不同波段激光的吸收和散射存在差别,这种差别随原始微粒粒径及数目变化而变化。这一工作为研究电磁波在烟尘中的传输特性提供重要参考数据。     

Bandwidth enhancement of transformation optics-based cloak with reduced parameters

In this paper, dispersive cloak design with broad bandwidth and minimal scattering cross section is proposed by appropriately selecting a radial permeability for each shell in a discretized reduced cloak. The dispersive medium is constructed by artificially varying the inner radius of the cloak with frequency, and this variation results into unique material properties at every frequency. The variation of inner radius of the cloak with frequency is artificial since the actual physical dimension of inner radius remains invariant. The relation between bandwidth and geometrical parameters of cloak is obtained by ensuring that transformation media must satisfy the condition that group velocity must remain less than the speed of light along every direction for a finite frequency range. The proposed cloak provides \(8.9\,\%\) bandwidth with respect to the center frequency for \(50\,\%\) reduction in total scattering cross section, and at the design frequency, the minimum scattering cross section obtained is \(0.266\). The proposed dispersive cloak design is verified by numerical full-wave simulations results which also confirm good cloaking performance.     

Theory of scattering of electromagnetic waves of the microwave range in a turbid medium

The coefficient of extinction of electromagnetic waves of the microwave range due to their scattering from clusters suspended in an amorphous medium and responsible for turbidity is calculated. Turbidity resembles the case when butter clusters transform water into milk. In the case under investigation, the clusters are conductors (metallic or semiconducting). The extinction coefficient is connected in a familiar way with the cross section of light scattering from an individual cluster. A new formula is derived for the light scattering cross section in the case when damping of oscillations of an electron is due only to spontaneous emission of light quanta. In this case, the resonant scattering cross section for light can be very large. It is shown that this can be observed only in a whisker nanocluster. In addition, the phonon energy on a whisker segment must be higher than the photon energy, which is close to the spacing between the electron energy levels in the cluster.     

Absorption,extinction and phase function measurements for algal suspensions of chlorella pyrenoidosa

Optical property measurements have been made for unicellular algal suspensions of C. pyrenoidosa in the spectral range from 380 to 720 nm. The measurements include the extinction and absorption cross sections and the scattering phase function. Although the spectral dependence of the extinction cross section is weak, there is a strong wavelength dependence for absorption which is related to cell pigment content. The absorption cross section increases with increasing cell size and pigment content. The scattering albedo is approximately 0.9 over the entire spectrum, and the scattering phase function is strongly peaked in the forward direction.     

Sensitive search for neutral resonances in Bhabha scattering around 1.8 MeV/c2

The excitation function for Bhabha scattering was measured in the energy range between 760 and 840 keV (CM) in 1.4 keV steps with beams of monochromatic positrons scattered on a Be foil at angles between 79° and 101° (CM). The statistical accuracy was 0.5%. Up to an upper limit of 1.0 b eV/sr (CM) for the energy-integrated differential cross section (97% CL) no resonant scattering was observed. This limit corresponds to a maximal resonance contribution of 0.7% relatively to the Bhabha cross section, consistent with the upper bound, derived from precision measurements of the electron (g-2) factor.     

Generalization of the optical theorem for an arbitrary multipole in the presence of a transparent half-space

The optical theorem is generalized to the case of excitation of a local inhomogeneity introduced in a transparent substrate by a multipole of arbitrary order. It is shown that, to calculate the generalized extinction cross section, it is sufficient to calculate the derivatives of the scattered field at a single point by adding a constant and a definite integral. Apart from general scientific interest, the proposed generalization makes it possible to calculate the absorption cross section by subtracting the scattering cross section from the extinction cross section. The latter fact is important, because the scattered field in the far zone contains no Sommerfeld integrals. In addition, the proposed generalization allows one to test computer modules for the case where a lossless inhomogeneity is considered.     

硅酸盐粒子的远红外散射特性研究

本文首先将T矩阵方法程序的计算结果分别和Mie理论以及文献进行了对比,证实了程序的有效性。接着研究了在红外波段硅酸盐粒子的形状(非球形)对散射特性的影响,并用T矩阵方法对λ=10.6μm波段的硅酸盐粒子进行了一系列计算分析。计算结果表明:随着a/b值得减小,粒子的最佳单位体积消光截面逐渐增大。     

The application of a fractal model to the scattering of ultrasound in biological media

The implications of a fractal model of medium structure for the angular dependence of the ultrasonic differential scattering cross section and for the frequency dependence of the total ultrasonic scattering cross section are explored. The results for the differential scattering cross section agree well with published experimental values for liver and those for the total scattering cross section are not inconsistent with the literature data.     

On acoustic scattering by a shell-covered seafloor

Acoustic scattering by the seafloor is sometimes influenced, if not dominated, by the presence of discrete volumetric objects such as shells. A series of measurements of target strength of a type of benthic shelled animal and associated scattering modeling have recently been completed (Stanton et al., "Acoustic scattering by benthic and planktonic shelled animals," J. Acoust. Soc. Am., this issue). The results of that study are used herein to estimate the scattering by the seafloor with a covering of shells at high acoustic frequencies. A simple formulation is derived that expresses the area scattering strength of the seafloor in terms of the average reduced target strength or material properties of the discrete scatterers and their packing factor (where the reduced target strength is the target strength normalized by the geometric cross section of the scatterers and the averaging is done over orientation and/or a narrow range of size or frequency). The formula shows that, to first order, the backscattering at high acoustic frequencies by a layer of shells (or other discrete bodies such as rocks) depends principally upon material properties of the objects and packing factor and is independent of size and acoustic frequency. Estimates of area scattering strength using this formula and measured values of the target strength of shelled bodies from Stanton et al. (this issue) are close to or consistent with observed area scattering strengths due to shell-covered seafloors published in other papers.     

Scattering of ultrashort laser pulses on “ion‐sphere” in dense plasmas

Scattering of ultrashort electromagnetic pulses on the dense strongly coupled plasma is under consideration in the frame of hard ion sphere model. The electron distribution inside the ion sphere is obtained from self‐consistent solution of the Shrodinger equation for bound electrons and the Poisson equation for free electrons. The electron density distribution is determined by plasma electron temperatures. The ion density of Al plasmas under consideration is of the order of 10²⁰–10²² cm^?3, the electron temperature changes between 54 and 816 eV. Dynamical polarizability of the hard sphere determining the scattering cross sections is calculated using the modified local plasma frequency approximation. The spectrum of scattering cross section has maxima in the vicinity of the mean plasma frequency. Dependencies of scattering probability on carrier frequency and pulse duration are analysed in detail. The transition of the total scattering probabilities from nonlinear time dependence at small times to standard linear ones with the increase of pulse duration is demonstrated.     

Electronic and optical properties of Cu, CuO and Cu2O studied by electron spectroscopy

The electronic and optical properties of Cu, CuO and Cu(2)O were studied by x-ray photoelectron spectroscopy (XPS) and reflection electron energy-loss spectroscopy (REELS). We report detailed Cu 2p, Cu LVV, O 1s and O KLL spectra which are in good agreement with previous results. REELS spectra, recorded for primary energies in the range from 150 to 2000 eV, were corrected for multiple inelastically scattered electrons to determine the effective inelastic scattering cross section. The dielectric functions and optical properties were determined by comparing the experimental inelastic electron scattering cross section with a simulated cross section calculated within the semi-classical dielectric response model in which the only input is Im(-1/ε) by using the QUEELS-ε(k,ω)-REELS software package. By Kramers-Kronig transformation of the determined Im(-1/ε), the real and imaginary parts (ε(1) and ε(2)) of the dielectric function, and the refractive index n and extinction coefficient k were determined for Cu, CuO, and Cu(2)O in the 0-100 eV energy range. Observed differences between Cu, CuO and Cu(2)O are mainly due to modifications of the 3d and O 2p electron configurations.     

The scattering of low energy electrons by electric field fluctuations near crystal surfaces

We present a discussion of the interaction of low energy electrons with surface lattice vibrations, with emphasis on the role played in this interaction by electric field fluctuations set up outside the crystal by the lattice vibration. We review briefly the experimental data presently available, along with theoretical interpretations that have been offered. We also examine in a manner more general than earlier discussions the contribution to the small angle scattering cross section from scattering by electric field fluctuations. After certain assumptions are introduced, we find that this contribution to the cross section is proportional to Im $\text{[1 + ?(Ω)]}{}^{\mathrm{?1}}$, where ?(Ω) is the frequency dependent dielectric constant of the material. We also exhibit an expression for the cross section for scattering from electric field fluctuations set up by particle—hole excitations.