Far infrared absorption of amorphous GaAs and Ge

Far infrared reflection spectra of amorphous GaAs and Ge have been obtained in the frequency region from 30–600 cm^?1. For each material, curves of ω?₂ vs frequency have been obtained whose corresponding reflectivity curves give a best fit to the data. The peak value of the abdorption coefficient is about 4000 cm^?1 for GaAs and 160 cm^?1 for Ge. The results are compared with Raman spectra and with theoretical calculations.     

A new method to measure permittivity and permeability in nanopowder materials in microwave range

A new method for measuring the electromagnetic properties (permittivity and permeability) of nanopowder materials in a wide microwave region is presented. Unlike previously developed systems, our experimental setup is based on reflection measurements over a short-circuited transmission line combined with the application of a uniform magnetostatic field. When this field is sufficiently high to saturate the material, the effective permeability of the sample equals the permeability of free space, without modifying its electrical properties. Hence, for each frequency, the permittivity can be obtained through the measurement of a single scattering parameter, such as the reflection coefficient. After this first measurement, and once the external field is removed, the reflection coefficient can be used again to obtain the permeability by means of the permittivity value obtained before. As the major advantage, this procedure allows the recording of the experimental data in just one sweep, using one-port measurements, and without modifying the geometrical characteristics of the sample holder. Hence, the measurement process can be easily automated.     

“Right-handed” and “left-handed” polaritons in material media

The dispersion law for polariton waves is deduced from analysis of Maxwell equations in a dielectric medium characterized by the presence of resonance in the frequency range of lattice vibrations or exciton transitions. The theory considers polariton waves with right- and left-oriented vectors $\vec E,\vec H,\vec k$ , corresponding to “right-handed” and “left-handed” polaritons. Dispersion dependences of group velocity of polariton waves and effective mass are established for “right-handed” and “left-handed” polaritons. Expressions are obtained for the effective refractive index and reflection coefficient in a wide spectral range including the resonance region. The specific features of lattice reflection spectra in alkali halide crystals are explained using the proposed theory.     

基于小波分解的医学显微图像融合

提出了基于小波分解的医学显微图像融合。通过先将图像进行小波分解,并对小波分解后的不同频率域分别进行了讨论,即分别讨论了选择高频系数和低频系数的原则。选择低频系数时,设计了两种选择方案,即区域能量法和边缘选择法。选择高频系数时,设计了两种选择方案,即区域能量法和目标清晰与模糊判据法。最后选择了在低频系数时采用边缘选择法,在高频系数时采用目标清晰与模糊判据法。从仿真结果的比较来看,在将多个目标聚焦图像融合后,图像能很好地保留多幅原来图像清晰部分的信息,融合后,提高了整幅图像的信噪比,从而获得了整幅高清晰图像。     

Space-time structure of the low-frequency acoustic field in an underwater sound channel

Experimental data are presented on the fine structure of the sound field in an underwater sound channel for low and infralow sound frequencies. The experiments are performed in the Black Sea, on a 600-km-long path, with explosive sound sources. The intensity, space-time, and frequency characteristics of the sound field are analyzed. The geometric dispersion of the first normal wave is experimentally studied. The role of the channel inhomogeneities in the violation of the sound field coherence is determined for different frequency bands. On the basis of the experimental data, the vertical distribution of the critical frequencies of the waveguide is obtained, and the validity limits are established for the wave and ray calculation methods. The applicability of the phase methods for calculating the sound fields in waveguides with dispersion is discussed. The frequency-angular dependence of the effective sound attenuation coefficient in an underwater waveguide is revealed and explained.     

Visibility

A clear, precise definition of visibility is given. It depends on the modulation transfer function of the light transmitting medium (or contrast loss ratio), the object-to-image distance, the spatial frequency, the medium dispersion (not scattering) coefficient and the medium absorption coefficient. The absorption coefficient, a, and the dispersion coefficient, β, are computed from the experimental modulation transfer function for a region of the ocean. The results obtained confirm the validity of the visibility defining equation.     

Dielectric substrate effect on the metamaterial resonances in terahertz frequency range

We report on the effect of dielectric substrate on the resonance shift of metallic metamaterials operating in the terahertz frequency region. The resonance frequencies for various metamaterials obtained by time-domain spectroscopy agree well with calculations based on the finite-difference time-domain method. The dependencies of the resonance frequency to substrate index are studied, and are systematically determined by introducing an effective substrate index. The relative contributions of the substrate index for various metamaterials are obtained by fitting the numerically obtained effective refractive indices as a function of substrate index, which is found to be ∼0.63 regardless of the metamaterial geometry.     

分形分层粗糙面电磁波透射特性的混合算法

运用基于矩量法(method of moment,MoM)结合基尔霍夫近似(Kirchhoff approximation,KA)的混合算法研究一维带限Weierstrass分形分层介质粗糙面的电磁波透射问题.在混合算法中将上层粗糙面和下层粗糙面分别划分到MoM区域和KA区域,数值计算得到透射系数随透射波的散射角的变化,讨论粗糙面分维数、高度起伏均方根、底层介质介电常数、中间层介质介电常数和厚度、入射波频率对透射系数的影响,结果表明上层粗糙面参数对透射系数有显著影响,而下层粗糙面参数对透射系数影响较小.     

Ab initio calculation of the refractivity and hyperpolarizability second virial coefficients of neon gas

Second virial coefficients for the density dependence of a number of electric properties are calculated for neon gas. Employing an accurate CCSD(T) potential for the Ne² van der Waals dimer and interaction-induced electric dipole polarizabilities and hyperpolarizabilities obtained from CCSD response theory, we evaluated the dielectric, refractivity, Kerr and ESHG second virial coefficients using both a semiclassical and a quantum statistical approach. The results cover a wide range of temperatures and are expected to be more reliable than the available experimental and empirical data. Quantum effects are found to be important only for temperatures below 100 K. The frequency-dependence of the refractivity virial coefficient is found to be small, but not negligible. For frequencies in the visible region it accounts for a few percent of the final results. For the ESHG virial coefficient of neon, frequency dependence is found to be very important, accounting for 20–25% of the second virial coefficient at the typical frequencies employed in experiments.     

Long-range sound propagation in the central region of the Baltic Sea

The data obtained from a set of experiments on the long-range, low-frequency (<5 kHz) sound propagation in the central region of the Baltic Sea are analyzed. The experiments were carried out in the summer season, with a fully developed underwater sound channel. Experimental data on the sound attenuation are presented. A significant excess of the attenuation coefficients over the predicted absorption coefficients is obtained. The quantitative estimates indicate that the sound scattering by internal waves is the most probable mechanism responsible for the observed excessive sound attenuation. The frequency dependence of the attenuation coefficient exhibits a minimum whose position on the frequency axis at the beginning of the summer season noticeably differs from that at the end of summer. The analysis of the propagation conditions allows one to relate the position of this minimum to the critical frequency of the water modes. In addition to the intensity parameters of the sound field, the formation of the time structure of explosion-generated signals propagating in the Baltic underwater sound channel is considered for the case of the sound propagation along the 360-km path crossing the Gotland Hollow. The specific role of the bottom waves in the time structure formation at short distances from the sound source is demonstrated.     

径向复合圆柱压电超声换能器

提出一种三元径向复合圆柱压电超声换能器,并对其径向振动特性进行了研究。基于弹性力学理论及机电类比原理,导出了柱坐标系中分割处理径向极化压电陶瓷管准厚度模振动及薄壁短圆管径向振动的机电等效电路;利用径向力和振速连续的边界条件,得出了径向复合圆柱压电换能器系统的径向振动机电等效电路及其共振频率方程。探讨了换能器径向共振频率及有效机电耦合系数随其几何尺寸的变化关系。研究表明,换能器的径向共振频率及有效机电耦合系数随其内芯半径和预应力管壁厚度增大而降低。研制了一些径向复合圆柱压电换能器,并对其径向共振频率进行了测试。结果表明,理论与实验结果基本一致。      

Shape effect of elongated grains on ultrasonic attenuation in polycrystalline materials

Longitudinal and transverse wave attenuation coefficients are obtained in a simple integral form for ultrasonic waves in cubic polycrystalline materials with elongated grains. Dependences of attenuation on frequency and grain shape are described in detail. The explicit analytical solutions for ellipsoidal grains in the Rayleigh and stochastic frequency limits are given for a wave propagating in an arbitrary direction relative to ellipsoid axes. The attenuation exhibits classic frequency dependence in those frequency limits. However, the dependence on the grain shape in the stochastic limits is unexpected: it is independent of the cross-section of the ellipsoidal grains and depends only on the grain dimension in the propagation direction. In the Rayleigh region attenuation is proportional to effective volume of the ellipsoidal grain and is independent of its shape. A complex behavior of attenuation on the grain shape/size and frequency is exhibited in the transition region. The results obtained reduce to the classic dependences of attenuation on parameters for polycrystals with equiaxed grains.     

Far infrared optical properties of amorphous semiconductors As?S systems

Far infrared reflection, Raman, and Brillouin spectra of amorphous semiconductor As–S systems are measured. From the reflection spectra, the optical constants are calculatured by the Kramers Kronig analysis. The existence of some intermediate range interaction among molecular units AsS₃ is suggested. From the analysis of the Boson peak at low frequency region in Raman scattering, the correlation length is obtained to be ca. 6 Å. From the absorption coefficient in the low frequency region about 3% fluctuation of charge is suggested for As₂S₃ by schlömann theory.     

声子色散对量子点中弱耦合磁极化子性质的影响(英文)

利用线性组合算符和幺正变换的方法,研究声子色散对磁场中抛物量子点弱耦合磁极化子性质的影响.计及LO声子色散,在抛物近似下导出了基态能量与量子点有效受限长度、声子色散系数、磁场强度以及耦合强度之间的关系.我们可以得到在弱耦合情况下抛物量子点中磁极化子的基态能量E0随量子点有效受限长度l0、电子声子耦合常数α和声子色散系数γ的增大而减小,随磁场回旋频率ωc增大而增大.自陷能Eotr随声子色散系数γ增大而增大.     

2×2熔锥型单模光纤耦合器的模型

选择适当的连续函数描述了熔锥型耦合器熔锥形状的渐变特性及其相互位置关系,然后分别应用局部模式理论和变分法分析了横截面沿纵向变化的锥形区域和横截面近似不变的耦合区域内的耦合行为,得出了耦合器中任一横截面处的耦合系数和耦合功率表达式计算表明,熔锥型耦合器的总功率耦合主要发生在耦合区域,在两锥形区域,对于纤芯归一化频率小于1.08的部分,也发生了相当程度的耦合为了使耦合器的理论模型更接近于实际物理情况,锥形区域的耦合作用也是不容忽略的,从而得到了一个更精细的熔锥型光纤耦合器的模型.     

复合圆环弯曲振动四极子模态分析

针对压电圆环弯曲振动机电转换性能较差的问题,提出了一种复合圆环弯曲振动换能器,它由一个径向极化的压电陶瓷内圆环和一个金属外圆环复合而成。基于能量原理推导得到了复合圆环弯曲振动的谐振频率和有效机电耦合系数,探讨了弯曲振动四极子模态特性与其结构尺寸间的关系。当压电圆环尺寸不变时,随外侧金属圆环壁厚增加,复合圆环弯曲振动四极子模态谐振频率上升,有效机电耦合系数迅速上升到极大值后缓慢下降。最后,设计制作了圆环换能器并对其谐振频率和有效机电耦合系数进行了实验测试,测试结果与解析结果和数值模拟结果吻合得较好。      

Influence of an interlayer bonding on the magnetoelectric effect in the layered magnetostrictive-piezoelectric structure

The theory of the magnetoelectric effect in magnetostrictive-piezoelectric bilayer structure is presented considering the interlayer bonding material. The expressions for the dispersion relation and the frequency dependence of the magnetoelectric voltage coefficient in the electromechanical resonance region are obtained, using the simultaneous solution of the motion equation and material equations taking into account the interlayer bonding phase. The theoretical and experimental dependence of the magnetoelectric effect on the thickness of the bonding material layer are presented. Results are in good agreement with the theory and experiment.     

Collisional recombination coefficient in an ultracold plasma: Calculation by the molecular dynamics method

New results of the calculations of the distribution function and the electron diffusion coefficient in energy space are presented. We analyze all of the data obtained and calculate the temperature dependence of the recombination coefficient. This dependence coincides with the Gurevich-Pitaevsky analytical formula in the region of a weakly coupled plasma and begins to differ in the direction of a decline in the region of a strongly coupled plasma; the difference can reach several orders of magnitude.     

Analytical Dielectric Spectrum Formula Based on Representative Frequencies

基于电介质复折射率的实验数据,采用最小二乘法结合选取的代表频率拟合得到了电介质的介电谱解析表达式,可以得到电介质在全频段的光频常数和介电常数. 研究了三个典型液态电介质,水、乙醇和甲苯的介电谱. 在实验数据可以获取的频率范围内,采用解析表达式得到的光频常数与采用Kramers-Kronig转换得到的折射率实部和实验得到的折射率虚部吻合得很好. 基于介电谱解析表达式可以预测目前实验无法测量的区域的介电谱.     

圆锥型复合变幅杆优化及动力学特性

针对应用传统解析法设计复合变幅杆过于繁琐的问题,提出采用简便、有效的微积法对圆锥复合变幅杆进行设计,之后通过Ansys软件对其结构进行有限元分析。此外,基于Ansys优化设计功能对变幅杆进行优化修整,对优化前后变幅杆进行阻抗分析和振动性能测试,测试结果表明优化后变幅杆振动效果更佳。最后,应用Ansys软件对变幅杆进行动力学特性分析,得到了不同直径、不同面积系数下变幅杆放大系数、最大应力值,并对仿真结果进行分析后得出了其动力学参数与大端直径及面积系数之间的关系。