Diffraction effects on bulk-wave ultrasonic velocity and attenuation measurements

The loss and phase advance due to diffraction are experimentally observed by measuring the amplitude and phase of radio frequency (rf) tone burst signals in the VHF range, in an ultrasonic transmission line consisting of a buffer rod with an ultrasonic transducer on one end, a couplant of water, and a solid specimen of synthetic silica glass. The measured results agree well with the calculated results from the exact integral expression of diffraction. The diffraction effects on the velocity and attenuation measured in this frequency range and their corrections are investigated to realize more accurate measurements. It is shown that attenuation measurements are influenced by diffraction losses and can be corrected by numerical calculations, and that velocity measurements are affected by the phase advance caused by diffraction. This investigation demonstrates that, in complex-mode velocity measurements, in which the velocity is determined from the measured phase of the signals, the true velocity at each frequency can be obtained by correction using the numerical calculation of diffraction. Based on this result, a new correction method in amplitude-mode velocity measurements is also proposed. In this new method, the velocity is determined from the intervals of interference output obtained by sweeping the ultrasonic frequency for the superposed signals generated by the double-pulse method. Velocity may be measured accurately at frequencies in the Fresnel region, and diffraction correction is essential to obtain highly accurate values with five significant figures or more.     

Successive diffraction model based on Fourier optics as a tool for the studies of light interaction with arbitrary ultrasonic field

In this paper we presented theoretical predictions of light diffraction by complex acoustic fields such as: two adjacent and superposed ultrasonic beams or ultrasound of cylindrical symmetry. The numerical calculations of light intensity of diffraction orders were performed by means of two methods: Nth Order Approximation (NOA) and Successive Diffraction Model (SDM) based on Fourier optics. The latter technique involves dividing the ultrasonic field into several adjacent sections provided that in each section the interaction of light and ultrasound is considered to fulfill Raman-Nath approximation. Propagation of light inside and behind the region with ultrasonic wave was described within the formalism of Fourier optics so that only both transfer and transmission functions had to be appropriately defined. It was revealed that SDM method based on Fourier optics might be applied in the studies of acousto-optic interaction in the cases of complex ultrasonic fields.     

超声光栅与平面透射光栅衍射图样的比较研究

分析了超声光栅的形成机理,给出了超声光栅衍射图样光强分布的解析表示,通过理论分析和数值模拟对超声光栅与平面透射光栅的衍射图样进行了比较研究.结果表明其衍射主极大满足类似的光栅方程,但衍射条纹的强度分布不同;都能产生缺级现象,但规律不同;超声光栅是一种动态光栅,各级衍射谱线的频率不同.     

Coherent Transmission and Reflection by a monolayer of discrete scatterers

Coherent transmission and reflection of a plane wave through a monolayer of discrete particles are considered on the basis of simple and physically transparent formulae for the single scattering approximation (SSA) corrected by introducing a multiple scattering permittivity factor. This factor allows for multiple scattering of Waves between monolayer particles, opposite to the SSA. The multiple scattering permittivity factor is considered on the basis of the quasi-crystalline approximation (QCA) via ? matrix formalism. The multiple scattering permittivity factor and parameters for obtaining coherent transmission and reflection coefficients (the effective extinction coefficient and the transmission and reflection coefficients due to rescattering) are calculated within the scope of QCA and plotted for comparison with SSA results. The expressions for these values are simplified for small Rayleigh particles to simple analytical formulae.     

掺杂氧的Ge-Sb-Te相变薄膜的光学性质

研究了氧掺入Ge-Sb-Te射频溅射相变薄膜在400nm-800nm区域的光学常数（n,k）和反射、透射光谱，发现适当的氧掺入能大大增加退火前后反射率对比度，因此可通过氧掺入改良Ge-Sb-Te相变材料的光存储性能。     

Composites of resonant dielectric rods: A test of their behavior as metamaterial refractive elements

We report numerical experiments of optical wave propagation in composites of high refractive index dielectric rods at frequencies where their first electric and magnetic Mie resonances are excited. The arrays of these particles have been extensively studied and proposed as non-absorbing and isotropic metamaterials. We show that negative refraction, observed in such ordered particle arrays, is due to diffraction and that an effective medium theory (EMT) yields constitutive parameters that do not reproduce the observations in these composites, whose transmission also depends on the sample shape. This is further confirmed by disordering the arrays, a case in which large transmission losses appear due to extinction by resonant scattering from the particles. Therefore, these composites, although having little absorption, give rise to large extinction due to scattering and do not constitute an improvement, as low loss refractive elements, upon all previously designed highly absorbing metamaterials.     

Extinction of Radiation in Sterically Interacting Systems of Monodisperse Spheres. Part 2: Experimental results

The Bouguer-Lambert-Beer law (BLBL), commonly used to describe the extinction of radiation in disperse systems, is known to be valid in the limit of low concentrations only. In Part [1] it was shown that steric particle-particle interactions lead to important effects in the range of high particle concentration and two versions of a more general extinction equation including steric interactions were derived for the regimes of fraunhofer diffraction and ray propagation in this part experimental evidence for the effects of steric interactions on the extinction of radiation in suspensions of monodisperse spherical particles is given. Various experiments on light extinction and on ultrasonic extinction are combined to cover a broad range of particle size parameters, mechanisms of particle-wave interaction and receiver apertures.     

Transmission X‐ray scattering as a probe for complex liquid‐surface structures

The need for functional materials calls for increasing complexity in self‐assembly systems. As a result, the ability to probe both local structure and heterogeneities, such as phase‐coexistence and domain morphologies, has become increasingly important to controlling self‐assembly processes, including those at liquid surfaces. The traditional X‐ray scattering methods for liquid surfaces, such as specular reflectivity and grazing‐incidence diffraction, are not well suited to spatially resolving lateral heterogeneities due to large illuminated footprint. A possible alternative approach is to use scanning transmission X‐ray scattering to simultaneously probe local intermolecular structures and heterogeneous domain morphologies on liquid surfaces. To test the feasibility of this approach, transmission small‐ and wide‐angle X‐ray scattering (TSAXS/TWAXS) studies of Langmuir films formed on water meniscus against a vertically immersed hydrophilic Si substrate were recently carried out. First‐order diffraction rings were observed in TSAXS patterns from a monolayer of hexagonally packed gold nanoparticles and in TWAXS patterns from a monolayer of fluorinated fatty acids, both as a Langmuir monolayer on water meniscus and as a Langmuir–Blodgett monolayer on the substrate. The patterns taken at multiple spots have been analyzed to extract the shape of the meniscus surface and the ordered‐monolayer coverage as a function of spot position. These results, together with continual improvement in the brightness and spot size of X‐ray beams available at synchrotron facilities, support the possibility of using scanning‐probe TSAXS/TWAXS to characterize heterogeneous structures at liquid surfaces.     

The layer multiple-scattering method for calculating transmission coefficients of 2D phononic crystals

We extend the layer multiple-scattering theory (LMST) to elastic waves propagating in two-dimensional (2D) periodical composites. The formalism to calculate the reflection and transmission coefficients for elastic waves through finite slabs is presented. In this spirit, the crystal is viewed as a sequence of identical monolayer which has one-dimensional (1D) periodicity along a given direction. The reflection and transmission coefficients for a multilayer slab can be obtained by a double-layer scheme through the calculation of the scattering matrix of a monolayer. To demonstrate the application of this formalism, we calculate transmission coefficients for systems consisting of pure solid components or mixing (solid and fluid) components. The validity of this method is checked by both band structure calculations and transmission measurement of ultrasonic experiment.     

Neutron thermometry on polarized silver nuclei at sub-microkelvin spin temperatures

We have utilized both neutron transmission and neutron diffraction to measure the spin polarization in a single crystal of ¹⁰⁹Ag, whereby the nuclear spin temperature could be obtained. Transmission of a polarized neutron beam provides an accurate and convenient primary thermometer. The feasibility of unpolarized neutron transmission was demonstrated as well. The diffraction measurements were strongly influenced by extinction, whence this method had to be calibrated against another thermometer. In this way we obtained information about extinction in the crystal.     

Prediction and suppression of strong dispersive coupling in microracetrack channel drop filters

For a four-port microracetrack channel drop filter, unexpected transmission characteristics due to strong dispersive coupling are demonstrated by the light tunneling between the input-output waveguides and the resonator, where a large dropping transmission at off-resonance wavelengths is observed by finite-difference time-domain simulation. It causes a severe decline of the extinction ratio and finesse. An appropriate decrease of the coupling strength is found to suppress the dispersive coupling and greately increase the extinction ratio and finesse, a decreased coupling strength can be realized by the application of an asymmetrical coupling waveguide structure. In addition, the profile of the coupling dispersion in the transmission spectra can be predicted based on a coupled mode theory analysis of an equivalent system consisting of two coupling straight waveguides. The effects of structure parameters on the transmission spectra obtained by this method agree well with the numerical results. It is useful to avoid the strong dispersive coupling region in the filter design.     

Reconstruction of the order parameter of oriented liquid crystal droplets

An optical method is proposed to reconstruct the order parameter for liquid crystal droplets with rigidly fixed poles. The method is based on measurements of the coherent transmission coefficient of a oneatom layer (monolayer) of polymer-dispersed liquid crystal droplets and on comparison with the results of the solution of the direct problem. In solving the direct problem for determining the coherent transmission coefficient of the polymer-dispersed liquid crystal monolayer, the anomalous diffraction approximation and the approximation of the effective refractive indices for liquid crystal droplets were used. The capabilities of the method are illustrated by the reconstruction of one component of the diagonal tensor of the droplet order parameter.     

Effects of ultrasonic treatment on amylose-lipid complex formation and properties of sweet potato starch-based films

To investigate the effect of ultrasonic treatment on the properties of sweet potato starch and sweet potato starch-based films, the complexing index, thermograms and diffractograms of the sweet potato starch-lauric acid composite were tested, and light transmission, microstructure, and mechanical and moisture barrier properties of the films were measured. The results indicated that the low power density ultrasound was beneficial to the formation of an inclusion complex. In thermograms, the gelatinization enthalpies of the ultrasonically treated starches were lower than those of the untreated sample. With the ultrasonic amplitude increased from 40% to 70%, the melting enthalpy (ΔH) of the inclusion complex gradually decreased. X-ray diffraction revealed that the diffraction intensity of the untreated samples was weaker than that of the ultrasonically treated samples. When the ultrasonic amplitude was above 40%, the diffraction intensity and relative crystallinity of inclusion complex gradually decreased. The scanning electronic microscope showed that the surface of the composite films became smooth after being treated by ultrasonication. Ultrasonication led to a reduction in film surface roughness under atomic force microscopy analysis. The films with ultrasonic treatment exhibited higher light transmission, lower elongation at break, higher tensile strength and better moisture barrier property than those without ultrasonic treatment.     

Experimental study of light diffraction by standing ultrasonic wave with cylindrical symmetry

We report a new acousto-optic arrangement based on ultrasonic wave with cylindrical symmetry. The theory of light interaction with standing cylindrical ultrasonic wave is experimentally verified in the Fraunhofer region. A very good agreement of experimental results with numerical calculations based on the proposed theory is found. The diffraction pattern consists of ring-shaped diffraction orders which posses a fine structure. The time average light intensity of the whole zeroth diffraction order as a function of the Raman-Nath parameter is investigated. The modulation properties of presented system are examined by means of single photon counting technique. Finally, some potentially useful applications in the laser and fibre technology are suggested.     

室温离子液体中超声制备纳米氧化锌掺杂材料及机理研究

在室温离子液体1-丁基-3-甲基咪唑四氟硼酸盐(ImBF_4)中超声制备镧系离子掺杂的纳米氧化锌材料,采用X射线衍射、透射电镜、光声光谱和荧光光谱技术表征样品结构和发光性能。为研究反应机理,采用甲基自由基复合法对离子液体的空化温度进行间接实测,进一步与空化动力学的数值分析结果进行比较。实验结果表明镧系离子掺杂可以有效地调控氧化锌的发光。与传统溶剂不同,离子液体空化温度不随超声时间的延长而降低,并随环境温度升高略有上升。反应机理的研究表明样品的形成生长与超声辐照和离子液体的共同作用密不可分。     

超声衰减和速度谱测量亚微米乳液的粒径

研究了超声衰减谱法和相速度测量高浓度多分散脂肪两相乳浊液的粒径分布问题.在理论分析基础上,实验测量了多个浓度(2%～20%)脂肪乳试样在2～13 MHz频带下超声衰减和相速度谱,结合反演算法由实验数据计算出乳浊液颗粒的粒径分布,并讨论了相速度法的意义和特点.将原始浓度(20%)下测得粒径分布与消光法在稀释条件下相同试样测量结果作对比,二者比较吻合,表明超声谱法可在无稀释的高浓度条件下作乳浊液粒径分布的表征.     

掺杂对2H-MoTe2光电特性影响的第一性原理研究

MoTe₂是一种非空间反演对称性半导体,由线性偏振光照射,在无偏压条件下可以直接产生光电流,但是非常微弱.掺杂可以改变电子能带结构和降低空间反演对称性,从而有效的增强光电流.本文基于非平衡格林函数-密度泛函理论,采用第一性原理,计算了本征、Nb掺杂、Ti掺杂和W掺杂2H-MoTe₂的能带结构、透射谱和光电流.能带结构表明：Nb掺杂使半导体2H-MoTe₂能带穿越费米能级,转变为金属特性;Ti和W掺杂减小了2H-MoTe₂的带隙,能带没有穿越费米能级,依然为半导体.掺杂都降低2H-MoTe₂的反演对称对称性,从本征的D_3h转变为C_s.从而在线偏振光的照射下可以有效的提高2H-MoTe₂的光电流.同时,发现掺杂可以提高单层2H-MoTe₂在低光子能量下的消光比,如Nb和Ti掺杂单层2H-MoTe₂分别在光子能量1.1 eV和1.2 eV处取得39.48和28.48的高消光比...     

高消光比双通马赫-曾德尔干涉仪梳状滤波器

提出了一种新颖结构的高消光比双通马赫-曾德尔干涉仪梳状滤波器，将常规单通马赫-曾德尔干涉仪的两个输出端与光隔离器连接，构成一个双通马赫-曾德尔干涉仪梳妆滤波器，对滤波器的输出特性进行了理论分析和实验研究。数值模拟表明，与常规的单通马赫-曾德尔干涉仪相比，在相同参量的情况下该双通马赫-曾德尔干涉仪梳妆滤器的消光比得到了大大改善，消光比的提高取决于干涉仪的臂长差，当干涉仪的臂长差为0．7366mm时，消光比提高了20dB。实验结果表明，双通马赫-曾德尔干涉仪梳妆滤器的消光比为25．8dB，消光比提高了13dB。     

特殊几何构件超声衍射时差法技术有限元模拟

衍射时差法(Time of flight diffraction,TOFD)技术是一种能够探测和精确测量缺陷尺寸的新型超声无损检测方法,针对该检测方法具有缺陷检出率高、缺陷定位定量精度高等优点。本文采用商业有限元软件ABAQUS,对汽包筒体与球形封头不等厚对接和超高压水晶釜两种特殊几何构件进行了超声TOFD技术的二维数值模拟研究,分析了超声波在这两种构件中的传播特性和规律。经分析从构件表面不同位置接收到的波型,当构件中存在损伤时,通过接收损伤所引起的衍射波,可判断构件中是否存在缺陷。模拟结果表明能够将超声TOFD技术应用于这两种特殊结构的压力容器构件,可扩大超声TOFD技术的应用范围。     

Change in adsorption bond length with coverage for KAl(111)

The local bond geometry of K adsorbed on Al(111) at low temperature has been studied by photoelectron diffraction (PED) as a function of K coverage. It is found that K atoms occupy on-top sites in the coverage range 0.05-0.4 monolayer and that the KAl bond length increases by 0.17 Å over this coverage range. The reliability of this result is supported by PED studies of the (√3 × √3)R30° structures formed by adsorption of one-third monolayer Na and K at 300 K, and K at 150 K, which give results in quantitative agreement with previous structure determinations by SEXAFS and LEED.