The accuracy of Kirchhoff's approximation in describing the far field speckles produced by random self-affine fractal surfaces

We describe an array of microscopic atom traps formed by a pattern of magnetisation on a piece of videotape. We describe the way in which cold atoms are loaded into one of these micro-traps and how the trapped atom cloud is used to explore the properties of the trap. Evaporative cooling in the micro-trap down to a temperature of K allows us to probe the smoothness of the trapping potential and reveals some inhomogeneity produced by the magnetic film. We discuss future prospects for atom chips based on microscopic permanent-magnet structures.     

The autocorrelation of speckles in deep Fresnel diffraction region and characterizations of random self-affine fractal surfaces

This paper studies the correlation properties of the speckles in the deep Fresnel diffraction region produced by the scattering of rough self-affine fractal surfaces. The autocorrelation function of the speckle intensities is formulated by the combination of the light scattering theory of Kirchhoff approximation and the principles of speckle statistics. We propose a method for extracting the three surface parameters, i.e. the roughness w, the lateral correlation length ξ and the roughness exponent α, from the autocorrelation functions of speckles. This method is verified by simulating the speckle intensities and calculating the speckle autocorrelation function. We also find the phenomenon that for rough surfaces with α= 1, the structure of the speckles resembles that of the surface heights, which results from the effect of the peak and the valley parts of the surface, acting as micro-lenses converging and diverging the light waves.     

强散射体产生的像面散斑对比度与随机表面及成像系统关系的研究

在对4f光学成像系统中强散射体形成的像面散斑的统计特性的研究中, 首先通过散斑场光波复振幅的一般形式和双重指数函数近似求出散斑光强的系综平均, 然后利用散斑场光波复振幅的实部和虚部的旋转变换法求出散斑光强的方差, 最后得出了散斑对比度与随机表面统计参量和系统参量的直接表达式. 本结果与现有文献中包含随机表面相关面积或散射粒子数目的隐含表达式相比具有明显的改进, 并对标定随机表面的散斑对比度法具有重要意义. 关键词： 随机表面 像面散斑 对比度     

Far-field intensity of electromagnetic waves scattered from random,self-affine fractal metal surfaces

Abstract

By means of the rigorous Green theorem integral equation formulation, we study the far-field intensity of linearly polarized, monochromatic electromagnetic waves scattered from a one-dimensionally rough silver surface characterized by a self-affine fractal structure. These surface fractal properties are ensured for the entire range of relevant length scales, from the illuminated spot size down to a sufficiently small (in terms of the wavelength) lower cut-off length. A peak in the specular direction is found in the angular distribution of the diffuse component of the mean scattered intensity, which becomes broader and smaller with increasing fractal dimension. For large fractal dimensions, enhanced backscattering in the case of p-polarization is observed owing to the roughness-induced excitation of surface plasmon polaritons. The interplay of different length scales of the fractal surface in the scattering process is analysed for an intermediate fractal dimension.     

Texture analysis of speckles due to random Koch fractals by lacunarity

We analysed the textures of speckles produced from random Koch fractals to quantitatively confirm the existence of speckle clustering which we investigated in a previous paper. The texture analysis involved using the lacunarity of these speckles, which is defined as fluctuations of the intensity integrated within a box, and comparing this quantity with that for ordinary fully-developed speckles. The lacunarity of clustered speckles was found to be greater than that of ordinary speckles throughout the whole range of the size of the box under examination, indicating the existence of long correlation tails in speckles. We also discuss the physical origin of these long correlation tails.     

Influence of nanoscale cutoff in random self-affine fractal silver surfaces on the excitation of localized optical modes

We report rigorous numerical calculations of the near field scattered from rough, one-dimensional self-affine fractal silver surfaces. We show that fractal lower-scale cutoff (decreased to the order of tens of nanometers) has a strong effect on excitation and strength of localized optical modes, leading to very large enhancements of the intensity (larger than 10(4)) and fluctuations of the electric field.     

高斯相关随机表面光散射散斑场相位奇异及其特性的理论研究

通过对二维高斯相关随机表面在远场平面上产生的散斑场及其相位的计算模拟, 发现在某一平面上除了实部零值线与虚部零值线有传统相交之外, 还有相切和重合的情况.切点和重合线也可以形成相位奇异, 并且其周围相位分布与传统的实部零值线与虚部零值线相交形成的奇点周围相位的螺旋变化不同, 呈现出对称性和不连续性的特征. 随着光波的传播, 在不同的观察面上散斑场复振幅的实部零值线和虚部零值线的相对位置经历了由相切到重合再到相交的演变过程, 相位奇异现象也随之发生变化. 关键词： 相位奇异 相位跃变 散斑场     

Statistical property of dynamic speckles produced by a rotating cylinder object

高斯光束照射以固定角速度旋转的粗糙圆柱体表面,反射空间形成随着被测表面运动变化的动态散斑.研究了照射面积大于圆柱曲面条件下在远场衍射区形成的动态散斑统计特性,得到了不同入射条件及不同圆柱时动态散斑强度起伏空间-时间归一化相关函数.结果表明:随着圆柱半径、旋转速度以及照射光斑有效面积的增大,动态散斑强度起伏相关性快速减小;当圆柱旋转速度恒定,动态散斑光强起伏相关函数的峰值随着探测点之间距离的增大而减小,但峰值的位置随之而增大;在近似点照射情况下随着圆柱半径的增大空间相关长度基本不变,而相关时间性明显增大.     

旋转圆柱目标的动态散斑统计特性(英)

高斯光束照射以固定角速度旋转的粗糙圆柱体表面,反射空间形成随着被测表面运动变化的动态散斑。研究了照射面积大于圆柱曲面条件下在远场衍射区形成的动态散斑统计特性,得到了不同入射条件及不同圆柱时动态散斑强度起伏空间-时间归一化相关函数。结果表明：随着圆柱半径、旋转速度以及照射光斑有效面积的增大,动态散斑强度起伏相关性快速减小;当圆柱旋转速度恒定,动态散斑光强起伏相关函数的峰值随着探测点之间距离的增大而减小,但峰值的位置随之而增大;在近似点照射情况下随着圆柱半径的增大空间相关长度基本不变,而相关时间性明显增大。     

Exact multiple scattering of waves from random rough surfaces: Speckle contrast

An exact formula for the mean multiple scattered intensity is presented by using normal statistics, as well as a gaussian correlation function, for the random rough surface in the limit of large correlation distance, T → ∞. Comparisons with the Kirchoff-Beckman approximation are also done. Further, we obtain an exact expression for the speckle contrast which explains experimental data and draws conclusions on the physical properties of the surface.     

近场X射线位相衬度成像及位相恢复算法

理论分析了X射线衍射位相成像和近场位相恢复算法。定义了一个最佳成像距离，给出了基于最佳成像距离和特征空间频率的衍射场光强分布新的表达形式。基于模拟位相物体给出了数值模拟结果。由此得出了探测距离和多色辐照对相衬图像和位相恢复结果的影响。本文给出的结果将对同轴x射线相衬成像实验具有一定的指导作用。     

Crystalline phase transitions produced by ion implantation

Elastic material properties of metamict titanite (sample E2312) during thermally induced stepwise recrystallization are measured using nanoindentation. Changes of the elastic modulus (E) and the hardness (H) are related to increasing long-range order and vanishing amorphous interface areas. Metamict titanite shows H and E values close to titanite glass. H decreases on annealing until ca. 950?K to 9.08?GPa and increases at higher temperatures, while E increases continuously on annealing up to ca. 168.4?GPa at 1220?K. Crystalline titanite from Rauris shows strong anisotropy and H and E values are clearly larger than those of E2312.     

Spacetime correlation functions of dynamic speckles produced by a series of moving phase screens

Spacetime correlation functions of the time-varying speckle intensity have been evaluated for the case where speckle patterns are produced in the Fresnel diffraction region by light propagating through a series of moving random-phase screens. The correlation functions are given by a set of recursion formulae which are useful for numerical computations. Three statistical quantities relating to the motion of speckle patterns are derived to examine the relationship between the velocity distribution of the phase screens and the temporal behaviour of resultant speckle patterns. It is found that the velocity differences between the phase screens have a strong effect on both the decorrelation of speckle patterns and the fluctuating speed of the speckle intensity.     

Spacetime correlation functions of dynamic speckles produced by a series of moving phase screens

Abstract

Spacetime correlation functions of the time-varying speckle intensity have been evaluated for the case where speckle patterns are produced in the Fresnel diffraction region by light propagating through a series of moving random-phase screens. The correlation functions are given by a set of recursion formulae which are useful for numerical computations. Three statistical quantities relating to the motion of speckle patterns are derived to examine the relationship between the velocity distribution of the phase screens and the temporal behaviour of resultant speckle patterns. It is found that the velocity differences between the phase screens have a strong effect on both the decorrelation of speckle patterns and the fluctuating speed of the speckle intensity.     

A comparison between transitions induced by random and periodic fluctuations

It is shown that a certain class of nonlinear systems possesses a unique and stable stationary state when subjected to periodic dichotomous modulations of an external parameter. This result enables us to define a probability density for the system and to characterize its shape and support. We compare this probability density with the one obtained in the case that the external parameter fluctuates randomly like a Markovian dichotomous noise and discuss various fluctuation-induced transition phenomena. The effects of these two types of fluctuations are quite dissimilar: the random fluctuations give rise to a richer behavior. The results are applied to the Freedericksz transition in nematic liquid crystals.Fellow of the University of Texas at Austin.     

半导体硅重构表面及其相变动力学的研究进展

简单介绍了文章作者在半导体硅重构表面及其相变动力学研究方面的进展.近期si(111)(7×7)-(1×1)相变的实验研究发现,将温度升高到相变温度以上时,7×7岛面积以恒定的衰减速率随时间减小至零,且初始面积越大的岛这个衰减速率就越大.文章作者分析了大量的实验事实,由此提出了一个双速相场模型来解释这个重要而令人困惑的现象.模型重点是:在相变过程中,7×7关键结构变化较快,随后的层错消解过程要慢得多.这个模型完美地解释了相关实验现象,说明该模型抓住了关键物理要素,这种相场方法也可以用于其他半导体表面相变研究.     

Effects of imaging properties on dynamic speckles produced by a set of moving phase screens

Space-time correlation functions have been derived for the time-varying intensities of speckle patterns in image space produced by a series of moving random-phase screens. The time-correlation length, which measures the changing rate of intensity fluctuations, is evaluated from the correlation functions to examine the effects of the imaging properties on the temporal behaviour of the speckles resulting from the multiple scattering. It is shown that the average velocity and velocity dispersion of the phase screens can be measured from the time-correlation length by using imaging systems which have a small and a large point spread at the front focal plane of the systems, respectively. Theoretical results for the relationship between the focal position of the imaging lens and the time-correlation length are also presented.     

半导体硅重构表面及其相变动力学的研究进展

简单介绍了文章作者在半导体硅重构表面及其相变动力学研究方面的进展.近期Si(111) (7×7)-(1×1)相变的实验研究发现,将温度升高到相变温度以上时,7×7岛面积以恒定的衰减速率随时间减小至零,且初始面积越大的岛这个衰减速率就越大.文章作者分析了大量的实验事实,由此提出了一个双速相场模型来解释这个重要而令人困惑的现象.模型重点是：在相变过程中,7×7关键结构变化较快,随后的层错消解过程要慢得多.这个模型完美地解释了相关实验现象,说明该模型抓住了关键物理要素,这种相场方法也可以用于其他半导体表面相变研究.