Parametric interactions in highly purified water in intense optical radiation field: Degassing effect

Excitation of optical breakdown and four-photon parametric processes in water in a pump field at a wavelength of 1.064 μm with pulse duration of 20 ps was experimentally studied. It was shown that degassing and helium washing of a water sample result in optical breakdown suppression; in this case, the processes of the four-photon parametric interaction are excited most efficiently. This effect is explained by the fact that pump self-focusing in water degassed and washed by helium is regular under given experimental conditions.     

Angular structure of radiation scattered by monolayer of polydisperse droplets of nematic liquid crystal

We considered light scattering by a polydisperse ensemble of droplets of a nematic liquid crystal. To model light scattering by a monolayer of polymer-dispersed spherical droplets of a nematic liquid crystal with cylindrical symmetry of its internal structure, we proposed a semianalytical modeling method. The method is based on interference approximation of the theory of multiple wave scattering, anomalous diffraction approximation, and effective-medium approximation. The method takes into account cooperative optical effects in concentrated, partially ordered layers and can be used to analyze the small-angle structure of the intensity of scattered radiation in relation to the concentration, size, polydispersity of liquid crystal droplets, orientation of their optical axes, and refractive indices of the liquid crystal and polymer. The obtained relations can be applied to solving direct and inverse problems of light scattering in composite liquid crystal materials using data of polarization measurements. We present graphical results of solving the direct problem for components of the polarization vector of scattered wave. These results illustrate the formation of an angular structure for monolayers with a high concentration of polydisperse droplets of the liquid crystal in the range of small scattering angles (0 < θ _s ≤ 8°).     

Experimental investigation on the improvement of SBS characteristics by purifying the mediums

The influence of optical breakdown on stimulated Brillouin scattering (SBS) process is investigated with purified CCl4, acetone, and CS2 mediums which are obtained through 0.22-μm pore-size filter. Experiments are conducted with a Nd:YAG Q-switched laser system. The optical breakdown threshold, the reflectivity and the stability of SBS in both purified and unpurified mediums are determined and compared. The improvement of optical breakdown threshold is observed more or less in all purified mediums.In the condition that optical breakdown does not occur after the medium purified, the energy reflectivity and the stability of SBS both show improvement. Therefore, the characteristics of SBS can be improved by purifying the mediums to obtain higher optical breakdown threshold which reduces the influence of optical breakdown on SBS process.     

杂质颗粒对受激布里渊散射介质光学击穿阈值影响的研究

建立了受激布里渊散射介质中热作用破坏的物理模型, 数值模拟了杂质颗粒的温度随其半径的变化曲线.结果显示,杂质颗粒存在一个最大热作用半径, 介质所含颗粒的尺寸在此半径附近时,介质最容易发生光学击穿现象, 其光学击穿阈值最低.在Continuum Nd: YAG种子注入式激光系统中,选取FC-3283, GF-180和HFE-7100介质, 通过不同孔径的过滤膜进行过滤,并研究了过滤前后的光学击穿阈值和能量反射率. 结果表明,随着过滤孔径的变小,介质光学击穿阈值逐渐提高, 且过滤之后介质的能量反射率有了明显的提高.介绍了一种利用He-Ne激光透射光光斑变化来判断是否发生光学击穿现象的方法,该方法具有方便、 准确的特点,可有效地减小由于肉眼观测引起的误差.     

Spontaneous hexagon formation in photorefractive crystal with a single pump wave

A new scheme for spontaneous formation of hexagonal optical patterns in photorefractive crystals with dominant reflection gratings and a single pump wave is proposed and realized experimentally. It is shown theoretically that spontaneous hexagon formation results from random small-angle scattering of the pump wave inside the crystal. The threshold of spontaneous hexagon formation is found.     

Small-angle polarized neutron scattering study of the mesostructure of phase precipitates in the steel P91 after heat treatment

This paper reports on the results of the analysis of the mesostructure of phase precipitates formed in the steel P91 during heat treatment in the temperature range of 600–820°C after quenching at 1050°C for 30 min. Measurements have been performed by the small-angle polarized neutron scattering (SAPNS) method with the separation of nuclear-magnetic interference scattering. The characteristic sizes of the phases precipitated during heat treatment have been determined to be in the range of 150–450 Å. It has been demonstrated that the method is highly effective for solving problems of this class, which is based on the possibility to separate and investigate nuclear-magnetic contrast scattering against the background of the total nuclear and magnetic small-angle scattering.     

Laser diagnostics of the Bubston phase in the bulk of aqueous salt solutions

Stable gas nanobubbles in the bulk of NaCl aqueous solutions and clusters of these nanobubbles have been investigated at different ion concentrations by four independent laser techniques (phase microscopy, dynamic light scattering, optical breakdown, and measurement of angular dependences of the light scattering matrix). The results obtained by these radically different techniques are in good agreement. It is found that the nanobubble size is practically constant and amounts to approximately 100nm in the range of ion concentrations 10^?6<C <1M. It is shown that a necessary condition for nanobubble nucleation is the saturation of solution with dissolved air. It is revealed that nanobubble clusters form a thermodynamically nonequilibrium phase with a lifetime of several months.     

光学击穿对受激布里渊散射特性的影响

把雪崩电离模型引入受激布里渊散射模型中，数值模拟了无光学击穿和有光学击穿时受激布里渊散射能量反射率随输入光功率密度的变化规律.实验上选取净化和未净化的CCl₄作为散射介质，在Nd:YAG调Q激光系统中进行了验证，理论与实验符合得很好.结果表明：当无光学击穿时，随着输入光功率密度的增大，能量反射率先是非线性增大，然后缓慢增大趋于饱和；而当有光学击穿时，随着输入光功率密度的增大，能量反射率先是非线性增大，达到最高值之后开始下降，最终趋于零.无光学击穿时的能量反射率及其稳定度明显高于有光学击穿时的情况. 关键词： 受激布里渊散射 光学击穿 雪崩电离     

X-ray scattering from freely suspended smectic-A films

This work is devoted to a theoretical analysis of specular reflection and diffuse scattering of x-rays from a freely suspended smectic-A film, taking into account its spatial inhomogeneity and deviations of orientational and position ordering in smectic layers from ideal ordering. The results of the calculations are compared with the experimental data on small-angle scattering of x-rays from freely suspended films of 7 AB liquid crystals. It is found that the theoretical results agree with the experimental data.     

水中受激拉曼散射的能量增强及受激布里渊散射的光学抑制

为提高液体介质中受激拉曼散射的输出能量,提出了通过温度调控来抑制受激布里渊散射的方法,设计了532 nm多纵模宽带脉冲激光泵浦的受激拉曼散射发生系统,测量了不同温度下水中前向受激拉曼散射及后向受激布里渊散射的输出能量,分析了水温、泵浦激光线宽及热散焦效应对受激拉曼散射输出能量影响的物理机制.实验结果表明:通过降低水温可实现对受激布里渊散射过程的有效抑制,同时减小热散焦效应带来的光束畸变,从而有效提高受激拉曼散射的输出能量.研究结果对液体介质中的受激拉曼散射多波长转换具有重要意义.     

沙尘天气下激光信号的传输特性

以小角度近似为条件,利用逐级递归的方法推导了激光信号在沙尘天气下的辐射传输方程,得到了多次散射下的光强分布函数,以及波长和不对称因子对光强的影响。同时,通过比较不同散射相位函数及沙尘粒子的散射特性,采用了修正的TTHG(Two Term Henyey-Greenstein)散射相位函数,更加全面地反映了沙粒散射后光强的变化规律。研究结果表明,随着光学厚度的增加,散射光强呈现出先增大后减小的趋势,且多次散射的比重相比于单次散射而言逐渐增大。当散射次数超过3次以上时,接收光强的变化可以忽略不计。相对于Mie理论下的结果而言,采用小角度近似理论,从辐射传输的角度分析沙粒的散射特性误差更小,实现了准确描述沙尘天气下激光信号传输特性的目的。     

Fluctuations of dielectric constant in a system of hard spheres

The correlation function for fluctuations of a dielectric constant in a latex-like suspension of spherical particles was calculated. An exact analytical expression was derived for the correlation function using the Percus-Yewick approximation for a system of hard spheres. The obtained results made it possible to calculate the indicatrix of single scattering, the extinction coefficient, and the transport mean path. It is shown that, starting with a volume content of about ten percent, the “gas” approximation becomes invalid, and optical parameters begin to depend on concentration in quite a complicated manner. In particular, the extinction length and the mean transport path, which are the basic parameters in describing the coherent effects in multiple scattering, vary nonmonotonically with concentration. It is found that there exists a range of sizes and concentrations of scattering particles in which an effect similar to the emergence of blue phase in liquid crystals can be observed.     

Investigation on pulse compression based on stimulated Brillouin scattering and optical breakdown

Pulse compression based on the combination of stimulated Brillouin scattering (SBS) and optical breakdown is investigated in this paper. The first pulse compression based on SBS features a sharp rising edge while the second based on optical breakdown is characterized by a sharp falling edge, therefore a symmetrical narrow pulse can be produced with this approach. In the experiment, FC-72 is used as the Brillouin medium in the first pulse compression, while HT-270 is chosen as the solvent for optical breakdown in the second pulse compression. A 12-ns pump pulse is first compressed to 6 ns by SBS and then further to about 2 ns with comparatively symmetrical edges after optical breakdown.     

Search for acoustic and optic vibrational modes in Zr<Subscript>40</Subscript>Be<Subscript>60</Subscript> metallic glass

Dispersion of collective modes in metallic glass (Zr₄₀Be₆₀, composed of disparate mass particles) was measured at small-angle spectrometer BRISP at the ILL what enabled us to extend to lower momentum transfers unlike to high-angle spectrometer IN4 at the ILL. It was shown that the behavior of the optical mode in the metallic glass is similar to the behavior of optic modes in the other systems with non-sized atoms (liquid LiPb, inert gas mixtures with high density (He₆₅Ne₃₅)).     

Turbid media optical properties derived from the characteristics of propagating laser radiation beams

The possibility is studied to develop a straightforward analytical approach to the determination of the optical properties of liquid turbid media having forward-peaked scattering indicatrices. The approach is based on investigating the in-depth behavior of the radius and the axial intensity of a laser radiation beam propagating through the turbid medium. Based on the small-angle approximation, the detected forward-propagating light power spatial distribution, at relatively small or large optical depths along the beam axis, is obtained asymptotically in analytical form allowing one to derive relatively simple expressions of the extinction, reduced-scattering and absorption coefficients and the anisotropy factor of the medium through the characteristics of the propagating light beam. Preliminary experiments have also been performed, using Intralipid dilutions of different relatively low concentrations and measuring the cross-sectional radial distribution of the detected light power at different depths along the beam axis. The corresponding on-axis detected light power profiles have been measured independently as well. The experimental results are consistent with the analytical expressions obtained that allow one to estimate the optical coefficients and the anisotropy factor of the investigated media on the basis of the measured beam characteristics. The values obtained are near those predicted by other researchers.     

In0.53Ga0.47As/In0.52Al0.48As量子阱中双子带占据的二维电子气的输运特性

研究了不同沟道厚度的In_0.53Ga_0.47As/In_0.52Al_0.48As量子阱中双子带占据的二维电子气的输运特性.在考虑了两个子带电子之间的磁致子带间散射效应后,通过分析Shubnikov-de Haas振荡一阶微分的快速傅里叶变换结果,获得了每个子带电子的浓度、输运散射时间、量子散射时间以及子带之间的散射时间.结果表明,对于所研究的样品,第一子带电子受到的小角散射更强,这与第一子带电子受到了更强的电离杂质散射有 关键词： 二维电子气 散射时间 自洽计算     

Diffraction limit of the theory of multiple small-angle neutron scattering by a dense system of scatterers

Multiple small-angle neutron scattering by a high-density system of inhomogeneities has been considered. A combined approach to the analysis of multiple small-angle neutron scattering has been proposed on the basis of the synthesis of the Zernike–Prince and Moliére formulas. This approach has been compared to the existing multiple small-angle neutron scattering theory based on the eikonal approximation. This comparison has shown that the results in the diffraction limit coincide, whereas differences exist in the refraction limit because the latter theory includes correlations between successive scattering events. It has been shown analytically that the existence of correlations in the spatial position of scatterers results in an increase in the number of unscattered neutrons. Thus, the narrowing of spectra of multiple small-angle neutron scattering observed experimentally and in numerical simulation has been explained.     

Optical Transfer Function of a Binary Markovian Mixture

A system of differential equations describing the optical transfer function (OTF) of a binary Markovian mixture is considered in a small-angle approximation. Its analytical solution has been found for the case where the mixture components have different extinction and scattering coefficients but identical scattering indicatrixes. The results of investigation show that the optical transfer function of the binary Markovian mixture depends strongly on the dimensions of nonuniformities and on the difference between the optical parameters of the mixture components, but it always markedly exceeds the optical transfer function of a uniform medium.     

Monitoring stimulated Raman scattering with photoacoustic detection

A capability of high-frequency ultrasound detection to monitor the process of energy deposition into a molecular system via Raman excitation is experimentally demonstrated. It is shown that the generated ultrasound signal is directly proportional to the optical signal generated in stimulated Raman scattering. Ultrasound detection provides a simple way to discriminate against laser-induced breakdown and allows for the quantification of the stimulated Raman scattering process where direct optical detection is not available. Additionally, it can be used for stimulated Raman imaging in deep tissue, provided that the generated photoacoustic signal is sufficiently strong.     

Molecular dynamics simulation analysis of small-angle neutron scattering by a solution of stearic acid in benzene

Data of small-angle neutron scattering by a diluted solution of stearic acid in deuterated benzene have been analyzed using the results of molecular dynamics simulation. The molecular dynamics simulation approach has been used to calculate the time-averaged distribution of the neutron scattering length density at the interface between the acid molecule and the solvent. It has been shown that the organization of the solvent at the interface with the acid molecule leads to a modulation of the neutron scattering length density and makes a significant contribution to the scattering. This contribution should be taken into account when interpreting the experimental small-angle neutron scattering curves for both the considered system and its analogues.