LBO晶体的布里渊散射与弹性和压电系数的测量

简述了应用布里渊散射测量晶体的弹性和压电系数的方法,推导了C_2v点群晶体(100),(010)和(001)晶面内任意波矢方向的布里渊散射张量及声速各向异性方程.根据上述方法及结果,用180°散射测量了LBO晶体不同波矢方向的布里渊散射谱,得到了LBO晶体在特超声频率下所有独立的弹性和压电系数,并依此计算了该晶体的声速各向异性曲线. 关键词：     

用布里渊散射研究晶体相变

在研究相变和临界现象中,光散射实验起着十分重要的作用.在晶体的结构相变研究中,高分辨的喇曼和布里渊散射谱能够提供有关“软模”的许多知识.软模是一种对温度有着很强依赖性的晶格振动,其振动频率在相变点变为零.软模的凝聚导致自发对称破缺. 当晶体发生相变时,弹性性质通常表现出反常.由于布里渊散射实验能够探测长波声学模行为,因此在过去的十六年中,布里渊散射光谱学被广泛地应用来研究固体相变.它有力地补充了与此紧密相近的声共振和超声传播测量手段所获得资料.这些实验结果,与喇曼散射、非弹性中子散射和X射线测量结果综合起来,使…     

L.布里渊与布里渊散射

非弹性光散射中两个最具有代表性的现象就是拉曼散射和布里渊散射,它们分别以印度科学家拉曼(C.V.Raman)和法裔美籍科学家布里渊(L.Brillouin)的名字命名的。人们对前者的认知远比后者多(包括相应的散射现象的理解和认识)。为此,本文将对L.布里渊及以他名字命名的布里渊散射做详细的介绍和阐述。     

钽铌酸锂晶体的布里渊散射研究

研究了钽铌酸锂(LiTa_xNb_(1-x)O_3,x=0.0013,0.0052,0.015,0.06)晶体在室温的布里渊散射.给出了它们的弹性刚度常数和压电应力常数.测量了一种组份晶体的布里渊散射频移在50K～580K范围内的温度特性.发现某些声子散射谱中除三个正常峰外还存在一个异常峰.     

一套高温高压原位拉曼散射、布里渊散射测量系统

本文描述了一套利用激光加热技术、金刚石对顶砧高压技术,集成共聚焦拉曼光谱测量、六通道布里渊散射光谱测量、以及黑体辐射光谱测温,成功搭建起来的高温高压原位拉曼散射、布里渊散射的光学测量系统。利用该系统,并结合薄膜沉积金属、光刻技术改进了金刚石对顶砧中样品的装填方法,解决了样品透明不易加热的问题。已利用该系统得到液态氩在0.85 GPa常温状态下和14.58 GPa熔融状态下的体声速分别为1.85 Km/s和5.19 Km/s。     

用布里渊散射测量水的体粘滞系数

提出了一种直接测量水的体粘滞系数的实验方法－布里渊散射方法,通过分析剪切粘滞系数和体粘滞系数与布里渊散射线宽的关系,得到了布里渊散射线宽和频移与水的体粘滞系数关系的公式,建立了实验测量水的体粘滞系数的基础,给出了不同温度下水的体粘滞系数的实验测量结果,估算了这种方法的测量误差,实验和理论分析都表明这是一种直接测量水的体粘滞系数的快速而精确的方法。     

光子晶体光纤中布里渊散射声波模式特性的分析

推导了光子晶体光纤中声波微小位移波动方程; 研究了泵浦波长以及纤芯折射率对声波模式的影响; 应用石英圆柱模型研究了小芯径光子晶体光纤中纤芯直径对布里渊声波模式色散的影响. 结果表明在光子晶体光纤中, 纵向声波和横向声波共同作用产生质点声场, 两者相互耦合将产生混合声波模式; 可以通过改变泵浦波长或光子晶体光纤纤芯折射率来改变参与布里渊散射(BS) 过程的声波模式的传播常数; 随着光子晶体光纤(PCF) 纤芯直径的增大, 声波模式耦合程度得到加强, 相速度呈减小趋势, 且同一传播常数下, 声波模式数呈增多趋势; 随着泵浦波频率的增大, 声波相速度减小.     

受激布里渊散射瞬时频率测量的偏振优化

针对光纤偏振随机偏转的问题,提出了一种受激布里渊散射瞬时频率测量技术的优化方案。对比分析了单模光纤和保偏光纤用于实现受激布里渊散射瞬时频率测量的特性差异。实验结果表明:与单模光纤相比,保偏光纤中的偏振随机偏转现象得到有效抑制,测量系统的频率分辨率由225MHz提高至90MHz。     

基于自发瑞利-布里渊散射测量空气的温度

瑞利-布里渊散射的散射截面比拉曼散射大,因而其在大气散射中实现对大气对流层温度廓线的准确测量方面具有一定的优势,同时利用瑞利-布里渊散射实现高压环境下温度的准确测量对于航天飞机主引擎状态的监测和超燃发动机燃烧室参数测量方面具有重要意义。基于自发瑞利-布里渊散射分别采用反卷积方法和卷积方法来实现空气在不同压力条件下的温度反演,研究引起温度反演误差的原因,并对利用两种方法获得的温度测量结果进行了比较。在利用基于维纳滤波器的反卷积方法对测量光谱直接处理实现温度反演之前,首先利用反卷积方法对由自发瑞利-布里渊散射模型与仪器函数卷积得到的卷积光谱进行处理获得反卷积光谱,将反卷积光谱与未经卷积的理论计算光谱进行比较实现温度反演, 并基于温度反演误差小于1.0 K,光谱拟合误差相对较小,光谱处理时间短的参数优化原则对反卷积方法中的关键参数奇异值叠加数进行了优化处理,得到优化后的奇异值叠加数为150。随后实验测量了由532 nm波长的连续激光激发的纯净空气在温度为294.0 K,压强为1～7 bar条件下的自发瑞利-布里渊散射光谱,并结合理论计算光谱和最小χ2值原理对光谱信号散射角进行优化,优化值为90.7°,同时利用反卷积和卷积方法分别对实验测量光谱进行处理实现空气在不同压强下的温度反演。实验结果表明反卷积方法在一定程度上可以提高信号光谱分辨率,而且利用反卷积和卷积方法均可以实现空气在不同压力（1～7 bar）条件下温度的准确测量,温度测量的最大误差均小于2.0 K;利用反卷积方法的温度反演结果随着气体压强的增大随之得到改善,实现温度反演测量所需要的光谱处理时间减少;在空气压强较低（≤2 bar）时,由卷积方法获得的温度反演结果要优于反卷积方法,压强较高（>2 bar）时,两种方法的温度反演结果相近, 其绝对误差均小于1.0 K。通过分析得到引起两种方法温度反演误差的原因主要包括环境温度的波动（±0.2 K）,散射角存在一定的不确定度以及气体的各已知参数的微量偏差对温度测量结果的影响以及反卷积对光谱噪声的非线性放大引起的光谱扰动对温度测量结果的影响。在实验中可以通过提高测量光谱的信噪比、提高散射角的优化精度及改善反卷积方法来获得更加准确的参数测量结果。     

Elastic measurements of layered nanocomposite materials by Brillouin spectroscopy

Surface Brillouin spectroscopy makes it possible to measure surface elastic wave propagation parameters at frequencies up to 20 GHz or more. This enables us to measure the elastic properties of surface layers only a small fraction of a micrometre thick. The wavelength and incident angle of the light determine the wavenumber of surface elastic waves (SAW) that scatter the light inelastically, and their frequency can be found by measuring the change in wavelength of the scattered light. By analysing the elastic wave modes present in the surface, the elastic properties can be deduced. We have used this technique to measure the elastic properties of layered nanocomposite materials, which are widely used in the packaging industry. 12 microns polymer films (PET) were coated with glass oxide layers of thickness as little as 25 nm, to give transparent nanocomposite structures with excellent gas barrier properties. In order to understand and model the behaviour of these films under deformation, it is necessary to determine the elastic properties of the different layers. Evaluation of the elastic properties presents several challenges. First, the oxide layers are much thinner than the wavelengths of the surface phonons in surface Brillouin spectroscopy (and hence the depth probed), which usually lie in the range 250-500 nm. The anisotropic elastic properties of the PET substrate must therefore be measured accurately, and this can be done using bulk Brillouin spectroscopy. Second, a thin layer of metal (usually 10-20 nm) must be deposited on the glass surface so that the surface phonons scatter the light effectively. The elastic properties of the glass layer can then be deduced from surface Brillouin spectroscopy measurements, by simulating the surface wave modes of the metal/glass/polymer composite, and adjusting the parameters to give the best fit. In this way it is possible to observe how the properties of the glass vary as a function of thickness, and in turn to understand how to improve systematically the properties under deformation.     

Elastic constants of lithium thallium tartrate measured by Brillouin scattering

All components of the elastic stiffness tensor of lithium thallium tartrate at room temperature have been measured by Brillouin scattering techniques using two different scattering configurations. Refractive indices have been determined independently by optical measurement. The results are compared with previous work on lithium thallium tartrate and lithium ammonium tartrate.     

Elastic and photoelastic constants of NaCl,KBr and LiF by Brillouin scattering

Elastic and photoelastic constants of NaCl, KBr and LiF were determined from Brillouin frequency shifts and intensities. Reported differences between the hypersonic and ultrasonic elastic constants for KBr and LiF were not confirmed. For each material the constants were in good agreement with the ultrasonic values. Scattering from longitudinal, transverse and mixed phonon modes was clearly observed for each material.     

Equation of State and Elastic Constants of Compressed fcc Cu

First-principles calculations of equation of state and single-crystal elastic constants of copper are carried out up to twofold compression. The Helmholtz free energies are calculated using the quasi-harmonic phonon approach based on density-functional theory within both the local density approximation and the generalized gradient approximation （GGA）. We find that the results calculated within GGA agree better with the experimental measurements in overall. The equation of state and the zero-pressure single-crystal elastic constants are close to the experimental values.     

Elastic Constants of NaC1 under Pressure via First-Principles Calculations

The elastic constants of the B1 structure NaCl under pressure are obtained by using the ab initio plane-wave pseudopotential density functional theory method. The obtained zero pressure lattice constant and elastic constants are in good agreement with the available experimental data. It is found that the elastic constants C11 and C12 and the bulk modulus B increase monotonically with pressure P, however C44 increases monotonically when P_〈28 GPa and decreases when P〉28 GPa. Moreover, we discuss the B1-B2 structure phase transition of NaCl and obtain the transition pressure of 28.3 GPa.     

高压下铜的声子谱和弹性系数

 以Foiles、Baskes和Daw提出的嵌入原子势（EAM）为基础,并结合Vinet-Rose形式的普适物态方程,建立了金属铜原子间的EAM势。在此基础上,用EAM势和第一原理微扰理论方法计算了高压下铜的声子谱和弹性系数。通过适当地调整嵌入原子势参数,这两种方法几乎能够给出完全一致的结果。而且,晶格振动声子谱和比热的理论值与实验值符合得也非常好。