双反射膜双频He-Ne激光器双频特性及偏振特性的实验研究

本文对 5 MHz频差双反射膜双频 He- Ne激光器输出激光的双频特性及偏振特性进行了实验研究。研究结果表明 ,当磁场方向同反射膜的一个本征模方向基本重合时 ,有双频出现 ;当磁场方向同反射膜的一个本征模方向夹角在 45°左右时 ,无双频出现。输出激光的偏振方向由反射膜的本征模确定 ,其偏振非正交程度为 tgρ≤ 0 .0 0 87,椭圆化程度tgβ随激光管与磁场的相对角位置变化而发生变化。在合适的位置 ,tgβ可以小至 2 / 70 0。该种外差光源具有较好的偏振特性。     

偏振光的χ矢量表示及应用

从Jones矢量的定义中引入了χ矢量,利用该矢量的定义给出了偏振光的χ矢量复平面表示,在该二维复平面上,确定了各个点与偏振光的态势相对应的关系.同时证明了与Jones矢阵的本征态有关的几个重要的结论:任何光学系统的Jones矩阵都有两个本征态;旋光介质的两个本征态是左旋和右旋圆偏振光;偏振器的本征态是两个正交的线偏振光;双折射介质的两个本征态是分别沿快、慢轴方向的线偏振光.     

本征广义琼斯矩阵方法（英文）

为了描述完全偏振光在非线性晶体中传播时的偏振态及相位变化,本文基于Ortega-Quijano等人在推导非线性晶体的广义琼斯矩阵时采用的微分广义琼斯矩阵方法,提出了本征广义琼斯矩阵方法。与微分广义琼斯矩阵方法相比,本征广义琼斯矩阵方法使用了更精确的数学技巧,在描述光在非线性晶体中传播的物理过程上更为严谨。解决了微分广义琼斯矩阵不能计算斜入射光或者光轴与实验室坐标不重合时光的偏振变化的问题。首先,根据折射率椭球方程和光的入射方向,计算出非线性晶体中本征光的传播方向和折射率。然后,给出了本征光的本征广义琼斯矩阵。最后,计算了本征光的偏振态和相位变化。本文使用本征广义琼斯矩阵对带有一个奇点的涡旋光在KDP晶体中的传播情况进行模拟计算,计算结果表明,本征广义琼斯矩阵方法能够描述任意入射角度、任意光轴方向的完全偏振光在非线性晶体中的传播过程。     

二阶偏振模色散矢量致脉冲展宽的理论分析

我们首次提出了消偏矢量的概念，并用它来描述二阶偏振模色散中偏振模色散矢量方向的变化。本文导出了由于二阶偏振模色散所引起的脉冲展宽的解析表达式，结论指出，偏振相关的本征色散总是使脉冲展宽加剧；而偏振模色散矢量方向的变化（消偏矢量），却是使脉冲展宽减弱。二阶偏振模色散对脉冲的展宽，不仅与偏振相关的本征色散和消偏矢量有关，而且还与信号的传输速率以及初始一阶偏振模色散的大小有关。信号速率的提高，将明显的使二阶偏振模色散的影响增强。在偏振相关的本征色散不为零的情况下，通过调整初始偏振态矢量、初始一阶偏振模色散矢量以及消偏矢量三者的方向，使它们互相平行，可以获得最佳的色散补偿。但是却不能获得完全的色散补偿。在最佳色散补偿时的最小脉冲展宽为σ= (2^1/2/4)( DCF/T₀).     

考虑晶体应力情况下的单块非平面环形腔分析

简要分析了单块非平面环形腔单频激光器的基本原理,并采用琼斯矩阵计算了单块单向行波非平面环形腔内的本征偏振态。深入分析了磁场强度、输出面偏振反射系数、入射角、非平面角及腔内其它参量对各本征偏振态的损耗及两本征偏振态间损耗差的影响,并且指出如何选择各参量以降低本征偏振态的损耗且增大两本征偏振态间的损耗差。最后还通过计算晶体应力在两个本征偏振态方向引起的相位差,讨论了应力作用方向以及应力双折射系数对腔内本征偏振态的损耗和两本征偏振态间损耗差的影响。     

双折射双频激光器频差特性分析

用激光原理和晶体光学原理分析了腔内加旋光晶体输出光的频差特性，分析表明：当晶片旋转时，理论计算的频差永远小于晶体两本征模通过晶片位相差所决定的频差，有一特殊因子Ｇ＾２起作用。Ｇ＾２的物理意义是激光本征模在晶体两本征模上的投影比平方。这表明一个激光本征模在通过晶体时是部分以快模形式，部分以慢模形式，实际位相延迟介于两者之间，由于两者都介于快模和慢模之间，因而实际频差小于由快模和慢模决定的频差，理论计     

光子晶体光纤的正交函数模型

提出了一种用于分析光子晶体光纤的正交函数模型。采用一种新型超格子的构造方法,将光子晶体光纤的横向介电常量表示为两种周期性结构叠加。将横向电场以厄密一高斯函数展开,利用正交函数的性质,将全矢量波动方程转化为矩阵本征值问题,求解本征值问题可得到模式的传输常量及模场分布。利用此模型举例讨论了椭圆孔三角格子光子晶体光纤的模场分布和偏振特性以及三角格子光子晶体光纤的色散特性和有效面积等传输特性。作为一种普适的模型,此方法还可适用于四方结构、蜂窝结构及椭圆孔等多种结构光子晶体光纤。     

椭圆孔光子晶体光纤的本地正交函数模型

提出了一种用于分析椭圆孔光子晶体光纤的正交函数模型.发展了一种新型超格子的构造方法，将光子晶体光纤的横向介电常数表示为两种周期性结构叠加，这两种周期性结构分别用余弦函数展开；同时将横向电场以Hermite-Gaussian函数展开.利用正交函数的性质，将全矢量波动方程转化为矩阵本征值问题，求得两偏振模式传输常数.利用此模型可以研究圆孔及椭圆孔光子晶体光纤的模式特性、色散特性、偏振特性等. 关键词： 光子晶体光纤 超格子 正交函数 本征值问题     

双折射双频激光器输出光偏振特性的实验研究

报道了双折射双频激光器输出光偏振特性的实验研究，实验发现双折射双频激光器两端输出的线偏振光的偏振方向并不一致，而相互有一定的旋转角度，实验也再次证明了激光器输出的是正交线偏振光，而不是晶体本征模的椭圆化了的偏振光。     

单色平面波从各向同性介质入射到晶体界面上折射波的求解方法

从麦克斯韦方程组出发,结合旋光晶体的物质方程以及单色平面波从各向同性介质入射到晶体界面时电场的连续性边界条件,通过晶体主轴坐标系和界面坐标系的坐标变换,推导了任意入射方向的单色平面波在晶体界面上的折射方向的求解公式,进而利用所得的折射方向求解晶体中折射波的电场本征模、电位移矢量本征模和光线方向。该方法以波动光学和晶体的物理性质为理论基础,推导过程没做近似,对双轴、单轴和各向同性晶体以及晶体是否具有旋光性都适用,并可通过计算机编程求出精确的数值解。     

Angular spectrum approach for the computation of group and phase velocity surfaces of acoustic waves in anisotropic materials

The decomposition of an acoustic wave into its angular spectrum representation creates an effective base for the calculation of wave propagation effects in anisotropic media. In this method, the distribution of acoustic fields is calculated in arbitrary planes from the superposition of the planar components with proper phase shifts. These phase shifts depend on the ratio of the distance between the planes to the normal component of the phase slowness vector. In anisotropic media, the phase shifts depend additionally on the changes of the slowness with respect to the direction of the propagation vector and the polarization. Those relations are obtained from the Christoffel equation. The method employing the fast Fourier transformation algorithm is especially suited for volume imaging in anisotropic media, based on holographic detection in transmission of acoustic waves generated by a point source. This technique is compared with measurements on crystals performed by phase-sensitive scanning acoustic microscopy.     

Acoustic waves and phonon focus in Li crystal: the first principle study

The acoustic waves in Li are investigated by solving the Christoffel equations.Theoretical calculations of the three dimensional slowness surfaces give insights into themixing of longitudinal and transverse modes and show the origin of the phonon caustics.The results are explained in terms of phonon focusing due to the anisotropy of crystals.The propagation of elastic waves in Li is affected by the elastic anisotropy of thelattice.     

Dispersion of an acoustic pulse passing through a large-grained polycrystalline film

Propagation of a short acoustic pulse through a polycrystalline film comprised of large randomly oriented elastically anisotropic grains is analyzed theoretically. For average grain size much larger than the film thickness, a short acoustic pulse launched normally into the film will traverse each grain in a time determined by the acoustic slowness in the direction normal to the film, which will depend on the local grain orientation. A typical measurement averages over a large number of grains resulting in the broadening of the composite output pulse. The resulting pulse shape is characterized by distinct features related to stationary values of the directionally dependent acoustic slowness of the crystalline material. Maxima and minima in the slowness yield discontinuities in the pulse shape, while saddle points yield logarithmic singularities. For cubic and hexagonal crystals, power law singularities result from cones of directions in which the slowness is a maximum or minimum. Numerical results, taking into account Gaussian broadening of the input pulse, are presented for thin film materials commonly encountered in picosecond ultrasonic experiments, such as copper, gold, and aluminum.     

On interference effects in the elastodynamic Green's functions G33(x, omega) of Si and GaAs

In this paper, we analyze interference effects present in the elastodynamic Green's functions G33(x,omega) of the cubic crystals Si and GaAs, which are associated with folded portions of the wave surface of the slow transverse (ST) acoustic mode. G33(x,omega) represents the three dimensional extension of the amplitude distribution imaged in the transmission acoustic microscopy of these crystals. The intensity contrast for oscillations of a particular wave vector k in the interference pattern is determined essentially by the 3D Fourier transform of G33(x,omega)G33*(x,omega). According to the Fourier autocorrelation theorem, that transform is equivalent to the autocorrelation function of the corresponding distribution G(33)(k,omega) in k-space. We show that due to the linear mapping between k-space and the slowness vector s-space, the interference phenomena discussed here are related to geometrical features of the slowness surface of the ST mode. We present calculations of these effects based on the angular spectrum technique.     

Modeling of Coulomb coupling and acoustic wave propagation in LiNbO3

The angular spectrum propagation technique is applied to modeling of wave propagation through piezoelectric media. The calculations of the angular spectrum propagator are based on the relevant equation for the slowness surface resulting from the solution of the Christoffel equation with piezoelectrically stiffened elastic constants. A two-dimensional FFT algorithm is applied in the final field superposition. We concentrate on the case of Coulomb coupling through local electrical point contacts on both the excitation and detection side. To model that case we superpose solutions for acoustic Green's functions of different propagation modes convoluted with equivalent distributed effective sources. Calculated results are in good agreement with the measured ones.     

Density-functional theory study of the effect of pressure on the elastic properties of CaB6

Under high pressure, the long believed single-phase material CaB6 was latterly discovered to have a new phase tI56. Based on the density-functional theory, the pressure effects on the structural and elastic properties of CaB6 are obtained. The calculated bulk, shear, and Young's moduli of the recently synthesized high pressure phase tI56-CaB6 are larger than those of the low pressure phase. Moreover, the high pressure phase of CaB6 has ductile behaviors, and its ductility increases with the increase of pressure. On the contrary, the calculated results indicate that the low pressure phase of CaB6 is brittle. The calculated Debye temperature indicates that the thermal conductivity of CaB6 is not very good. Furthermore, based on the Christoffel equation, the slowness surface of the acoustic waves is obtained.     

Effects of oxygen deficiency on the acoustic anomalies and phase transition behaviors of barium titanate single crystals

The acoustic behaviors of oxygen-reduced barium titanate (BaTiO_3-δ) single crystals with δ∼0.04 were investigated as a function of temperature by using Brillouin spectroscopy. The longitudinal acoustic mode of the moderately-reduced BaTiO₃ (BTO) showed two pronounced anomalies at approximately 112 °C and −11 °C, which correspond to the cubic-tetragonal and tetragonal-orthorhombic phase transition temperature, respectively. These temperatures were lower by more than 10 °C compared to those of the pure BaTiO₃ suggesting that the disorder introduced by oxygen vacancies lowers the phase transition temperatures. The paraelectric phase of the reduced BaTiO₃ were characterized by substantial softening of the longitudinal acoustic mode and the growth of central peaks centered at zero frequency. These anomalies were observed in a certain temperature range above the Curie temperature, indicating that pretransitional precursor polar clusters exist in the cubic phase and that their dynamics are responsible for the acoustic anomalies caused by electrostrictive coupling between the strain and the polarization. The relaxation time of the precursor polar clusters derived from the central peak exhibited a critical slowing-down behavior showing that their dynamics becomes more sluggish as temperature approaches the Curie point.     

Effect of Sn substitution on the elastic anomalies and phase transition behaviors of antiferroelectric PbHfO3 single crystals studied by Brillouin scattering

The phase transition sequence of a PbHf_0.7Sn_0.3O₃ single crystal was investigated by using Brillouin spectroscopy. The longitudinal acoustic mode showed three distinct changes at ∼370 K, ∼455 K and ∼495 K which corresponded to the antiferroelectric 1 → antiferroelectric 2 → intermediate → paraelectric phase transitions upon heating. The paraelectric phase was characterized by softening acoustic mode, increasing acoustic damping and the appearance of quasielastic central peaks which were observed upon approaching the phase transition temperature. The relaxation times derived from the acoustic mode anomalies and the central peak were consistent with each other and showed a slowing-down behavior. These precursor phenomena were attributed to the polar clusters having broken inversion symmetry. The substitution of Hf with Sn induced a very soft intermediate phase between the paraelectric and the antiferroelectric phases, where the longitudinal acoustic mode exhibited the lowest mode frequency along with substantially high acoustic damping. The transverse acoustic mode, which was observed in the two antiferroelectric phases, did not appear in the intermediate phase. These results indicated that the average symmetry of the intermediate phase may be cubic consisting of fine ferroelastic domains.     

Caustic and anticaustic points in the phonon focusing patterns of cubic crystals

Phonon focusing patterns are dependent on the existence of concave/saddle regions and acoustic axes in the slowness surface. The main feature of the focusing patterns in cubic crystals can be characterized by the caustic and anticaustic points in the symmetry planes. By applying the Stroh formalism, the caustic and anticaustic points in the symmetry planes are investigated in relation to degeneracies in the Stroh eigenvalue equation. A set of analytical expressions for the locations of the caustic and anticaustic points is derived for cubic crystals.     

Elastic,Piezoelectric and Acoustic Properties of Wurtzite MnO from Density Functional Calculation

The wurtzite MnO has been obtained lately and is expected to have large potential in varies applications.Since elastic properties are the bases of various applications, we calculate these properties of wurtzite MnO based on the density-functional theory and compare it with other two phases of MnO (rocksalt and zinc-blende MnO). The Young's modulus of wurtzite and zinc-blende MnO are 65.6 GPa and 73.4 GPa, respectively, which are much lower than those of rocksalt MnO (177.6 GPa). More importantly, both the Poisson ratio and the bulk modulus to shear modulus ratio indicate that wurtzite MnO should have much better ductile properties than rocksalt MnO. The calculated piezoelectric constants of wurtzite MnO are comparable to those of ZnO. This suggests wurtzite MnO is a good piezoelectric material.Furthermore, the slowness surfaces of acoustic waves of them are given from Christoffel equation.