单环多段光强分布检测光学涡旋拓扑荷值

针对涡旋光束检测范围局限这一问题, 提出了一种新的光学涡旋拓扑荷值检测方法-单环多段光强分布检测法, 它以分段数和环半径为两大检测常数, 将检测涡旋光束拓扑荷值范围扩大到了128种, 与以往利用旁瓣调控光学涡旋检测拓扑荷值方法相比, 检测范围扩大了1个数量级. 单环多段光强分布是基于计算机全息图实现在远场衍射焦平面上环半径相等的两束携带不同拓扑荷数的涡旋光束叠加后形成的光强分布. 计算机模拟和光学实验验证了所提出方法的可行性, 该方法在自由空间光通信领域具有一定的研究价值和应用潜力.     

涡旋光束形成的散斑场光强和相位的分布特性

通过对不同拓扑荷的涡旋光束经随机表面散射后在衍射区形成的横向和纵向光强分布的模拟,发现散斑颗粒的横向和纵向的平均尺寸比用高斯光束照明随机表面产生的散斑颗粒平均尺寸小很多,并且随着涡旋光束拓扑荷和光斑半径的增大而减小,同时散斑场相位涡旋的密度随着涡旋光束拓扑荷和光斑半径的增大而增大。利用这种方法可以方便地选择不同拓扑荷和光斑半径的涡旋光束照射随机表面,得到合适的散斑颗粒来捕获更小的微粒。这种结果还可以用来降低散斑噪声。     

超高斯涡旋光束在空间中的传输特性

研究了超高斯涡旋光束光强最大值、光斑半径以及环围能量半径等参数随传输距离和拓扑荷数的变化规律,并与高斯涡旋光束做了比较,结果表明:超高斯涡旋光束的光斑半径和环围能量半径随拓扑荷数及传输距离呈近似线性关系;对同一拓扑荷数和传输距离,高斯涡旋光束的能量较超高斯涡旋光束要发散;当拓扑荷数较大时,超高斯涡旋光束的光斑半径比高斯涡旋光束更大。针对光束质量研究了广义光束质量因子随传输距离和拓扑荷数的变化,结果表明传输距离足够远时,拓扑荷数较小的超高斯涡旋光束具有更好的光束质量。     

利用柱透镜调控涡旋光束的拓扑结构

提出了一种利用柱透镜调控涡旋光束拓扑结构的方法.利用计算全息法制作的叉形光栅掩模板,实验获得了具有不同拓扑荷的涡旋光束,分析了涡旋光束通过柱透镜变换后的强度和相位分布.结果表明,涡旋光束经柱透镜变换后,其拓扑荷符号将发生改变,并且高阶涡旋光束退化为多个分离的一阶涡旋光束.利用高阶激光模式的线性叠加特性以及古依相移对实验结果进行了理论解释,并通过数值模拟对实验结果进行了验证.     

平面涡旋光干涉的分析

光学涡旋具有独特的相位奇点和螺旋相位结构, 多个涡旋光场之间的干涉呈现出新颖的强度和相位分布特征.通过在平面波背景中嵌入涡旋相位产生平面涡旋光场, 采用数值模拟方法研究了多个平面涡旋光场之间的干涉, 并分析了两个平面涡旋光场的中心间距及拓扑荷值对涡旋产生和湮灭的影响.进一步数值研究了对称分布的多个点涡旋光之间的干涉, 结果表明通过改变涡旋光束数目或者拓扑荷值, 可获得不同分布的对称涡旋阵列光场.利用计算全息并通过空间光调制器, 实验上实现了具有不同拓扑荷值的多个对称点涡旋光场的干涉, 其干涉图样与模拟结果吻合.实验结果不仅证实了数值模拟结果, 也为实验研究复杂涡旋光场的干涉提供了一种有效方法.     

球形粒子在聚焦拉盖尔-高斯光束中的散射特性研究

研究了球形粒子在聚焦拉盖尔-高斯光束中的散射特性. 根据广义Mie理论, 推导出球形粒子在聚焦拉盖尔-高斯光束中散射系数的解析公式. 针对光束的电场分布及粒子散射强度进行了数值仿真, 讨论了散射强度随散射角、散射球粒子半径和拓扑荷的变化特性, 并通过散射系数解释了散射强度分布的振荡现象. 结果表明, 在聚焦拉盖尔-高斯光束照射下, 球形粒子的后向散射强度随着粒子半径的增大而逐渐增大; 后向散射强度开始增大时对应的粒子半径与拓扑荷有关. 通过与高斯光束的对比, 可以看出球形粒子在聚焦拉盖尔-高斯光束中散射特性的差异, 使其在粒径测量、光通信和大气后向散射探测等方面具有潜在应用价值.     

大气湍流中部分相干聚焦涡旋光束的传输特性

基于广义惠更斯-菲涅耳原理和相位结构函数的平方近似,研究了部分相干高斯-谢尔模型涡旋光束被聚焦后在大气湍流中的传输特性,得到了焦平面上光强解析表达式.利用该表达式,详细研究了该类光束在大气湍流中传输焦平面上的光强分布特性.结果表明：在大气湍流中,随着传输距离的增加,涡旋光束的奇异性逐渐降低.对于拓扑荷大的以及空间相干长度较长的涡旋光束,光束奇异性的保持相对要好.在一定的焦距长度和湍流大气条件下,我们可以通过调整光源的拓扑荷和相干长度控制焦面光强分布和焦斑大小.另外,有一定拓扑荷的涡旋光束可以在一定程度上降低大气湍流对传输光束焦面光强分布的影响.     

完美涡旋光拓扑荷的原位测定

为了实现对完美涡旋光拓扑荷的快速测定,提出了一种同轴干涉测定方法,其基本思路是利用一个空间光调制器同时调制产生完美涡旋光与球面波,调制球面波的发散角使两者发生干涉,利用干涉条纹数来实现拓扑荷数的直接快速测定.理论模拟和实验测定结果表明,利用该方法得到的干涉条纹可以测定完美涡旋光的拓扑荷,包括大小与符号.进一步,利用该方法测量了完美涡旋光阵列与球面波的干涉图样,实现了对每个完美涡旋光的拓扑荷的快速测定.该原位测定方法简单、有效,对于利用完美涡旋光实现轨道角动量控制和信息编码等应用具有参考意义.     

等离子体中涡旋光束自聚焦与成丝现象的模拟研究

涡旋光束在低密度等离子体传输过程中出现的非线性物理效应与光子携带的轨道角动量(拓扑荷)存在密切关联.基于亥姆霍兹方程理论获得描述涡旋光束传输的旁轴近似方程,解析求出涡旋光束在低密度等离子体中传输的自聚焦临界功率表达式.利用分步傅里叶数值方法,针对不同参数条件下涡旋光束的自聚焦和成丝现象进行了模拟分析.研究结果表明,涡旋光束的拓扑荷数决定了自聚焦临界功率大小.涡旋光束发生成丝不稳定性现象的功率阈值和最大成丝数目与拓扑荷数存在密切关联.     

涡旋光束的产生与干涉

分别从理论上和实验上研究了涡旋光束的产生和干涉现象.理论上分析了分数阶和整数阶涡旋光束同球面波以及平面波的干涉情况,并从实验上得出了其干涉图形.实验结果和理论模拟基本上一致.研究表明,随着涡旋光束拓扑荷数的变化,干涉图形也会产生变化.这一现象可用于测定分数阶涡旋光束的拓扑荷数.     

Arrays of Charged (±2π)-Twist Disclinations in Ferroelectric Liquid Crystals

The notion of an electrostatic charge of (±2)-twist disclinations is used to approximate the evaluation of the electrostatic interaction energy among disclinations forming arrays in finite samples of ferroelectric chiral smectic C liquid crystals. Screening effects of free charges in a material surrounding the disclination are taken into account by introducing a phenomenological depolarisation factor.The electrostatic interaction energy is important in chiral smectic C materials with high values of the spontaneous polarisation when screening effects of free charges are small. Then the electrostatic interaction leads to elimination of disclinations from the sample. When there is a high concentration of free charges in the sample (smaller value of depolarisation factor), the electrostatic interaction energy is of the order of the elastic interaction energy of disclinations what influences the equilibrium of disclination arrays in the sample. Two disclination configurations are considered. In the Brunet-Williams configuration the disclinations of opposite topological charge have also the opposite electrostatic charge so their attraction is augmented. This attraction can be balanced by the helical structure in the central part of the sample when the sample thickness is rather high.On the contrary, in the Glogarová-Pavel configuration the disclinations of opposite topological charge have the electrostatic charge of the same sign. The equilibrium in this configuration is either a balance of elastic attraction and electrostatic repulsion if elastic and Coulomb forces are of the same order or it is governed by the value of the anchoring energy when electrostatic interaction prevails over the elastic one.     

Iterative extended Huckel calculation on hexagonal boron nitride

Some properties of two-dimensional boron nitride are treated in a truncated crystal approach, employing the iterative extended Huckel method in a charge self-consistent calculation. The values of the band width, band gap, charges, equilibrium distance, and sublimation energy are in satisfactory agreement with experiment.     

Colloidal interactions in two-dimensional nematics

The interaction between two disks immersed in a 2D nematic is investigated i) analytically using the tensor order parameter formalism for the nematic configuration around isolated disks and ii) numerically using finite-element methods with adaptive meshing to minimize the corresponding Landau-de Gennes free energy. For strong homeotropic anchoring, each disk generates a pair of defects with one-half topological charge responsible for the 2D quadrupolar interaction between the disks at large distances. At short distance, the position of the defects may change, leading to unexpected complex interactions with the quadrupolar repulsive interactions becoming attractive. This short-range attraction in all directions is still anisotropic. As the distance between the disks decreases, their preferred relative orientation with respect to the far-field nematic director changes from oblique to perpendicular. Received 1 October 2002 and Received in final form 12 November 2002 RID="a" ID="a"e-mail: miko@cii.fc.ul.pt     

Separation-independent attractive force between like particles mediated by nematic-liquid-crystal distortions

We investigate numerically with the aid of the Landau-de Gennes continuum theory the interaction between two spherical particles carrying the same topological charges +1 mediated by the elastic distortion of a nematic liquid crystal. We consider the case where an escaped nontopological ring disclination is situated between the particles; the director is continuous everywhere and no counterdefects are present. We find that the interaction is attractive and its potential energy depends linearly on the interparticle distance D. This behavior yields the D-independent interaction force, which was observed experimentally by Poulin, Cabuil, and Weitz [Phys. Rev. Lett. 79, 4862 (1997)] in the presence of narrow strings of birefringent regions ("bubble-gum" configuration) between the particles.     

Measurement of elastic forces between iron colloidal particles in a nematic liquid crystal

The forces that arise between two iron particles in a nematic liquid crystal with a strong homeotropic anchoring were studied. For the first time, the short range repulsive force resulting from the presence of a hedgehog defect between two particles was precisely determined thanks to application of a small magnetic field and observation of the equilibrium position resulting from the balance between the elastic and magnetic forces. Above a given threshold force, the particles stuck together whereas the hedgehog defect was expelled and transformed into a Saturn ring located between the particles. The attractive part of the interparticle force was determined with the same method on the entire range of separation distances; we found that the equilibrium distance between two particles was r = 1.19 +/- 0.05 ( was the average diameter of the pair of particles).     

Pair interactions of heavy vortices in quantum fluids

The dynamics of quantum vortex pairs carrying heavy doping matter trapped inside their cores is studied. The nonlinear classical matter field formalism is used to build a universal mathematical model of a heavy vortex applicable to different types of quantum mixtures. It is shown how the usual vortex dynamics typical for undoped pairs qualitatively changes when heavy dopants are used: heavy vortices with opposite topological charges (chiralities) attract each other, while vortices with the same charge are repelled. The force responsible for such behavior appears as a result of superposition of vortices velocity fields in the presence of doping substance and can be considered as a special realization of the Magnus effect. The force is evaluated quantitatively and its inverse proportionality to the distance is demonstrated. The mechanism described in this paper gives an example of how a light nonlinear classical field may realize repulsive and attractive interactions between embedded heavy impurities.     

Interaction of two macroparticles in a nonequilibrium plasma

The interaction of two macroparticles in a nonequilibrium plasma at elevated pressures has been investigated. An asymptotic theory of screening, which leads to a two-exponential dependence of the macroparticle potential on distance with different screening constants, is used to determine the electrostatic energy of the system of charges associated with the two macroparticles. The dependence of the electrostatic energy on interparticle distance has been found to have a minimum, as in an equilibrium plasma. The interaction force between the macroparticles has been determined; it turned out to be asymmetric—for different charges, the forces acting on the first and second macroparticles are not equal. This is the result of an asymmetric charge separation near macroparticles with differing charges and indicates that the interaction force in a nonequilibrium plasma is nonpotential. The forces are equal for identical macroparticles or in an equilibrium plasma and the potential energy of the interaction between the macroparticles has been determined for these cases. Attraction between likely charged particles with different (in magnitude) charges has been found to be possible when they come very close together. Relations to determine the modified coupling parameter for an interaction potential that consists of two exponential terms with different screening constants have been derived.     

Strong pressure-energy correlations in van der Waals liquids

Strong correlations between equilibrium fluctuations of the configurational parts of pressure and energy are found in computer simulations of the Lennard-Jones liquid and other simple liquids, but not for hydrogen-bonding liquids such as methanol and water. The correlations that are present also in the crystal and glass phases reflect an effective inverse power-law repulsive potential dominating fluctuations, even at zero and slightly negative pressure. In experimental data for supercritical argon, the correlations are found to be approximately 96%. Consequences for viscous liquid dynamics are discussed.     

On the charge and shape polarization of fission fragments,fusioning nuclei,or scattering heavy ions near contact

The polarization of the fragments from binary fission or of scattering or fusioning heavy nuclei is investigated in the liquid drop model. Due to the mutual Coulomb repulsion near contact the fragments may be polarized with respect to their charges (electric dipole moments from inhomogeneous charge distributions) as well as with respect to their shapes (quadrupole and octupole deformations). The lowering of the minimum energy near contact due to charge polarization is in the order of 1 MeV if one takes into account the energy from the giant dipole restoring force derived from the volume symmetry energy in addition to the liquid drop energies. The question whether one obtains prolate or oblate shapes is entirely due to the restriction in deformation space (fixed distance between centers-of mass or between the tips of the fragments).