相反拓扑指数的Laguerre-Gaussian模的产生和模间相互作用的实验研究

实验研究了带有相反拓扑指数的Laguerre-Gaussian(LG)模间的相互作用，发现含有相反拓扑指数的两个LG模在传播过程中其光场发生变化，观察到随着传播距离的增加，两个LG模的中心位相奇点会逐渐连成为一个弧形暗线形状.实验上验证了带有相反拓扑指数的光学涡流中的螺旋位错在传播过程中会演变为边位错这一理论结果.     

光波场中多边位错向螺旋位错的转化

实验研究及理论分析了光波场中边位错向螺旋位错的转化行为. 设计了围绕中心沿角向周期排列的多边位错相位结构,实验上通过计算全息图法研究了其对高斯光束的调制作用以及调制光束的传播特性. 结果表明,随着调制光束逐渐演化为类涡旋光束,多边位错结构可以转化为螺旋位错,且位错结构的总相位变化守恒. 结合光场的线动量密度图详细分析了该演化过程,并进一步讨论了多边位错结构中的相位突变值对螺旋位错转化的影响,给出了决定螺旋位错拓扑荷值的定量关系式. 关键词： 边位错 螺旋位错 涡旋光束 拓扑荷     

BeAl_2O_4:Cr晶体的缺陷研究

用化学腐蚀、光学显微镜、X射线形貌术以及电子显微分析等方法研究了引上法生长的BeAl_2O_4:Cr晶体中的缺陷.其结果如下:1)获得了对低指数面(100)、(010)及(001)面位错的理想腐蚀剂,2)晶体中的位错分布不均匀,位错产生的来源有二:一是从晶种中,另一是由包裹物引起.大多数位错是刃型位错,其柏格氏矢量为〈100〉方向、滑移面为(010)面.3)在晶体中常出现的包裹物如气泡,管状物及第二相沉积物,它们是由于原料中的杂质及生长过程中的组分过冷引起的.     

光学总论

光学概论O432007032001相反拓扑指数的Laguerre-Gaussian模的产生和模间相互作用的实验研究=Experi mental investigation of interac-tion of two Laguerre-Gaussian modes carrying opposite to-pological charges[刊,中]/张晓波(中国科学技术大学物理系.安徽,合肥(230026)),张巍…//物理学报.?2007,56(1).?213-217实验研究了带有相反拓扑指数的Laguerre-Gaussian(LG)模间的相互作用,发现含有相反拓扑指数的两个LG模在传播过程中其光场发生变化,观察到随着传播距离的增加,两个LG模的中心位相奇点会逐渐连成为一个弧形暗线形状。实验…     

广义双曲正弦-高斯光束的Gyrator变换性质和暗空心光束产生

基于Gyrator变换,推导了广义双曲正弦-高斯光束场分布的解析表达式,研究了广义双曲正弦-高斯光束在Gyrator变换平面上的光强分布和相位特性.结果表明,在Gyrator变换过程中,具有边缘位错相位特性的双曲正弦-高斯光束能转换为具有涡旋的暗空心光束,并确定产生的暗空心光束的拓扑荷指数为一,而不具有边缘位错相位特性的双曲余弦-高斯光束则不可能出现空心结构.对影响变换场强度和相位分布的束结构参数及系统参数进行了分析讨论.     

拉盖尔-高斯模叠加而成的部分相干光的相干涡旋

对Bogatyryova等人近来提出的一种可分离位相的部分相干光的相干涡旋作了详细研究.这类新的光束由相同角向指数的拉盖尔-高斯(LG)模非相干叠加而成.研究表明，叠加LG模式的模指数，权重因子和参考点位置的选择都会影响圆刃型位错出现的位置，使得涡旋消失或出现多个相干涡旋.而且，LG模叠加而成的位相不可分离的部分相干光也存在相干涡旋. 关键词： 奇点光学 一类新的部分相干光束 相干涡旋 圆刃型位错     

界面形核时间对GaN薄膜晶体质量的影响

利用金属有机化学气相沉积技术系统研究了界面形核时间对c面蓝宝石衬底上外延生长GaN薄膜晶体质量的影响机理. 用原子力显微镜、扫描电子显微镜、高分辨X射线衍射仪以及光致发光光谱仪表征材料的晶体质量以及光学性质. 随着形核时间的延长, 退火后形成的形核岛密度减小、尺寸增大、均匀性变差, 使得形核岛合并过程中产生的界面数量先减小后增大, 导致GaN外延层的螺位错和刃位错密度先减小后增大, 这与室温光致发光光谱中得到的带边发光峰与黄带发光峰的比值先增大后降低一致. 研究结果表明, 外延生长过程中, 界面形核时间会对GaN薄膜中的位错演变施加巨大影响, 从而导致GaN外延层的晶体质量以及光学性质的差异.     

动力学淬火过程中的不动点及衍生拓扑现象

本文对近两年来有关淬火动力学过程中拓扑现象的研究做简要综述.这些动力学拓扑现象被动力学过程中的衍生拓扑不变量保护,与淬火前后体系的拓扑性质有密切关系.基于人工量子模拟平台的高度可控性,已在诸如超冷原子、超导量子比特、核磁共振、线性光学等众多物理体系中,通过对人工拓扑体系动力学过程的调控,观测到如动力学涡旋、动量-时间域的Hopf映射及环绕数、拓扑保护的自旋环结构、动力学量子相变、动量-时间斯格明子等诸多动力学拓扑现象.其中某些拓扑结构还可以在非幺正动力学淬火过程中稳定存在.这些研究将人们对拓扑物相的认识和研究从平衡态推广到非平衡动力学领域,具有重要的科学价值.     

二维光子拓扑绝缘体研究进展

受凝聚态拓扑绝缘体研究的启发,整数量子霍尔效应、量子自旋霍尔效应、拓扑半金属、高阶拓扑绝缘体等拓扑物理相继在光学系统中实现。光子系统因能带干净,样品设计简单且制作精度高等优势,逐渐成为研究物理拓扑模型和新型拓扑效应的重要平台。拓扑光子学提供了全新的调控光场和操控光子的方法,其拓扑保护的边界态可实现光子对材料杂质缺陷免疫的传播,这种传统光子系统不具备的理想的传输态有望驱动新型光学集成器件的变革。本文将从二维光学体系出发,简要介绍几种典型的光拓扑绝缘体的最新进展,例如光整数量子霍尔效应、光量子自旋霍尔效应、光Floquet拓扑绝缘体、拓扑安德森绝缘体和高阶拓扑绝缘体。文中重点介绍了上述几种光拓扑绝缘体的拓扑模型及其新型的拓扑现象,并在最后展望了新型光学拓扑效应及其在光学器件中的应用前景。     

刃型位错高斯光束的Riemann-Silberstein涡旋通过双焦透镜的聚焦特性

基于时间平均复标量场的零值点,推导出寄居于高斯光束中的刃型位错线形成的Riemann-Silberstein (RS)涡旋通过双焦透镜传输时的复标量场。详细研究了刃型位错高斯光束形成的RS涡旋通过双焦透镜的聚焦特性,分析了传输距离和双焦透镜在x方向的焦距对RS涡旋的影响。研究发现RS涡旋通过双焦透镜后会出现RS涡旋的移动、新产生一对含有相反拓扑电荷的RS涡旋、两个含有相反拓扑电荷的RS涡旋逐渐靠近至湮灭,但是,在整个聚焦传输变化过程中,RS涡旋的总拓扑电荷守恒。特别地,当RS涡旋通过理想透镜时,复标量场中始终只有4个位于x轴上的RS涡旋。随着传输距离增加,这4个RS涡旋先逐渐靠近原点(0, 0),又逐渐远离原点(0, 0),但每个RS涡旋的拓扑电荷一直保持不变,因此,总拓扑电荷守恒。     

Conversion of topological charge of optical vortices in a parametric frequency converter

It is shown that sum-frequency generation of light beams with vortices allows to construct vortices of different topological charges. An optical parametric amplifier enables to produce a vortex with opposite topological charge (to the reverse vortex). In this way, a frequency converter can be seen as a prototype of an optical processor which allows to produce simple arithmetical operations with topological charges of the interacting vortices.     

利用全息法在偶氮聚合物薄膜中记录涡旋光场

利用全息技术在偶氮聚合物薄膜中记录了拓扑荷数q=–1,1,2,4的涡旋光场,并将记录的原始叉形光栅与计算全息光栅进行对比,对不同拓扑荷数涡旋光的记录速率和偶氮材料的可重复擦写性能进行了测试;记录完成后,将复现涡旋光与高斯光束干涉,并与原始涡旋光和原始叉形光栅对比,分析了记录质量.实验结果表明:高阶涡旋光场的全息叉形光栅会在记录过程中发生劈裂,轻微劈裂的涡旋光束仍维持一个稳定的环状结构;全息记录过程中不同拓扑荷数的涡旋光束记录速率较为统一,偶氮材料可经过上百次的擦写而不出现疲劳;再现涡旋光与原始涡旋光在光强分布结构上保持高度一致,再现涡旋光的干涉条纹与原始涡旋全息光栅保持高度一致,涡旋光及其携带的拓扑荷信息可被有效记录和读取.     

Nonautonomous Vortices in (2+1)-Dimensional Graded-Index Waveguide

With the help of self-similarity transformation, we construct and study the nonautonomous vortices with different topological charges inside a planar graded-index nonlinear waveguide, analytically, and numerically. Although these vortices are approximate, they can reflect the real properties of self-similar optical beam during a short-term propagation. Existence of these autonomous vortices require delicate balances between the system parameters such as diffraction, nonlinearity, gain, and external potential. We are concerned with some special but interesting situations, and discussing the changes of the height, width, energy, and central position of the vortices as the increase of propagation distance. Moreover, we are also interested in the azimuthal modulational instability of the system, and comparing our prediction for the modulational instability growth rates to numerical results.     

Quasi-two-dimensional dark spatial solitons and generation of mixed phase dislocations

This paper presents experimental evidence that orthogonally crossed dark soliton stripes form quasi-two-dimensional spatial solitons with a soliton constant equal to that of singly charged optical vortices. Besides the pairs of oppositely charged optical vortex solitons, the snake instability of the dark formation at moderate saturation is found to lead to generation of steering mixed edge–screw phase dislocations with zero total topological charges. Received: 26 October 1998 / Revised version: 19 January 1999 / Published online: 12 May 1999     

Selective generation of high-order optical vortices from a single phase wedge

We demonstrate that it is possible to generate high-order optical vortices from a single phase wedge by applying an incident beam with an annular intensity distribution. Various topological charges of optical vortices are realized by a static phase wedge when the position and radius of the annular illumination are changed accordingly.     

Generation of Circular Optical Vortex Array

We report on a novel optical vortex array named circular optical vortex array, which is generated by the superposition of two concentric perfect optical vortices. The circular optical vortex array has a constant topological charge of +1 or ?1, the number and sign of which are determined by the topological charges of the two perfect optical vortices. Moreover, the radius of the circular optical vortex array is easily adjusted by using the cone angle of an axicon. Furthermore, the circular optical vortex array and multiple circular optical vortex array can be rotated by changing the initial phase difference of the perfect optical vortices on demand. This work demonstrates a complex structured optical field, which is of significance for applications such as optical tweezers, micro‐particle manipulation, and optical imaging.     

Dynamics of topological multipoles: II. Creation, annihilation, and evolution of nonparaxial optical vortices

The behavior of nonparaxial combined beams transferring two or four optical vortices with opposite topological charges (topological dipole or quadrupole) is studied. The paraxial and nonparaxial approaches are compared. It is shown that the behavior of a topologically neutral wave system is well characterized by the position of a representative point on the parametric plane. It is found that there exists a large region on this plane for which spatial trajectories of the optical vortices, both for dipoles and quadrupoles, do not intersect the focal plane, i.e., the vortices cannot exist within the forbidden zone. At the edges of the forbidden zone, the vortices are either created or annihilated. In contrast, there exists a region on the parametric plane for which the optical vortices can exist only near the waist plane.     

Doing arithmetic with nonlinear acoustic vortices

Phase singularities of wave-front-like screw dislocations or vortices possess a well-defined quantity that can only take integer value: the topological charge. In the nonlinear regime, it has been demonstrated that optical or acoustical vortices interact and the topological charge follows a conservation law. Here this facility is used in nonlinear parametric interaction of two vortices shifted in frequency to perform sums and subtractions of the topological charge. Thus, we experimentally demonstrate a new technique to perform wave computation in the group of integer Z. When the two vortices have commensurable frequencies, different combinations give the same frequencies but different tolopological charges may occur. We show that an energy criterion can be used to predict the outcome. A corollary is that a modulation of amplitude of the vortices switches from one result to the other.     

Tight focusing properties of linearly polarized Gaussian beam with a pair of vortices

The properties of a pair of vortices embedded in a Gaussian beam focused by a high numerical-aperture are studied on the basis of vector Debye integral. The vortices move and rotate in the vicinity of the focal plane for a pair of vortices with equal topological charges. For incident beam with a pair of vortices with opposite topological charges, the vortices move toward each other, annihilate and revive in the vicinity of focal plane.     

Superfluidlike motion of vortices in light condensates

We demonstrate, through numerical simulations, the generation of stable vortex lattices in light condensates. This can be achieved by propagating several concentric laser beams with nested vortices of different topological charges in an optical material with a cubic-quintic nonlinearity. We have considered several initial conditions, and in all the cases the net topological charges of the resulting lattice is equal to the topological charge of the initial outer vortex. The lattice exhibits rotation similar to vortex motion in superfluids. These vortex arrays could be used to implement all-optical photonic crystal fibers. Our results also apply to Bose-Einstein condensates in the presence of three-body elastic interactions.