Trapping two types of particles using a focused partially coherent elegant Laguerre-Gaussian beam

The radiation forces on a Rayleigh dielectric sphere induced by a focused partially coherent elegant Laguerre-Gaussian (ELG) beam are investigated by using the Rayleigh scattering theory. It is found that a focused partially coherent ELG beam with suitable mode orders can be used to trap a Rayleigh particle whose refractive index is larger or smaller than that of the ambient by varying its initial spatial coherence width. Therefore, one can use one optical-trap system to trap two types of particles with different refractive indices.     

Efficient conversion of a radially polarized beam to a nearly Gaussian beam

We demonstrate an efficient method for transformation of a radially polarized Laguerre-Gaussian beam to a nearly Gaussian beam with much higher beam quality. The method is based on separation of the radially polarized mode into two degenerate modes and coherent addition of the modes after phase flattening. We transformed a high-power Nd:YAG radially polarized (0,1)(*) Laguerre-Gaussian beam with M(2)=2.52 and power of 30 W into a nearly Gaussian beam with M(2)=1.3. As a result, the brightness increased by a factor of approximately 2.5.     

拉盖尔-高斯光束的近场矢量结构特征

直接以麦克斯韦方程组的解表征拉盖尔-高斯光束。基于麦克斯韦方程组解的矢量角谱表述和电磁光束的矢量结构理论,利用一些数学技巧导出了拉盖尔-高斯光束的TE项和TM项在近场的解析表达式。利用所导出的公式,在近场描绘了拉盖尔-高斯光束的TE项、TM项及整个光束的光强分布。并对角度依赖分别为余弦和正弦关系的拉盖尔-高斯光束的矢量结构进行了比较,结果显示两者整个光束的光斑完全相似,唯一的区别是子瓣的空间方位不同且两者的内部矢量结构完全不相同。     

The effective torque on patchy spheres nearby a flat substrate induced by infinitely thin rods with adsorptive ends

The system studied here is the mixture of infinitely thin rods and a patchy sphere nearby a flat substrate. The adsorption between the rod ends and the patch can induce an effective torque on the sphere, which are studied by the Monte Carlo method. At the proper patch angle, the effective torque makes the patchy surface face away from the substrate, and the effective interaction between the bare surface and the substrate is attractive at the same time. The effective torque strengthens with the approach between the sphere and the substrate, while first strengthens and then weakens with the increase of the included angle between the axis of the patch and the normal of the substrate. The relationships between the effective torque and the adsorptive strength, the patch angle and the diameter of the sphere have also been studied. The effective torque discussed here may play a role in the biological lock-and-key mechanism and be used to adjust the two-dimensional self-assembly of the colloidal particles with the anisotropic surface.     

均匀球扭矩德拜级数分析(英文)

基于会聚光束所产生的扭矩来实现对小粒子的操纵已在物理学、生物学等领域得到了广泛的应用。为了分离出单个散射过程对扭矩的贡献,给出扭矩物理机理的解释,本文引入德拜级数分析了高斯波束对均匀球粒子所产生的扭矩。计算表明,当德拜项p从1取到一个足够大的值后,德拜级数计算结果与广义米氏理论结果吻合。文中重点分析了单阶p散射过程对横向扭矩的贡献,结果表明:当线极化光束入射时,p=1～5散射过程都可以产生横向扭矩,但扭矩的方向不同;当圆极化光束入射时,p=-1和0对应的扭矩远大于p=1～4对应的扭矩,且p=0过程产生与其他p过程相反方向的扭矩。     

Field Enhancement by a Metallic Sphere on Dielectric Substrates

We calculate the scattered local field around a small gold sphere on dielectric substrates which is illuminated by a collimated monochromatic beam. The numerical results show the field between the sphere and the substrate is much enhanced when the sphere is illuminated by a p-polarized beam. The enhanced area is very localized between the sphere and the substrate and the diameter of the enhanced area on the surface of the substrate is one fifth of the sphere diameter. The field increases as the gap distance decreases or as the refractive index of the substrate increases.This paper was originally presented at the 5th International Conference on NEAR FIELD OPTICS and RELATED TECHNOLOGIES (NFO-5), which was held on December 6–10, 1998 at Coganoi Bay Hotel, Shirahama, Japan, in cooperation with the Japan Society of Applied Physics and Mombusho Grant-in Aid for Scientific Research on Priority Area “Near-field Nano-optics” Project, sponsored by Japan Society for Promotion of Science.     

Conversion of high-order Laguerre-Gaussian beams into Bessel beams of increased, reduced or zeroth order by use of a helical axicon

We propose a new method for transformation of a Laguerre-Gaussian beam of azimuthal index l and radial index n = 0 (LG_l,0) into a vortex, diverging or nondiverging Bessel beam, which can have increased or decreased phase singularity order, or into a zeroth order Bessel beam, by means of a helical axicon. The Bessel beam divergence or nondivergence depends upon the waist position of the input Laguerre-Gaussian beam, regarding the plane where the helical axicon is situated.The expressions for the amplitude and the intensity distribution of the diffracted wave field, in the process of Fresnel diffraction, are deduced using the stationary phase method. The theoretical analysis for the vortex radius and the maximum propagation distance of the Bessel beams obtained is presented.     

Characteristics of gain spectrum of modulation instability in soft matter

We investigate the spatial modulation instability in colloidal suspension based on the nonlinear Schrödinger equation. By a standard linear stability analysis, we obtain the expression for instability gain and identified the relationship between the measurable instability characteristic parameters and the particle radius, the optical beam width. It is shown that the modulation growth rate, the maximum growth rate, cut-off frequency and the fastest growth frequency increase as the particle radius decreases and (or) the beam width increases if the optical intensity is constant. Moreover, the ultimate values of the cut-off frequency and the fastest growth frequency are proportional to the beam width, while are independent of the sphere radius. The results provide a new approach to measure the sphere radius and to control the characteristic parameters of modulation instability through the beam width.     

在大气湍流斜程传输中拉盖高斯光束的轨道角动量的研究

大气湍流引起大气折射率随机变化,导致空间不均匀性.拉盖高斯光束在大气湍流中传输时,空间不均匀性会使光子波函数改变,引起轨道角动量的变化.本文讨论了拉盖尔高斯光束在大气斜程传输时,湍流介质改变光子轨道角动量而形成不同的光子态.计算了螺旋谐波各分量所占光束总能量的权重,分析了拉盖高斯光束的轨道角动量的变化规律.     

基于拉盖尔-高斯光束的单光子捕获理论研究

基于TEM₁₀模拉盖尔-高斯光束,推导自由空间量子密钥分配中单光子捕获概率表达式。针对低轨卫星-地面站间激光链路进行单光子捕获分析。结果表明：采用可高度衰减激光脉冲的TEM₁₀模拉盖尔-高斯光束作为单光子源,单光子捕获采用前驱波参考脉冲设置时间窗口的方法,可使卫星上接收机以最大概率捕获光子。与基模高斯光束相比,采用TEM₁₀模拉盖尔-高斯光束的优点是,不会由于卫星运动而增加单光子捕获概率的损耗。     

Transverse mode control by manipulating gain distribution in a Yb:YAG ceramic thin disk

We demonstrated that transverse mode can be controlled by manipulating gain distribution in a Yb:YAG ceramic thin disk. Several elongated higher order transverse modes of a Hermite-Gaussian beam were observed when a straight-line-shaped gain region was created by using a diode laser output from an optical fiber. An orthogonally crossing pair of straight-line-shaped gain regions created with an additional optical fiber output resulted in the generation of a doughnut-shaped Laguerre-Gaussian beam. This implies that easy and quick switching of Hermite-Gaussian and Laguerre-Gaussian beams is possible without mechanical handling.     

均匀球扭矩德拜级数分析

基于会聚光束所产生的扭矩来实现对小粒子的操纵已在物理学、生物学等领域得到了广泛的应用。为了分离出单个散射过程对扭矩的贡献,给出扭矩物理机理的解释,本文引入德拜级数分析了高斯波束对均匀球粒子所产生的扭矩。计算表明,当德拜项P从1取到一个足够大的值后,德拜级数计算结果与广义米氏理论结果吻合。文中重点分析了单阶P散射过程对横向扭矩的贡献,结果表明：当线极化光束入射时,P=1～5散射过程都可以产生横向扭矩,但扭矩的方向不同;当圆极化光束入射时,P=一1和0对应的扭矩远大于P=1～4对应的扭矩,且P=0过程产生与其他P过程相反方向的扭矩。     

Laguerre-Gaussian supercontinuum

We show what is believed to be the first coherent white-light optical vortices generated from supercontinuum that have the azimuthally varying phase structure consistent with a monochromatic Laguerre-Gaussian beam. Two methods of Laguerre-Gaussian supercontinuum generation are discussed and contrasted. We use a computer-generated hologram to convert a Gaussian white-light supercontinuum source into Laguerre-Gaussian supercontinuum.     

In situ monitoring of laser cleaning by coupling a pulsed laser beam with a scanning electron microscope

Lasers have proved to be effective tools for material processing at the micron and nanometer scales. In particular, laser interaction with nanostructures offers the unique advantage of highly localized excitation and heating. In this study, a short-pulsed laser beam is coupled to a scanning electron microscope, without disturbing the microscopy function, in order to study in situ laser cleaning of individual submicron particles from a silicon substrate. The substrate conditions before and after particle removal were inspected by electron microscopy. The mechanisms of particle removal and the underlying dynamic coupling of the laser radiation associated with particle cleaning are investigated. PACS 42.62.Cf; 42.82.Gw; 81.65.Cf; 07.78.+s     

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.     

光线庞加莱球法构建的结构光场及其传输特性研究

结构光场在光信息传输、显微成像以及微粒俘获中有重要作用.本文基于光线庞加莱球法结合不同花瓣数的梅花曲线构建了一类结构光束.根据光线庞加莱球法可计算这类光束在束腰面上的光强与相位分布,以及光束内外焦散线的分布.这些焦散线的特征表明梅花形结构光束具有无衍射与自修复的特性.进一步采用角谱衍射法和光线追迹研究了这类光束在空间中的传输特性.当梅花花瓣数为0时,该光束退化为拉盖尔-高斯光束;当花瓣数为1时,内焦散线汇集到两点,使光束具有无衍射特性.通过光线庞加莱球法获得其光线在空间传播的轨迹,直观地展示了光束被遮挡后的自修复特性.此外,本文还展示了花瓣数为5的结构光束,其内焦散线为十角星结构,该光束同样具有自修复特性.通过修改梅花曲线参数或选择其他庞加莱球面曲线可以构造更加复杂的结构光场.     

纳米结构Si表面增强拉曼散射特性研究

为了实现低成本高灵敏度的表面增强拉曼散射效应,制备了一种基于硅表面纳米结构的表面增强拉曼散射效应（SERS）衬底。首先利用低能反应离子注入的方法对单晶硅进行表面处理,制作高陡值度的墙壁结构。然后采用电子束蒸发的方式在硅片表面蒸镀银膜,高密度的银纳米点阵列出现在侧壁表面,形成大量的热点。实验采用罗丹明6G（R6G）作为探针分子进行表征,发现获得最强拉曼信号的银膜厚度为40 nm,R6G的探测极限能达到10^-14 mol/L;同时分析衬底的重复性和稳定性,发现在614 cm^-1和1 650 cm^-1处的拉曼信号特征峰的相对标准偏差分别达到12.3%和14.3%,保存一个月的衬底测得的拉曼信号强度保持不变。本研究提供了一种操作简单、成本低的制备高灵敏度增强拉曼效应衬底的方法,制备的衬底具有高信号可重复性和高稳定性的优点。     

Higher-order complex source for elegant Laguerre-Gaussian waves

We introduce a higher-order complex source that generates elegant Laguerre-Gaussian waves with radial mode number n and angular mode number m. We derive the integral and differential representations for the elegant Laguerre-Gaussian wave that in the appropriate limit yields the corresponding elegant Laguerre-Gaussian beam. From the spectral representation of the elegant Lauguerre-Gaussian wave we determine the first three orders of nonparaxial corrections for the corresponding paraxial elegant Laguerre-Gaussian beam.     

Symmetry and the generation and measurement of optical torque

A key element in the generation of optical torque in optical traps, which occurs when electromagnetic angular momentum is transferred from the trapping beam to the trapped particle by scattering, is the symmetries of the scattering particle and the trapping beam. We discuss the effect of such symmetries on the generation and measurement of optical torque in optical tweezers, and some consequent general principles for the design of optically driven micromachines.     

均匀椭球粒子对拉盖尔-高斯光束的散射特性研究

基于广义Mie理论, 研究了椭球粒子对在轴入射的拉盖尔-高斯光束的散射特性. 通过局域近似法求解椭球坐标系中的波束因子, 计算得到了波束因子之间满足的普遍关系. 对散射强度随椭球粒子不同尺寸参数和扁圆程度的变化特性进行了数值计算, 并针对不同拓扑荷时的散射强度进行了对比分析. 结果表明: 当椭球粒子尺寸在与入射光波长可比拟的范围内变化时, 散射强度随尺寸参数的增大而增大, 随椭球长短轴之比和拓扑荷的增大而减小. 本文的理论研究能够为拉盖尔-高斯光束在粒径测量、大气激光通信、 大气遥感等领域的应用提供更准确的粒子模型和参考价值. 关键词： 椭球粒子 拉盖尔-高斯光束 波束因子 散射强度