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。     

在轴高斯波束入射无限长多层圆柱散射的德拜级数解

不均匀柱形粒子的光散射特性研究对复杂结构粒子参量的反演具有重要意义。基于德拜理论,对在轴高斯波束垂直入射无限长多层圆柱的散射特性进行了讨论。获得了散射系数的德拜级数展开式;并利用该公式分别计算了均匀和双层圆柱的总散射强度角分布,德拜级数单阶散射强度角分布;总散射强度结果与广义米氏理论(GLMT)进行了比较,两者吻合很好。分析表明圆柱散射强度不同散射角区间的值来自德拜级数不同阶的散射强度贡献;双层圆柱各层半径和折射率的值对德拜级数二阶散射强度角分布中峰值的出现起决定作用。当双层圆柱的外层较薄时,在120°~150°之间会出现两个明显峰值,即出现双重一阶彩虹峰值;反之,当外层厚度大于内层时,只有一个峰值存在。     

基于一阶彩虹区域高斯光散射的液滴测量

为实现雾化过程中局域内单液滴的测量,采用德拜级数展开研究了高斯光照射下球形液滴一阶彩虹区域的散射光强分布,以及高斯光束腰大小对光强分布峰值角度的影响.根据德拜级数展开计算的散射光强分布反演液滴的折射率和粒径,证明了根据高斯光的彩虹散射反演液滴信息的可行性.基于广义洛伦兹-米氏理论计算一阶彩虹区域的总光强分布,根据总光强分布反演液滴折射率和粒径,讨论了高斯光束位置对反演液滴信息的影响.对于半径在200~1 000μm区间的液滴,高斯光束位于中心入射时,反演折射率的误差小于2.38×10-4,粒径的相对误差在-3.31%~3.31%之间.与采用平行光彩虹技术相比,采用高斯光束为入射光可以得到较高的光能聚集区,较好地定义测量区大小,既可以有效避免多个液滴同时出现在测量区的情况、减小颗粒之间复散射的影响,又可以提高信号强度.     

可调谐艾里光束及其传输特性

艾里光束是一种新型无衍射光束,解决了激光在传播过程中的衍射效应。基于艾里变换技术,介绍一种可以通过椭圆平顶高斯光束产生的新型艾里光束,即可调谐艾里光束。通过理论计算,可以看出调节椭圆平顶高斯光束的束宽比可获得单尾艾里光束,同时,可调谐艾里光束的尾长可以通过调节入射椭圆平顶高斯光束的阶数来控制。通过研究,当束宽比p=1时,艾里光束的加速方向沿x轴45方向;p=1/2时,加速方向沿x轴31方向,p=1/3时,加速方向为沿x正方向,可见调节束宽比p的大小可以改变艾里光束的加速方向。     

大气泡散射的几何物理模型数值计算

米氏模型和德拜级数展开模型在气泡尺寸达到μm量级时不能很好地解释海水中大尺寸空泡的后向散射现象，计算速度较慢且容易溢出。提出使用几何物理光学模型解释大尺寸空泡的后向散射现象，给出了一种基于几何物理光学模型的球形大气泡散射特性的数值计算方法，并与德拜级数展开式进行了比较，计算结果基本吻合。该方法在计算大尺寸空泡的后向散射时不受微粒尺寸参数和折射率的限制，且避免了复杂的级数形式，与基于米散射理论或德拜级数展开的算法相比，具有更快的运算速度。最后给出了一些计算实例。     

双环径向偏振涡旋光束经介质界面的深聚焦

基于矢量德拜理论,研究了双环径向偏振涡旋光束经介质分界面的深聚焦特性。当选取适当的入射光束拦截比时(即透镜孔径半径与入射光束的束腰半径的比),在聚焦场中可以得到一个极小的局域空心光束。局域空心光束的大小不仅与透镜的数值孔径有关,还与聚焦场介质的折射率有关。另外,还研究了透镜的数值孔径、入射光束的拦截比以及探测深度对聚焦光束实际焦点位置的影响。通过对聚焦光束的实际焦点位置的计算发现:当选取一定的光束拦截比时,聚焦光束存在一个焦点开关。     

基于Pancharatnam-Berry相位调控产生混合偏振矢量光束

提出了一种基于Pancharatnam-Berry(PB)相位调控产生混合偏振矢量光束的方法.按照光轴随空间坐标变化的规律,用相位延迟为π的PB相位元件对PB相位进行操控,获得局域偏振为线偏振的矢量光束,将此矢量光束入射到四分之一波片,产生混合偏振矢量光束.通过测定斯托克斯参数重构输出光场的偏振分布,实验结果表明:当以θ0=0的线偏振光入射时,输出光场包含庞加莱球上垂直于S3轴经线圆上所有偏振态的混合矢量光场;当以θ0=π/2的线偏振光入射时,输出光场包含庞加莱球上垂直于S1轴经线圆上所有偏振态的混合矢量光场.     

二氧化硅粉尘对有形激光的反射和透射特性研究

利用蒙特卡罗方法研究了二氧化硅粉尘层对无限窄准直光束的反射和透射特性,根据Mie理论计算了单次散射相函数,并利用其对光子散射方向进行了抽样,分析了消光系数和粒子尺寸对粉尘层反射和透射特性的影响,对于浓度较大的粉尘层,激光在传输过程中,其光斑的大小和形状将对散射特性产生影响,将无限窄准直光束入射情况下的散射函数作为脉冲响应,与有形激光光束进行卷积,得到有形激光束入射情况下的多次散射特性,研究了高斯形和圆盘形两种激光光束入射时,光斑形状和大小对散射特性的影响。     

具有轨道角动量光束入射下的单球粒子散射研究

根据广义Mie理论,研究了具有轨道角动量拉盖尔-高斯光束（LGB）的空间传输特性以及单粒子散射特性.在单体球粒子对高斯光束散射研究的基础上,分析了在自由空间不同传输距离LGB光束的光强分布情况,在不考虑散射和波束相移关系的情况下,将LGB作为入射高斯光束,通过对波束入射时的散射衰减截面求解得到波束因子,利用矢量球谐函数对入射高斯波束进行展开,从而研究了单球粒子在在轴条件下对具轨道角动量高斯波束入射的散射问题.通过数值计算,讨论了散射强度及角分布在不同波束宽度情况下对其散射特性的影响,并与平面波的情况做了对比.结果表明,当波束束腰半径较小时,束腰半径对衰减率的影响较大,而当束腰半径远大于粒子半径时,接近于平面波的情况. 关键词： 广义Mie理论 轨道角动量 Laguerre-Gauss光束 单球粒子     

超衍射极限相干反斯托克斯拉曼散射显微成像技术中空心光束的形成

空心光束的质量是超衍射极限相干反斯托克斯拉曼散射显微成像技术中决定成像质量的一个至关重要的因素. 本文基于菲涅耳衍射理论,分析了螺旋相位片法产生空心光束的物理机理,并且模拟了不同的入射条件对产生的空心光束的影响. 模拟结果表明:波长与相位片中心波长匹配且光强呈圆对称分布的高斯光垂直入射到相位片上,当高斯光束中心与相位片中心完全对准时,可获得较理想的空心光束;入射光光强分布的圆对称性以及入射光中心与相位片中心的对准程度都会影响产生的空心光束的强度分布;同时,高斯光束小角度倾斜入射时,空心光的强度分布仍呈圆对称,却在观察面发生一定的位移;此外,入射光中心波长偏离相位片中心波长不大时,对产生的空心光束的强度分布几乎没有影响. 上述分析结果对用于超衍射相干反斯托克斯拉曼散射显微成像技术中理想空心光束的获取具有重要的指导意义. 关键词： 空心光束 超衍射极限 相干反斯托克斯拉曼散射 螺旋相位片     

Gauss声束对离轴椭圆柱的声辐射力矩

利用部分波展开法求解得到了Gauss声束入射下刚性和非刚性椭圆柱的声散射系数,推导了一般情况下的声辐射力矩表达式.在此基础上,通过一系列数值仿真详细分析了离轴距离、入射角度和束腰半径对声辐射力矩的影响.结果表明:正向与负向声辐射力矩均可以在一定条件下存在;低频情况下刚性椭圆柱比非刚性椭圆柱更容易产生较强的声辐射力矩;特定频率的入射声场可以激发出非刚性椭圆柱不同阶的共振散射模式,因而非刚性椭圆柱的声辐射力矩峰值与频率的关系更密切;增加束腰半径有利于扩大散射截面,进而增加椭圆柱的声辐射力矩.该研究结果预期可以为利用声辐射力矩实现粒子的可控旋转和流体黏度的反演提供一定的理论指导.     

Analyses of radiation force and torque on a spherical particle near a substrate illuminated by a focused Laguerre-Gaussian beam

We present the analyses of radiation force and torque on a spherical particle illuminated by Laguerre-Gaussian beam. Since Laguerre-Gaussian beam has angular momentum, the particle can be rotated by illumination of Laguerre-Gaussian beam. Radiation forces and torques for a silver sphere and a silicon sphere near the silver substrate are analyzed. We found that the maximum torque on silicon sphere is about five times higher than the maximum torque on silver sphere and fine control of the focused spot is required to push the particle to the substrate.     

Bessel驻波对椭球形液滴的声辐射转矩

在实际的声操控中,由于声辐射力、表面张力和重力的共同作用,液滴往往呈现出椭球的形状,在螺旋声场中会受到力矩的作用而发生转动。从声波的散射理论出发,根据部分波展开法求解得到了椭球形液滴在Bessel驻波场中的声散射系数,并给出了其受到的声辐射转矩的解析式。在此基础上,对椭球形不可压缩液滴和椭球形可压缩液滴分别进行数值计算。仿真结果表明,不可压缩液滴的声辐射转矩与声束半锥角的关系更密切,而可压缩液滴则更依赖于特定的频率;提升Bessel驻波场的阶数有利于增强声辐射转矩的峰值,但在中低频处较难对可压缩液滴产生明显的力矩。该研究结果预期对利用螺旋声场进行液滴的操控具有理论指导作用。      

Acoustic rotational manipulation using orbital angular momentum transfer

We report on the first quantitative test of acoustic orbital angular momentum transfer to a sound absorbing object immersed in a viscous liquid. This is done by realizing an original experiment that is to spin a millimeter-size target disk using an ultrasonic vortex beam. We demonstrate the balance between the acoustic radiation torque calculated from the Brillouin stress tensor and the viscous torque evaluated from the steady state spinning frequency. Moreover, we unveil a rotational acoustic streaming phenomenon that results from the acoustic angular momentum transfer to the host fluid. We show that it lowers the viscous torque, thereby restoring the torque balance.     

Leed measurements of the surface debye temperature for the (110) nickel face

Measurements of the surface Debye temperature for the (110) nickel face were performed in such a manner that difficulties in the interpretation known from the literature seem to have been avoided. A specular beam could reach a fluorescent screen for incidence angles 30° < β < 70°. A strong (00) beam was observed for (β ≈ 60° and electron energy E = 100–110 eV. For this beam, the dependence of its intensity on the sample temperature was measured and the effective Debye temperature was calculated to be equal to not more than 216 ± 10 K. Owing to the large β, the penetration depth of electrons should be equal to about 1.2 Å and only the first layer of atoms is responsible for diffraction in this case. Besides, it is shown that single scattering processes are dominant in our experimental conditions. So, the effective Debye temperature measured should be very close to the surface Debye temperature.     

Debye Series of Scattering by a Multi-Layered Cylinder in an Off-Axis 2D Gaussian Beam

The Debye series of light scattering by an infinite multi-layered cylinder in an off-axis 21） Gaussian beam is studied. A simplified but rigorous iterative formula for scattering coefficients is presented. The numerical calculations of scattering intensity by a cylinder in on-axis and off-axis beams are developed. It is indicated that the results of Debye series reach an agreement with those of generalized Lorenz-Mie theory and the off-axis distances vary the results to a great extent. The Debye series components of a two-layered cylinder are further discussed. The relations between them with rainbow phenomena are analysed.     

Temperature dependence of specular and non-specular leed beams from Cu3 Au

The temperature dependence of intensities of the prominent diffraction peaks in the specular (00) and one non-specular ($\text{11}$) beam from the (100) surface of Cu₃Au have been measured in the range of 300 to 673 K. The effective Debye temperature associated with the specular beam appears to increase continuously with energy below 50 eV but varies discontinuously at high energies. For the ($\text{11}$) beam, which was available only in the higher energy range of 65 to 136 eV, the effective Debye temperature varies discontinuously with energy. Parallel and normal components of the Debye temperature were deduced from the two sets of data from which it appears that the two mean square displacements are approximately equal, compared to the harmonic approximation which indicates a difference of 30 percent. The log of peak intensity versus temperature (Debye plot) deviates from the straight line at 60° below the disordering temperature for all beams and all energies.     

Debye series expansion of shaped beam scattering by GI-POF

We derive Debye series expansion (DSE) for infinitely long multilayered cylinders normally incident by shaped beam. Typically the interaction between multilayered cylinders and Gaussian beam is derived in detail, and localized approximation is introduced to calculate the beam shaped coefficients. Finally DSE is employed to the study of rainbow scattering by graded-index polymer optical fiber (GI-POF).     

Relation between Debye series and generalized Lorentz--Mie theory of laser beam scattering by multilayer cylinder

With the recursive relations of Bessel function and applying inductive approach, the consistency between the scattering coefficient formula of Debye series and that of the generalized Lorentz-Mie theory (GLMT) in the scattering of a multilayer cylinder illuminated by a laser beam is proved in detail. As an application example, rainbow phenomenon in the scattering of a two-layer cylinder is discussed by the Debye series components. It can be concluded that the radius and the refractive index of a two-layer cylinder have an effect on the twin-primary rainbow, and the high-order Debye series components which are associated with the high-order rainbows are more complex than those of a homogeneous cylinder.