【作者】 赵承良；

【导师】 陆璇辉；

【作者基本信息】 浙江大学 ， 光学， 2009， 博士

【摘要】 自从A. Ashkin在1970年用激光的辐射力捕获和控制微小粒子以后引起了越来越多人的关注,被操控的粒子的范围很广,包括中性原子、分子、微型介质小球和活细胞等等。众所周知,光具有能量和动量,因此当光子和粒子发生作用时光子的动量和能量会和粒子发生相互交换,从而会产生激光束对粒子的梯度力和散射力。因此对激光束对粒子辐射力的研究显得十分重要。近年来,新型的复杂激光光束被大量研究,如空心光束、椭圆光束、平顶光束等,这些新型复杂激光光束的出现促使人们迫切地去研究它的传输变换特性以及将这些激光束应用到捕获微粒的辐射力研究上来。本文在理论上研究了空心高斯光束和贝塞尔高斯光束这两种空心光束模型通过失调透镜系统和光阑系统的传输特性；用多模光纤获得了新的产生空心光束的方法并且通过改变入射激光束的相干度来提高光束质量,获得了多级衍射涡旋光束的干涉条纹；在理论上对高聚焦空心高斯光束、贝塞尔高斯光束和平顶光束与瑞利微粒作用时的辐射力进行了研究。在过去的几十年里,部分相干光被大量的研究,并且已经被广泛的应用在很多领域,如自由空间光通讯、激光材料表面热处理、激光扫描、惯性约束核聚变、非线性光学、光学成像等。本文首次对光的相干性对微粒的辐射力的影响进行了研究,用部分相干高斯-谢尔模(GSM)光束和部分相干平顶光束计算了对瑞利微粒的辐射力。同时,在光镊发明之后,在实验上采用的大部分激光源都是连续激光(CW),但是随着应用对象的多样化研究者开始把脉冲激光应用到光镊装置上来,用脉冲激光束对生物细胞动手术等实验操作,本文也首次对脉冲激光束对微粒的辐射力的影响进行了研究。全文的组织结构为：第一章,分别综述了光捕获的原理,激光传输理论的发展历史,空心光束的研究动态,激光操控微粒的研究背景以及光镊技术的发展概述,介绍了目前国内外的研究状况,指出了本论文研究的内容、目的和创新点。第二章,分别介绍了激光传输的基础理论知识,部分相干光的基础理论,光辐射力的相关理论基础。第三章,分别介绍了Cai和Lu引入的描写圆对称空心高斯光束和贝塞尔高斯光束这两种空心光束的理论模型。导出了空心高斯光束和贝塞尔高斯光束通过失调光学系统的传输变换公式。利用导出的公式,计算分析了空心高斯光束和贝塞尔高斯光束在通过失调光学系统中的传输性质。结果表明,空心高斯光束通过失调的聚焦光学系统后在焦点处变成了实心而不再保持空心形状,贝塞尔高斯光束通过失调系统之后还是能够继续保持其空心形状,因此证明空心高斯光束和贝塞尔高斯光束都是非常好的描述空心光束的理论模型。第四章,介绍了空心光束的各种产生方法。在理论上我们提出用圆对称空心高斯光束通过三角棱镜的方法获得椭圆空心高斯光束,这是一种非常便利和有效的椭圆光束的获得方法。同时对用计算全息片获得的不同衍射级的涡旋光束的拓扑荷进行了研究,得出涡旋光束的拓扑荷不仅仅和计算全息片的拓扑荷有关而且与衍射级次也有关系。我们提出一种用多模光纤来产生空心光束的新方法,实验结果表明,通过改变入射光束的失调角度和位移能够获得空心光束,这是一种非常简捷的空心光束产生方法。同时我们通过降低入射光束的相干性来获得更佳质量的空心光束,实验得到的空心光束的光强分布更加均匀,光束质量更好。我们的实验方法为实验产生空心光束提供了一条很好的途径,为光镊装置提供了一种非常有效的空心光束光源。第五章,简述了光辐射力的定义和不同的计算方法。主要是用激光束对瑞利微粒的辐射力进行研究。针对空心高斯光束能够在焦点处获得实心光束,在焦点附近获得空心光束的光束传输特性,我们将空心高斯光束应用到激光束捕获微粒的研究中去,理论计算结果表明,我们可以用空心高斯光束在聚焦的焦点处捕获那些折射率大于周围介质的微粒,在焦点附近处能够捕获那些折射率小于周围介质的微粒。实现了用一种光束分别捕获不同折射率微粒的方法。同时我们根据平顶光束的光束分布特性,对平顶光束捕获瑞利微粒进行了研究,结果表明,平顶光束也可以稳定的捕获微粒,并且可以通过增加平顶光束的阶数来增加稳定捕获区域,这个优点在实验上非常明显并且非常有用。还对贝塞尔高斯光束捕获瑞利微粒的辐射力也进行了研究,并且对捕获时的稳定性进行了分析。研究结果表明,可以用这两种空心光束模型来稳定捕获或引导微粒,特别是空心高斯光束更是能够捕获不同折射率的微粒,该优点在实验上具有非常大的实用意义。第六章,简要概述了部分相干光的理论和实验研究状况,介绍了部分相干光的研究意义。我们将部分相干光应用到光捕获微粒的研究上来,采用部分相干高斯谢尔模(GSM)光束和部分相干平顶光束作为光源对瑞利微粒捕获时的辐射力进行了分析,同时对由部分相干GSM光束和部分相干平顶光束捕获微粒时的稳定性进行研究。研究结果表明,部分相干GSM光束和部分相干平顶光束都可以稳定捕获瑞利微粒,但是随着相干性的降低,微粒受到的辐射力也会相应的降低,但是同时发现,随着相干性的降低,光束捕获微粒的捕获区域会相应的增加。因此我们可以通过根据实际情况,在辐射力足够大的前提下,通过适当的降低相干性来获得比较大的稳定捕获区域。该研究成果将会对部分相干光应用到具体的光镊实验中具有指导意义。第七章,简要回顾了脉冲光束的研究历史和发展概况,并着重介绍了脉冲光束应用到光镊装置中的研究工作。在理论上推导了脉冲光束对瑞利微粒辐射力的解析表达式,并分析了脉冲光束对瑞利微粒产生的辐射力。研究结果表明,不同持续时间的脉冲激光将会为瑞利微粒产生不同效果的辐射力,如果脉冲持续时间太大,将达不到稳定捕获瑞利微粒的效果；如果脉冲序列太小,则微粒受到的散射力远大于梯度力,因此微粒将会在激光束的作用下加速而无法实现稳定捕获。所以需要选择适当的脉冲序列以达到最佳的稳定捕获,该理论结果将会对实验上利用脉冲激光束捕获微粒具有重要的指导意义。第八章,综述了光镊的实验装置,介绍了光镊装置所需的各个光学器件和要求,着重介绍了我们采用的各器件的性能,成功搭建了单光束光镊实验装置,实现了单光束对玻璃微珠的稳定捕获,得到了一些实验结果。更多还原

【Abstract】 Since A. Ashkin first demonstrated how to capture and manipulate micron-sized particles using the radiation force pressure, optical traps (or tweezers) have attracted intensive attention in lots of literature because of their wide-range applications in manipulating a wide of particles, including neutral atoms, molecules, micron-sized dielectric particles, and living biological cells. As we know, light has both energy and momentum, and light radiation force is produced by the exchange of momentum and energy between photons and particles. There are two types of the radiation force: such as scattering force and gradient force. In the past several years, with the development of science and technology, different new type of complex laser beams such as hollow beam, elliptical beam, flat-topped beam etc. are pushed in many applications. The appearance of new complex laser beams made it urgent to study their propagation and transformation and the radiation force caused by these beams in order to meet the requirements in practical applications.The propagation properties of hollow Gaussian beams and high-order Bessel-Gaussian beams through lens misaligned and a hard-aperture misaligned optical system have been studied in theoretic. The dark-hollow beams have been generated by using the multimode fibers, and the high quality dark-hollow beams have been obtained through decreasing the coherence of the input beam. The phase structures of optical vortices by computer-generated holograms have been obtained. The radiation force on a dielectric particle produced by highly focused hollow Gaussian beams and high-order Bessel-Gaussian beams has been studied too.Over the past decades, partially coherent beams have been extensively investigated and have found wide applications in many fields, such as free-space optical communications, material thermal processing by laser beam, laser scanning, inertial confinement fusion by laser, nonlinear optics and imaging applications. This thesis is mainly devoted to studying the effect of spatial coherence on radiation forces. The radiation force of partially coherent Gaussian Shell-modes (GSM) and coherent flat-topped beams on a Rayleigh particle has been studied.Usually optical trapping or tweezers in many experiments are construed by using the CW laser. However, as the development of the pulsed laser, more and more pulsed laser has been applied in the optical tweezers. This thesis first studied the effect caused the duration of the pulsed laser.This thesis is organized as follows:In chapter 1, the principle of the optical trapping, the progresses in the field of laser propagation theory, the development of the hollow beams, the background of the laser manipulating particles and the progresses in the field of optical tweezers are reviewed. The main content and originality of this thesis are presented.In chapter 2, the basic theory of the laser propagation, partially coherent beams and the radiation force are introduced.In chapter 3, the hollow Gaussian beams and high-order Bessel-Gaussian beams have been introduced. A generalized formula of hollow Gaussian beams and high-order Bessel-Gaussian beams through the first-order misaligned ABCD optical system is derived by using the generalized diffraction integral formula. The propagation properties of the hollow Gaussian beams and high-order Bessel-Gaussian beams through the misaligned optical system have been studied by using the obtained formula. It is shown that the hollow Gaussian beams become the maximal axial intensity distribution at focal plane and the hollow beams near the focal plane. The high-order Bessel-Gaussian beams still hollow beams in different propagation distances. And these two beams become a decentered hollow beam.In chapter 4, the different methods for creating dark-hollow beams have been introduced. Based on the tensor method, an analytical formula for hollow Gaussian beams generated by a triangular prism has been derived, and the propagation properties have been studied. It is shown that hollow elliptical Gaussian beams can be obtained in the near field after the prism. The interference patterns between the diffraction beams of different orders by the computer-generated hologram and the reference beam have been studied. The results can show the properties of the vortices of the different diffraction order. We introduced a new method for generating the dark-hollow beams by a coupling of a single fundamental mode He-Ne laser beam with a misaligned multimode fiber in a special way. The dark-hollow beams can be obtained by changing the misaligned angle and displacement. The better quality hollow beams can be generated through decreasing the spatial coherence of the input beams. Our results can be used for manipulating particles. Further investigation on trapping atoms by partially coherent dark-hollow beams will be carried out.In chapter 5, the definition of the radiation force has been introduced. We mainly studied the radiation force caused by the laser. The radiation force on a dielectric sphere produced by highly focused hollow Gaussian beams in the Rayleigh scattering regime is theoretically investigated. Numerical results demonstrate that, the high focused hollow Gaussian beams at the focus of the lens system becomes a peak-centered shape which can be used to stably trap and manipulate the particles with the refractive index larger than the ambient, and in the neighborhood of the focus the beam becomes a doughnut shape which can be used to guide the particles with refractive index lower than the ambient. We can manipulate two refractive index particles by using one beam. The radiation force of highly focused Bessel-Gaussian beams and flat-topped beams on a dielectric sphere in the Rayleigh scattering regime is also introduced. It is found that we can increase the transverse trapping range at the planes near the focal plane by increasing the flatness (i.e., beam order) of the flat-topped beams. Our results are interesting and useful for particle trapping.In chapter 6, the theory and experiment of the partially coherent beams have been reviewed. We studied the radiation force caused by the highly focused partially coherent GSM and partially coherent flat-topped beams. The trapping stability also has been analyzed. The results show that, the partially coherent GSM and the partially coherent flat-topped beams can stably trap the Rayleigh particles. However, the radiation force would decrease as decreasing the spatial coherence. We found that, the trapping ranges can be increased at the focal plane by decreasing the initial coherence. So it is necessary to choose suitable initial coherence of a partially coherent GSM and partially coherent flat-topped beams in order to trap a particle.In chapter 7, the progress and development of the pulsed laser have been reviewed. We mainly introduced the pulsed laser applied in the trapping field. We investigate the dynamic evolution of the radiation forces produced by the pulsed Gaussian beams acting on a Rayleigh dielectric sphere. We derive the analytical expressions for the scattering force and all components of the ponderomotive force induced by the pulsed Gaussian beams. Our analysis shows that the radiation force, for both the transverse and longitudinal components, can be greatly enhanced as the pulse duration decreases. It is further found that for the pulse with long pulse duration, it can be used for the stable trapping and manipulating the particle, while for the pulse with short pulse duration it may be used for guiding and moving the small dielectric particle. Finally we discuss the stability conditions of the effective manipulating the particle by the pulsed beam. Therefore, the pulsed laser can be used for trapping and manipulating the particles.In the chapter 8, the experimental set-up of the optical tweezers has been reviewed. We introduced the apparatus and their requisition. The experimental set-up of the optical tweezers has been successfully established. The glass particles have been stably trapped by using the 1064nm laser, some important results are obtained.更多还原