The Study Of The Laser Beam And The Radiation Force On The Particles

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.