Several Optical Micro Control Theory Research

Optical micromanipulation is devoted to the study of manipulating particles rang-ing in size from tens of nanometers to tens of micrometers by using optical force and torque, and involves the effect of optical force to the optical properties of micrometer-or nanometer-scale photonic crystal recently. Optical micromanipulation is based on the optical force and torque induced by laser, and its realization greatly profits from the development of laser technology. One of the most widely used optical manipula-tion techniques is the optical tweezer, which was introduced by Arthur Ashkin and his coworkers at AT&T Bell Laboratories. The simplest optical tweezer is to use the gra-dient force exerted by a strongly focused beam of light to trap and move small objects toward the focus. Optical micromanipulation is becoming mature recently, and there is an increasing interest in the manipulation of one or a large number of particles, optical binding of multi-particles, particle transportation and so on via structured light beams and multi-beams. Optical micromanipulation is widely used in colloid science, physics, biology, medicine and many other fields to manipulate particles and ev^n living cells. One can say that, optical manipulation is useful for any field involving small parti-cles. Optical micromanipulation has been developed into a rapidly expanding branch of experimental science. However, the theoretical development is lagging significantly behind. Indeed, the focused beams used in optical micromanipulation are difficult to model exactly in theory, and the precise description of the light path in experiment is also difficult.A new generation of optical micromanipulation is aimed at the simultaneous ma-nipulation of a large number of particles by using structured light field. It is a common view that structured light beams will be an important part in optical micromanipulation. This thesis is devoted to the study of optical micromanipulation and related problems in theory, and the exact description of structured beams and light path is the key point. We model the incident focused beam by using the highly accurate generalized vector Debye integral, and calculate the scattering field by using the Mie theory, and calculate the optical force by using the Maxwell stress tensor. This thesis is organized as follows.In chapter one we present the background introduction, which is made up of three parts:a sketch of the optical micromanipulation development; the classification of the optical micromanipulation which include optical trapping, optical binding and optical pulling; and the structure of this thesis.In chapter two we present the theoretical method used in this thesis, including the three dimensional multiple scattering theory; the optical force and torque exerted on spheres; and the description of several structured beams.In chapter three, we investigate the optical trapping of a Rayleigh particle by tightly focused Gaussian beam. In our calculation, the incident beam is described by the mixed dipole model, which proves efficient in modeling Gaussian beam near the focal region beyond the paraxial limit. Moreover, in order to guarantee the numerical accuracy, we use the Mie theory and the integral of the Maxwell stress tensor to calculate optical force. We then obtain approximate analytical expressions for the optical force, equi-librium position, and trap stiffness for a Rayleigh particle, which provide a convenient and reliable theory method for experiment.How to bind a large number of particles simultaneously is an important subject in optical binding, for a useful material is made up of massive particles. In chapter four, based on the exact theoretical description of the vector Bessel beams and the multiple scattering theory, we have investigated how one can tailor the interparticle interaction by overlaying different coatings. It is found that the Ag and low dielectric coating can increase the number of equilibrium positions, whereas the antireflection coating reduces it. It is clear that coatings play quite different roles in optical trapping and optical binding. Moreover, for the small core Bessel beams that we used, coatings have little effect on the maximum number of particles (Nmax) that can be bounded. While for a Bessel beam with wider core, the story is different. In the large core limit, Bessel beams resemble plane waves. Using Ag or low dielectric coatings can considerably enhance Nmax.In chapter five, we propose the concept of optical pulling force. For a beam with positive photon momentum incident on a particle, it may be anticipated that light will push on any object standing in its path by means of the scattering force. Can you image that, a particle can move oppositely against the propagation direction of a propagation invariant beam? In the absence of an intensity gradient, using a light beam to pull a particle backwards is counter-intuitive. Here, we show that it is possible to realize a backward scattering force that pulls a particle all the way towards the source without an equilibrium point. The optical pulling force we called here is totally the scattering force, which is different from the gradient force in optical trapping. We show explicitly that the necessary condition to realize a negative (pulling) optical force is the simul-taneous excitation of multipoles in the Mie particle, and if the projection of the total photon momentum along the propagation direction is small, an attractive optical force is possible. This possibility adds’pulling’as an additional degree of freedom to optical micromanipulation.In chapter six, we derived, from angular momentum conservation, that a beam with positive angular momentum can exert a negative torque on a transparent non-spherical particle. It is a consequence of retardation and closely related to the symmetry of the structure. Optical torque oscillates between positive and negative values due to re-tardation, and this is the reason that, in order to observe negative rotation, perforated structures are preferred. The negative torque will be obvious if the perforated structure has sufficiently high level of discrete rotational symmetries, even the absorptive par-ticles can have negative torque in this case. From this work, we can realize both the positive and negative rotation of a structure, which provide more flexibility and free degree to optical micromanipulation. Finally, we give a conclusion to this thesis.