Investigations On Trapping Force Exerting To The Mie Particles And The Rayleigh Particles In A High Focused Laser Beam

With the development of Laser Biography, the optical tweezers technology based on mechanical effect of micro-beam laser becomes an effective tool for life science and nanometer technology in the research of micromanipulation. The dielectric particles, the size range from nm toμm, can be trapped at the focus point by the gradient force of the strong focused laser beam. Optical tweezers can be used to capture and manipulate active cells with non-contact and no invasion. The interaction force between particles and micro-beam laser is very complicated. In present, there are two models for calculation of the trapping force. One is Ray-Optics Model (RO Model), another is Electromagnetic Model.In this paper,by using the RO model, the optical trapping force exerting on the Mie particles is calculated quantitatively. For the Rayleigh particles, based on EM Model, the optical trapping force is calculated. According to the results of the calculation, the relation between the optical trapping force and the main parameters of the system, including focus spot radius, the relative index of refractive, the laser wave-length and so on, is discussed.For the Mie particles, if the Gaussian laser beam is used as the trapping light, the relative index of refraction must be greater than 1, and there is an optimum relative index. And in this optimum relative index condition, the trapping force will be the steadiest. But, when the doughnut laser beam is used as trapping light, relative index can be less than 1.The beam waist of focused laser beam has a virtual affection for the trapping force. The less the beam waist is, the steadier the trapping force for the particles. Furthermore, the wavelength of laser and the size of microscopic particles have important affections to trapping force respectively.The trapping force will become greater when the power of laser increase. But the particles will be damaged as the same times. Now, we can use the hollow laser beam to capture the particles with little power.For Rayleigh particles, the beam waist and the relative index have important effect for trapping force. The less the beam waist is, the greater the trapping force. The greater the relative index is, the greater the trapping force. This paper is consisted of six chapters. In chapter one, the researching history, researching background and the researching progress of optical tweezers are introduced. In chapter two, the principles of the laser trapping technique are analyzed. In chapter three, the trapping force of strong focused laser beam exerting to Mie particles and Rayleigh particles is calculated. The relations between the optical trapping force and the main parameters of the system are discussed. In chapter four, the trapping force of hollow laser beam is calculated. And a comparison with the trapping force of the Gaussian laser beam is given. In chapter five the method about calculation of trapping force and the basic system setup are introduced. In chapter six, the conclusion of this paper is given and the future of optical tweezers is imaged.