Analysis For The Force Exerting To A Micro-particle In Femtosecond Laser Optical Field

Optical tweezers is based on the radiation pressure. However, its application was realized after the born of the laser. The optical tweezers make it possible to transit passive observation to active manipulation during investigation due to the precise orientation, unit selecting and untouched and scatheless manipulation to the sample. The force produced by optical tweezers is fit for research on biologic cell and sub-cell structure with high-precise force and displacement measurement. The application of optical tweezers is not only on the micromanipulation but on the mensuration on force and movement extent on the level of cell and molecule.The confinement of tradition optical tweezers to trapped particle appears. For example, particle in high-focused field may be damaged by optical heat because of heat assimilation, but the femtosecond laser force exerted on the trapped particle is pulsed. The particle is trapped by optical gradient force only during the pulse continuance phase, the area all around is nearly not hurt when femtosencond laser exerted on the biologic tissue. This is important for avoiding or reducing heat damage to trapped cell when trap and manipulate biologic cell at much low laser power.This paper is consisted of six chapters. In the first chapter, we introduced the background, development, and research progress of optical tweezers as well as the purpose and meaning. In the second chapter, we introduced the principle of optical tweezers, the planar optical trap and three-dimensional optical trap. In the third chapter, the character of laser, fundamental-mode Gaussian beam and high-order Gaussian beam are analyzed. In chapter four, the optical trapping forces exerting on Mie particles and Rayleigh particles are calculated, the results show the relationship between the optical trapping force and some parameters of instrument. We also simply introduce the T-matrix method for calculating optical trapping force. In the fourth chapter, we make a theoretical analysis on axial and lateral force in femtosecond laser and educe trap condition in which particle could be trapped in femtosecond laser. In the fifth chapter, we briefly introduce the analysis of force on particles that exposed in doughnut beam. In the sixth chapter , we give the conclusion of this paper of optical tweezers.