Research On The Characteristic Of The Force Exerted On A Micro-particle In The Optical Vortex

In 1986, Ashkin et al.[1] reported a technique for the optical trapping of microscopic particles which is commonly known as optical tweezers or single beam potential well. The ability of trapping microscopic particles make optical tweezers has wide applications in many fields, especially in biology and medicine. In conventional optical tweezers, the tightly focused TEM00 Gaussian beam will create an extremely high electric-field gradient near the focus. Any high-index particles falling within the laser beam will experience a force directed towards the focus of the beam. However, the conventional gradient-force trapping has some limitations. Trapped particles are susceptible to optical damage by absorptive heating because the center of the trap is located in the high-intensity focal region of the beam. Another is that it can not trap the low-index particles [2]. Compared with the conventional optical tweezers, the optical vortex will be more advantageous in trapping of microscopic particles. It can not only increase the trapping efficiency, but also can trap the low-index particles.This paper discusses the principle of optical tweezers, the numerical calculations of optical trap force and the instrumentation of single beam optical trap. By means of single beam tracing method, the axial force exerted on a micro-sphere particle in the Mie scattering field and induced by the TEM01* doughnut mode is theoretically investigated. The formula of the axial force is deduced and corresponding numerical simulation results is also given. The calculated results show that the optical vortex possesses two advantages in trapping the high-index micro-particles compared with that of the conventional optical tweezers. One is the axial force induced by the optical vortex is three times as great as that of the optical tweezers under the same power level. The other is there are two equilibrium positions in the optical vortex, which indicate optical vortex is more suitable in trapping particles. Conclusions are given accordingly. The stability of the potential well is enhanced with increasing the wavelength .With the decrease of the radius, the axial force will be reduced accordingly.The stability of the potential well will be strengthened with decreasing the beam radius. When n>1.45, the micro-particle can not be trapped in the TEM01* mode. So the trapping stability can be enhanced by selecting the corresponding parameters properly.This paper is consisted of five 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 axial force exerting to a micro-particle in the TEM01* doughnut mode is calculated by using the Ray-Optic Model. The relations between the optical trapping force and the main parameters of the system are discussed. Optical spanner and the basic system setup are introduced in chapter four and five, respectively.