Numerical Calculations For Optical Trap Force And Analyses For The Instrumentation Of Optical Tweezers

The optical tweezers based on radiation pressure of laser micro-beam is aneffective tool for biology and nanometer technology in the research ofmicromanipulation. The gradient force near the focus point that is caused by a strongfocused laser beam can capture the neutral dielectric particles the size range from nm toμm. Optical tweezers are non-contact and no invasion operating technique, which cancapture and manipulate active cells or microscopic objects. Optical tweezers areeffective tools in the research of life sciences and in biosynthesis. With the developmentof life sciences and nanometer technology, the optical tweezers technique will play animportant role in micromanipulation fields, and its applications will become morebroadly in the future.The capture ability of an optical tweezers is caused by optical gradient force. Theinteraction force between laser micro-beam and particles is very complicated. Theinteraction force in an optical tweezers is concerned with the laser wavelength, the laserpower, the distribution of intensity, the focus spot size, the relative refractive index ofparticles and so on. In present, two models for the calculations of optical trap force areused. One is Ray-Optics Model (RO Model) for the Mie particles, and the other one isElectromagnetic Model (EM Model) for the Rayleigh particles. In this paper, the opticaltrap force is calculated by an improved method of RO Model and EM Model.The principle of optical tweezers is discussed, and the optical trap force iscalculated. By means of single beam tracing method, the force on microscopic particlesis calculated under the approximation of geometrical optics for the Mie particles.Numerical results show that the relative refraction index has an important effect onoptical trap force. If the relative refraction index is greater than 1, the particles will becaptured. In the condition of the optimal relative refraction index, the largest optical trapforce will occur. The lesser the optical waist, the larger the optical trap force. Theabsorption of particles has a remarkable effect on the optical trap force, too. When theabsorption intense is great enough, the trap force will become so weak that can't tocapture particles. Besides the above effects, the laser wavelength and the size ofmicroscopic particles have some effects on optical trap force respectively, too. For theRayleigh particles the numerical calculation is based on EM Model. As the RO Modelfor Mie particles, the laser wavelength, the relative refraction index and the opticalwaist have effects on the optical trap force in EM Model, too. The construction of anoptical tweezers is considered and the selection of optical devices is analyzed primarily.This paper is consisted of five chapters. In chapter 1, the history, background andrecent research of optical tweezers are introduced. In chapter 2, the development andtheoretical principle of optical tweezers are analyzed briefly. In chapter 3, the numericalresult of optical trap force for the Mie particles and the Rayleigh particles are calculated;and the effects of parameters, include laser wavelength, waist size, relative refractionindex and so on, are discussed too. In chapter 4, the instrumentation of an opticaltweezers is considered. In chapter 5, some conclusions about optical trap force aregiven.