The study of optical trapping force from optical tweezers

Abstract

Optical tweezers use the forces exerted by a strongly focused beam of light to trap and move objects ranging in size from tens of nanometers to tens of micrometers. Since their introduction in 1986, the optical tweezers has become an important tool for research in the field of biology, physical chemistry and so on. So the study and measure of optical trapping forces are important currently, and those are also helpful for the development of optical tweezers. At present the theoretical model has been established about the calculation method of optical tweezers. This document shows a method of optical tweezers on axis. The optical trapping force was measured and estimated in experiment. The theoretical model of optical trapping force on axis is different with different size particle. One is Mie particle which size is much larger than the laser wavelength. We can calculate the optical trapping force by the theoretical model based on geometrical-optics. Another is Rayleigh particle which size is much smaller than the laser wavelength. We can calculate the optical trapping force by the theoretical model based on electromagnetics. A method was presented mainly about the calculation of optical trapping force on axis in this document. Under given parameters, the numerical simulation of the optical trapping force on particle was demonstrated. And the important impacts of the parameters were discussed including the radius of the beam waist, the laser wavelength, the laser power, particle radius and so on. Through the numerical simulation, they have the close relation between the different system parameters and the optical trapping force. The optical trap will change shallow along with the radius of the beam waist increasing. On the contrary, the optical trap will change deep along with the laser power increasing. They have a best optical trap scope between the laser wave length and the particle radius. Thus, we can choose the appropriate parameters in the experiment to obtain the best optical trapping force. We captured and moved a particle by a strongly focused beam of laser in experiment. In order to obtain the escaping speed, we control platform steadily and record the entire capture process through the CCD imaging system. Figuring out the escaping speed from the known two time interval and the granule displacement value, we can estimate the optical trapping force by Stocks formula.