Measurement of the optical trapping force on micro-particles immersed in fluids

A strongly focused laser beam through an objective microscope with high NA allows the trapping of dielectric particles with micrometric sizes. The trapping force is proportional to the power of the laser, the relative refractive index (the ratio between the refractive index of the particle and the refractive index of the medium surrounding it) and the trapping geometry (shape of the laser beam, shape of the particle, transmission and reflection coefficients). Numerical models to evaluate the trapping force can be developed for simple geometrical shapes of the trapped particle. For particles with complicated shapes the trapping force should be measured experimentally. The goal of this paper is to evaluate a measurement method based on the equilibrium between the drag force in a fluid with knows viscosity and the transversal trapping force. A particle with a known size is first trapped in a cell filled with water. After stable trapping, the cell is shifted with controlled velocities using piezoelectric actuators. If the velocity exceeds a certain threshold, the particle escapes from the trap. This threshold allows to determine the trapping force. Experimental results obtained with high and low index particles are presented and discussed.