Measurement of optical trapping force and stiffness of micro-particles with the drag-force method

By using the hydrodynamic method,maximal transverse trapping forces on three kinds of micro-particles with radii ranging from 0.5 μm to 22 μm were measured by dragging these transparent dielectric beads suspended in water with the focused laser beam.The magnitude of trapping forces obtained from the experiment was of the order of pico-Newton.For PMMA particles of large size,the offset Δx of the particle from the optical trapping center was measured with different dragging velocities,and the trap stiffness was calculated according to the experimental result.In addition,at the edge of optical trap,where Δx is larger than the radius of the micro-particle,the optical trapping force was also measured to obtain the domain of trapping force and the relationship between the force and Δx.Comparison of measured trapping forces and stiffness on yeast cells with different radii under the same condition shows that the critical velocity decreases,the maximal trapping force rises and the stiffness reduces with increase of the radius of the yeast cell.