Measurements of trapping efficiency and stiffness in optical tweezers

We report an experimental study concerning the radial forces of an optical tweezers acting on spherical polystyrene particles diluted in water solution. The radius of the trapped beads varied between 0.5 and 7.5 μm, i.e., in an intermediate range between Rayleigh and geometric optics regime. As a force calibration method we used the viscous drag exerted by a fluid flow. A parametric study of the transverse trapping forces was made as a function of bead radius and laser power for two objective lenses having numerical apertures of 0.65 (40×) and 1.25 (100×). Measured forces were compared with numerical estimates based on generalized Lorenz–Mie theory reported in literature. Optical tweezers were also characterized in terms of the optical potential well by measuring the displacement of trapped particles experiencing a viscous drag at a fluid flow below the critical velocity. Following this procedure, trap stiffness was determined for several bead radii and at different numerical apertures of the objective lenses.