Parametrization of trapping forces on microbubbles in scanning optical tweezers

We present the results of experiments to parametrize the trapping forces on microbubbles held in scanning optical tweezers. We determine the dependence on microbubble and trap dimensions of the maximum axial trapping force, and the dependence on bubble size of the maximum transverse drag force for which the bubble remains trapped. We have also determined the spring constant of the optical trap in the radial direction, which is the first measurement of this important parameter for a low refractive index particle, or for any object in a time-averaged optical potential.