Stiffness of optical traps: quantitative agreement between experiment and electromagnetic theory.

The first quantitative agreement between measured and calculated stiffnesses of optically trapped particles in the subwavelength regime is presented. It is shown for all three dimensions that the measured extent of harmonic optical trapping potentials for dielectric spheres comes very close to the theoretically predicted extent, provided all known instrumental parameters are considered. The recently predicted strong asymmetry of the trapping potential due to the electric field's linear polarization has been verified in all three directions. This effect vanishes for spheres with diameters d approximately lambda, which exhibit the strongest trap stiffnesses.