The force field of optical tweezers is commonly assumed to be conservative, neglecting the complex action of the scattering force. Using a novel method that extracts local forces from trajectories of an optically trapped particle, we measure the thre
e dimensional force field experienced by a Rayleigh particle with 10 nm spatial resolution and femtonewton precision in force. We find that the force field is nonconservative with the nonconservative component increasing radially away from the optical axis, in agreement with the Gaussian beam model of the optical trap. Together with thermal position fluctuations of the trapped particle, the presence of the nonconservative force can cause a complex flux of energy into the optical trap depending on the experimental conditions.
We demonstrate that the data presented in the manuscript by Y. Roichman et al. are not sufficient to show that the circulation of a trapped particle exists in a static optical trap.
