  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 three 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.
