We present an efficient, discrete-dipole-approximation-based method for computing gradient and nongradient contributions to the optically induced force on neutral, polarizable particles in a field. We compare numerical data from this method with those generated using previously devised computational approaches for computing total forces. The agreement is generally adequate, and rounding error is the likely cause for differences among results obtained from the three methods. For both one- and two-sphere targets, nongradient forces generally make a nonnegligible contribution. For spheres, the gradient force often nearly cancels a component of the nongradient force, so that the radiation-pressure component is approximately equal to the net force. These results are contrary to the commonly assumed dominance of the gradient force for nanometer-sized particles.
© 2006 Optical Society of AmericaPDF Article