Optical forces on small particles: attractive and repulsive nature and plasmon-resonance conditions.

A detailed study of time-averaged electromagnetic forces on subwavelength-sized particles is presented. An analytical decomposition of the force into gradient and scattering-plus-absorption components is carried out, on the basis of which the attractive or repulsive behavior of the force is explained. Small metallic particles are shown to experience both kinds of forces; which kind also depends on the excitation of surface plasmons. Resonances give rise to enhancements of both the scattering and the absorption forces, but the gradient force can become negligible. Also, close to resonant wavelengths, the gradient force can be maximum, while both the scattering and the absorption forces remain large. Comparisons of analytic results with rigorous calculations allow the establishment of ranges of validity of the dipolar approximation for these forces.