Particle Dynamics in Non-Conservative Optical Vortex Fields

Light forces on small (Rayleigh) particles are usually described as the sum of two terms: the dipolar or gradient force and the scattering or radiation pressure force. The scattering force is traditionally considered proportional to the Poynting vector, which gives the direction and magnitude of the momentum ∞ow. However, as we will show, when the light fleld has a non-uniform spatial distribution of spin angular momentum, an additional scattering force arises as a reaction of the particle against the rotation of the spin. This non-conservative force term is proportional to the curl of the spin angular momentum of the light fleld 1 . We will illustrate the relevance of the spin force in the particular simple case of a 2D fleld geometry arising in the intersection region of two standing waves 2 We will also discuss the peculiar particle dynamics in the non-conservative force fleld of an optical vortex lattice 3 . Radiation pressure in the whirllight fleld (arising in the intersection region of two crossed optical standing waves 2 ) plays an active role spinning the particles out of the whirls sites leading to a giant acceleration of free difiusion. Interestingly, we show that a simple combination of null-average conservative and nonconservative steady forces can rectify the ∞ow of damped particles. We propose a \deterministic ratchet" stemming from purely stationary forces 4 that represents a novel concept in dynamics with considerable potential for fundamental and practical implications.