Optical gradient force in the absence of light intensity gradient

Abstract

The working mechanism of optical tweezers is generally believed to be the gradient force arising from the light intensity inhomogeneity of an incident optical field. Here we demonstrate a somewhat counterintuitive phenomenon---optical trapping of chiral particles relying on the gradient force emerging in an optical field with homogeneous light intensity. This type of gradient force is shown to survive even in the Rayleigh limit, where the particle size is far smaller than the illuminating wavelength. By deriving a rigorous analytical expression for the optical force exerted on a chiral particle, the origin of such a gradient force is traced to the gradient of the magnetoelectric energy density, irrespective of the particle size. Besides enriching our understanding of the light-matter interaction, this gradient force is found to have magnitude comparable to the conventional intensity gradient-induced gradient force and to switch its direction for particles with opposite chiralities, rendering it applicable to particle chirality discrimination and identification.