Helicity-gradient force on chiral multipoles induced by optical vortex

The gradient in the helicity of optical field can exert the discriminatory force on the enantiomer, and this force is a concept well established within the dipole approximation for a Rayleigh particle. Here, with the aid of the multipole moment expansion theory, the formulae of the multipolar helicity-gradient force on a Mie chiral particle are derived. Moreover, such force induced by a tightly focused optical vortex is numerically simulated. The obtained results illustrate that the multipolar helicity-gradient force is not proportional to the gradient of optical helicity due to the nonlocal effect. The interception, recoil, electric, and magnetic components in such force are remarkably different. Besides, this force shows apparent dependence on the particles radius, beam topological charge and radio of the pupil radius to the beam waist radius. These findings can help us to understand the interplay of multipoles and may be exploited in chirality sensing, optomechanics, and manipulation.