Revision of the forces exerted in the Rayleigh regime by a tightly focused optical vortex tweezer

Optical tweezers use light from a tightly focused laser beam to manipulate the motion of tiny particles. When the laser light is strongly focused its electromagnetic field is characterized by a longitudinal component which is of a magnitude comparable to the transverse ones and which has been ignored in the theoretical analysis of the tweezing forces. In our paper we revise the calculations of the various components of the radiation pressure force, within the limits of the Rayleigh regime or dipole approximation, in the case where a tiny particle interacts, in free space, with a circularly polarized optical vortex beam, by taking into account this ignored field term. We show that this term is responsible for considerable modifications in the magnitude of the various components, and, moreover, also for the appearance of terms involving the coupling of the spin angular momentum and the orbital angular momentum of the photons of the vortex beam. We compare our findings with the ones taken ignoring the longitudinal field component and the wave-vector magnitude reduction.