Characterization of optical forces and torques in polystyrene spheres steered by tightly focused orbital angular momentum beams

Abstract

With orbital angular momentum, tightly focused vortex beams can impose optical torque on particles through momentum transfer. In this paper, based on the Richards-Wolf vector diffraction theory, we examine the forces and moments exerted on Mie particles by a tightly focused beam, including the impact of the beam's intensity distribution and the topological charge (TC) number on particle trapping stability. Numerical calculations are conducted to assess the effects of corresponding parameters on the characteristics of the optical radiation force and the tight focusing properties. Our resultsshow that the direction of the particle's rotation is determined by the sign of the TC numbers and that there is a direct relationship between the magnitude of the induced torque and the TCs. Furthermore, the stability of particle control can be improved by OAM beams with small TCs. These results have potential applications in light capture, optical storage, and optical imaging.