Observation of reversible optical lateral forces on Mie spheres

The inhomogeneity of spin angular momentum (SAM) of light generates optical forces by the transverse Belinfante spin momentum (BSM). This BSM-induced force, though has theoretically been predicted to reverse sign with the size and refractive index of the Mie particle, lacks experimental validation. Here, using a typical line-shaped beam that mitigates the influence of light gradient and spin-orbit interaction, we observe distinct inverse optical forces on Mie particles when their sizes are varied. In addition, BSM-induced forces on polystyrene and silica particles can exhibit either the same or opposite signs at certain particle sizes. The good agreement of experimental results with simulations in this work uncovers a generic reversible force effect on Mie particles, the exhibition of which can be expected in various types of optical forces, including optical gradient force, radiation pressure, pulling force, etc. This work enriches the physics of optical forces and has broad applications in optical sorting and microscale motors.