Shape-Dependent Light-Induced Rotation of Non-Birefringent Microparticles by Linearly Polarized Light.

Abstract

A linearly polarized Gaussian beam (LPGB), carrying no angular momentum (AM), is not expected to drive particle rotation. Here we demonstrate that a tightly focused LPGB can induce stable rotation of irregular, non-birefringent microparticles through shape-dependent optical torque. Asymmetric transverse force components act on irregular geometries, producing a net torque once axial gradient and scattering forces form a stable trap. Notably, a single particle can rotate clockwise or counterclockwise in different trials under identical optical fields with orientation determined by its geometry shape and trapping orientation. Circularly polarized beams introduce spin-to-orbit conversion that can modulate rotation speed but not orientation, while vortex beams with an intrinsic orbital AM can dominate and enforce a fixed rotation direction. This work demonstrates that net torque and stable rotation can arise from shape-optical coupling without relying on AM-carrying beams, offering a new paradigm for low-cost optical microdevices and the functional manipulation of irregular particles.