Polarization-dependent, morphology-driven alignment and stability of microparticles via optical tweezers

Abstract

In this Letter, we observed the polarization-dependent axial alignment effect of microparticles induced by a highly focused laser beam. We theoretically and experimentally verified that the microparticles align due to morphology-dependent optical torque generated by the interaction between shape-dependent polarizability and the longitudinal field component near the focal region. We found that a linearly polarized beam plays a significant role in stabilizing the post-alignment microparticles, whereas circular and elliptical polarization beams cause the microparticles to rotate rather than remain stable. We further show that particle size and moment of inertia significantly influence the alignment behavior, where larger particles exhibit a slower response and enhanced stability due to inertia. The proposed mechanism enhances the understanding of polarization-dependent alignment and particle-size effects, enabling precise control of the orientation dynamics of shape-dependent microparticles in microfluidic and micromechanical applications.