Impacts of particle morphology and rotation on optical manipulation

It is well known that nonspherical particles rotate continuously in light beams, with their rotation directions determined by the light spin. Here, we reveal that particle morphology (e.g., size, shape, length, etc.) and rotation angle are vital in influencing light-matter interactions, thereby optical torques (OTs) and forces, especially in a paraxial or slightly defocused optical system. Positive and negative OTs emerge with distinct rotation angles in light fields. Their competition can also freeze the particle's rotation, leading to the stable lateral drift effect. These extraordinary OTs arise from the nonuniformly distributed optical force vectors in the paraxial trapping. Experimentally, we observe positive and negative OTs, and bilateral drift (zero OT) of various particles (e.g., long and short cylinders, triangles, irregular shapes, etc.) under the influence of inverse optical forces and torques. Our works delve into a fascinating domain where morphologies and rotation angles of particles play essential roles in optical manipulation, enriching the fundamental understanding of optical forces and torques.