Multipole engineering of attractive-repulsive and bending optical forces

Abstract

Focused laser beams allow controlling the mechanical motion of objects and can serve as a tool for assembling micro and nanostructures in space. While small particles mainly experience attractive gradient forces and repulsive radiation pressure, introducing additional flexibility suggests approaching new capabilities. Herein, optical forces acting on a high refractive index sphere in a focused Gaussian beam are analyzed and new regimes are revealed. Multipolar analysis allows separating an optical force into interception and recoil components, resulting in different mechanical actions. In particular, interplaying interception radial forces and multipolar resonances within a particle can lead to either trapping or antitrapping, depending on the system parameters. At the same time, the recoil force generates a significant azimuthal component along with an angulardependent radial force. Those contributions enable enhancing either trapping or antitrapping and also introduce bending reactions. These effects are linked to the far-field multipole interference and, specifically, to asymmetric scattering patterns. The latter approach is extremely useful, as it allows assessing the nature of optomechanical motion by observing far-fields. Multipolar engineering of optical forces, being quite a general approach, is not necessarily linked to simple spherical shapes and paves a way to new possibilities in microfluidic applications, including sorting and microassembly