  Focused laser beams allow controlling mechanical motion of objects and can
serve as a tool for assembling complex micro and nano structures in space.
While in a vast majority of cases small particles experience attractive
gradient forces and repulsive radiation pressure, introduction of additional
degrees of freedom into optomechanical manipulation suggests approaching new
capabilities. Here we analyze optical forces acting on a high refractive index
silicon sphere in a focused Gaussian beam and reveal new regimes of particles
anti-trapping. Multipolar analysis allows separating an optical force into
interception and recoil components, which have a completely different physical
nature resulting in different mechanical actions. In particular, interplaying
interception radial forces and multipolar resonances within a particle can lead
to either trapping or anti-trapping scenarios, depending of the overall system
parameters. At the same time, the recoil force generates a significant
azimuthal component along with an angular-dependent radial force. Those
contribution enable enhancing either trapping or anti-trapping regimes and also
introduce bending reactions. These effects are linked to the far-field
multipole interference resulting and, specifically, to its asymmetric
scattering diagrams. The later approach is extremely useful, as it allows
assessing the nature of optomechanical motion by observing far-field patterns.
Multipolar engineering of optical forces, being quite general approach, is not
necessarily linked to simple spherical shapes and paves a way to new
possibilities in microfluidic applications, including sorting and micro
assembly of nontrivial volumetric geometries.
