Optical trapping by a highly focused laser beam has been extensively used for the manipulation of submicron-size particles and biological structures. Usually the gradient force will support a stable trapping while the scattering force will push the particles away and destroy the stable trapping. Metallic particles are generally considered difficult to trap due to strong scattering and absorption forces. As one class of spatially variant polarized beams, cylindrical vector beam (CV beam) is proven to have advantage for metallic particle trapping because the axial scattering force is identical zero.
In this work, CV beam is generated and applied to trap metallic nanoparticles while the expected stable trapping is not observed. Several possible reasons are examined and the argument that a curl scattering force is responsible for the trapping difficulty is proposed. The curl scattering force is usually neglected in previous research for linearly polarized beam trapping. Numerical simulation shows the curl scattering force can be significant when vectorial beams such as CV beams are used for trapping. After realizing the significance of the curl force, innovative methods to engineer the forces are proposed and investigated to find the fields distribution which could support three dimensional stable trapping.