Trapping metallic particles under resonant wavelength with 4π tight focusing of radially polarized beam.

Here we propose a new method for trapping the resonant metallic particles with the 4π tight focusing (high numerical-aperture (NA)) system, which is illuminated by radial polarization light. Numerical simulations have indicated the maximum total optical force is 16.1pN while with nearly zero scattering force under axis trapping, which keeps the gradient force predominant. Furthermore, the distribution of total force is centrosymmetric and odd. We also gain stable 3D trap with an equilibrium point along z axis and r axis as in normal optical tweezers. What's more, we obtain the nearly pure longitudinal field. The maximum transverse intensity is only 2.3 × 10^-3 and the transverse spot size reaches 0.36λ, which is below Abbe's diffraction limit.