Trapping nanoparticles with cylindrical polarization

Abstract

Metallic and magnetic nano-particles are generally considered difficult to trap due to strong scattering and absorption forces. In this paper, we propose new methods of trapping metallic and magnetic nano-particles using highly focused cylindrically polarized beams. It is shown that an optical tweezers using cylindrical polarization can stably trap metallic/magnetic particles in 3-dimension, owing to the unique focusing properties of cylindrically polarized beams. Metallic particles can be stably trapped by highly focused radial polarization. The extremely strong axial electrical field from a highly focused radially polarized beam provides a large gradient force. Meanwhile, this strong axial field component does not contribute to the Poynting vector along the optical axis. Consequently, it does not create axial scattering/absorption forces. Owing to such a spatial separation of the peaks of gradient force and scattering/absorption forces, a stable 3-D optical trap for metallic nano-particles can be formed. Similarly, an optical tweezers using azimuthal polarization may provide stable 3-D trapping of magnetic nano-particles due to an extremely strong axial magnetic field.