Optical trapping of a dielectric-covered metallic microsphere

Shubo Cheng, Shaohua Tao, Conghua Zhou and Liang Wu

In this paper we propose a ray-optics model of single-beam gradient trap for a dielectric-covered metallic microsphere and calculate the optical forces exerted by a single incident optical ray striking the microsphere. The axial trapping forces experienced by the copper and silver microspheres covered with polystyrene are analyzed, respectively. The results show that, unlike the case for a purely dielectric Mie particle, the trapping positions for a metallic microsphere covered with dielectric depend on the thickness of the dielectric shell. In the experiments copper particles covered with polymer are trapped in the bright region rather than in the dark region of a focused vortex beam and rotate owing to the orbital angular momentum of the beam. The experimental results are in good agreement with the theoretical analysis.

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