Trapping of low-index microparticles in an optical vortex

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Abstract
The equilibrium position of a low-index particle in an optical-vortex trap was experimentally measured for two different systems: a buoyant hollow glass sphere in water and a density-matched water droplet in acetophenone. Vortex traps are the only known static, single-beam configurations allowing three-dimensional trapping of such particles in the size range of 2–50 μm. The trap consists of a strongly focused Gaussian laser beam containing a holographically produced optical vortex. Using experimental and theoretical techniques, we also explored changes in the trapping efficiency owing to the vortex core size, the relative refractive index, and the numerical aperture of the focusing objective.