TRAPPING OF LOW-INDEX MICROPARTICLES IN AN OPTICAL VORTEX

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 ac etophenone. Vortex traps are the only known static, single-beam config urations allowing three-dimensional trapping of such particles in the size range of 2-50 mu m. The trap consists of a strongly focused Gauss ian laser beam containing a holographically produced optical vortex. U sing experimental and theoretical techniques, are also explored change s in the trapping efficiency owing to the vortex core size, the relati ve refractive index,and the numerical aperture of the focusing objecti ve. (C) 1998 Optical Society of America.