Trapping of lowindex microparticles in an （lowindex微粒的捕获）.pdf

524 J. Opt. Soc. Am. B/Vol. 15, No. 2/February 1998 K. T. Gahagan and G. A. Swartzlander Trapping of low-index microparticles in an optical vortex K. T. Gahagan and G. A. Swartzlander, Jr. Department of Physics, Worcester Polytechnic Institute, Worcester, Massachusetts 01609-2280 Received May 6, 1997; revised manuscript received October 6, 1997 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 acetophe- none. Vortex traps are the only known static, single-beam configurations allowing three-dimensional trap- ping of such particles in the size range of 2– 50 m. 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. © 1998 Optical Society of America [S0740-3224(98)03802-8] OCIS codes: 350.3950, 080.0080. 1. INTRODUCTION momentum transfer from the vortex beam.23– 26 The an- 1 gular momentum of a vortex may also be used to control Gradient force optical traps, also known as optical twee- the motion of other vortices.27,28 We recently reported zers, have become a valuable tool for researchers in di-