Optical micro-manipulation using Laguerre-Gaussian beams

In this work we investigate the features of single-ringed Laguerre-Gaussian (LG) beams, often referred to as optical vortices, for laser trapping and micro-manipulation experiments that can not be performed using Gaussian beams. LG beams, exhibiting "doughnut"-like transversal intensity distributions and carrying orbital angular momentum (OAM) about their axis, greatly extend the capabilities of laser tweezers. LG beams can be obtained by converting the Gaussian beam generated by a common laser source, by means of phase-only diffractive optical elements (DOEs). We present a trapping system based on DOEs implemented on a liquid crystal display. Trapping of small dielectric high-index particles on the "doughnut" profile is demonstrated. Orbital angular momentum transfer to trapped particles, which are caused to rotate, is studied as a function of the doughnut radius. Moreover, low-index particles, that would be rejected by a conventional Gaussian beam, are trapped in the zero intensity region of the doughnut. Finally, trapping of low-index particles with multi-LG beams, obtained by means of DOEs, is achieved.