Laser trapping and micro-manipulation using optical vortices

The aim of this work is to investigate the usefulness of the Laguerre-Gaussian (LG) beams, often referred to as optical vortices, for laser trapping and manipulation experiments that cannot be performed using Gaussian beams. Laguerre-Gaussian beams, exhibiting "doughnut"-like transversal intensity distributions and carrying orbital angular momentum (OAM), greatly extended the capabilities of laser tweezers. These beams can be obtained by converting the Gaussian beam generated by a common laser source, by means of properly designed diffractive optical elements (DOEs). We present two trapping systems, the first one based on amplitude DOEs, the second one based on phase DOEs. In both cases the DOE is implemented on a liquid crystal display. Trapping of small dielectric high-index particles on the "doughnut" profile is demonstrated. OAM transfer to trapped particles, that are caused to rotate, is observed as well. Moreover, low-index particles, that would be rejected by a conventional Gaussian beam, are trapped in the zero intensity region of the doughnut.