Trapping microscopic particles with singular beams

It has been shown previously that it is possible to two- dimensionally trap a microscopic absorbing particle against a substrate using a focused doughnut beam. Beam angular momentum associated with the phase singularity is transferred to the particle, causing it to rotate. A detailed consideration of the optical forces acting on a particle shows the importance of wavefront curvature for stable trapping and lead to a quantitative description of the motion of the particle in single and multiple beam traps.