Selective optical trapping using optical beams with fractional helical phase

The Laguerre-Gaussian beams or higher-order Bessel beams, which possess a full-doughnut shaped intensity pattern at the center, repel the low index particles away from the dark central spot due to radiation pressure. Optical beams with fractional helical phase are possible solutions to this problem, as they possess a clear opening slit. By using a spatial light modulator, we demonstrate the control of an opening slit by embedding different fractional helical phase into Bessel beams and Gaussian beams. This dynamic opening and closing of the slit will allow a possible selective process of the micro-particles. This is also a new way to control the guiding of particles in the transverse plane instead of the propagation direction.