Three-dimensional optical forces and transfer of orbital angular momentum from multiringed light beams to spherical microparticles

Experimental evidence of transfer of orbital angular momentum of multiringed beams to dielectric particles has been reported recently [e.g., J. Opt. B 4, S82 (2002); Phys. Rev. Lett. 91, 093602 (2003)]. Here we present a detailed theoretical examination of the forces involved in trapping and transferring orbital angular momentum to microparticles due to a multiringed light beam, particularly a Bessel beam. Our investigation gathers, in a more general way, the trapping forces for high-index and low-index dielectric transparent particles, as well as for reflective metallic particles, as a function of particle size and position relative to the dimensions of the rings of the beam. We find that particles can be trapped in different regions of the beam intensity profile according to their size and that an azimuthal force component opposite to the beam helicity may appear under certain circumstances, depending on the relative size and radial equilibrium position with respect to the beam for high-index spheres.