Three-dimensional optical confinement of micron-sized metal particles and the decoupling of the spin and orbital angular momentum within an optical spanner

We report a new mechanism for trapping metallic particles in inverted optical tweezers. The particles are loosely confined, in three dimensions, to an annular region just below the beam waist where gravity is counterbalanced by the scattering force. When using a Laguerre-Gaussian mode the trapping efficiency is improved and additionally we are able to observe an off-axis rotation of the particles around the beam circumference which is induced by the orbital angular momentum of the beam. Since there is no mechanism by which the spin angular momentum can contribute to the scattering force the spin and orbital angular momentum terms are decoupled and therefore the polarisation state does not influence the motion of the particles. (C) 2000 Elsevier Science B.V. All rights reserved.