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Proceedings Paper

Can integer and fractional optical vortices produce equally smooth optical manipulation? (Conference Presentation)
Author(s): Georgiy Tkachenko; Mingzhou Chen; Kishan Dholakia; Michael Mazilu

Paper Abstract

A photon can carry orbital angular momentum equal to an integer number of the reduced Planck’s constant. This principle expresses itself in geometrical quantization of optical vortex beams, which thus can propagate only in the form of fields having a helically wavefront characterized by an integer valued topological charge. However, one can create an optical vortex beam of an effective fractional charge by combining multiple integer vortices. Here, we investigate this apparent violation of the geometrical quantization of orbital angular momentum of light. Our approach relies on observation of the light-induced motion of a microscopic particle, which thus acts as an optomechanical probe for the optical vortex beam. A fractional topological charge corresponds to an abrupt jump in the helical phase front of the beam. This singularity expresses itself as an off-axis disturbance in the intensity profile, and thus complicates the optomechanical probing. We overcome this problem by distributing the disturbance along the vortex ring, so that a microparticle can continuously orbit due to the orbital angular momentum transfer. We demonstrate theoretically that whatever effort is put into smoothing the fractional vortex ring (as long as the net topological charge is fixed), the particle’s orbital motion cannot be as uniform as in the case of an integer vortex beam. We support this prediction by experimental proof. The experimental technique benefits from the recently introduced “perfect” vortex beams which allow an optically trapped particle to orbit along a constant trajectory irrespective of any topological charge.

Paper Details

Date Published: 10 November 2016
PDF: 1 pages
Proc. SPIE 9922, Optical Trapping and Optical Micromanipulation XIII, 99220Q (10 November 2016); doi: 10.1117/12.2239408
Show Author Affiliations
Georgiy Tkachenko, Univ. of St. Andrews (United Kingdom)
Mingzhou Chen, Univ. of St. Andrews (United Kingdom)
Kishan Dholakia, Univ. of St. Andrews (United Kingdom)
Michael Mazilu, Univ. of St. Andrews (United Kingdom)

Published in SPIE Proceedings Vol. 9922:
Optical Trapping and Optical Micromanipulation XIII
Kishan Dholakia; Gabriel C. Spalding, Editor(s)

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