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Mechanical Evidence of the Orbital Angular Momentum to Energy Ratio of Vortex Beams

Christine E. M. Demore, Zhengyi Yang, Alexander Volovick, Sandy Cochran, Michael P. MacDonald, and Gabriel C. Spalding
Phys. Rev. Lett. 108, 194301 – Published 8 May 2012
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Abstract

We measure, in a single experiment, both the radiation pressure and the torque due to a wide variety of propagating acoustic vortex beams. The results validate, for the first time directly, the theoretically predicted ratio of the orbital angular momentum to linear momentum in a propagating beam. We experimentally determine this ratio using simultaneous measurements of both the levitation force and the torque on an acoustic absorber exerted by a broad range of helical ultrasonic beams produced by a 1000-element matrix transducer array. In general, beams with helical phase fronts have been shown to contain orbital angular momentum as the result of the azimuthal component of the Poynting vector around the propagation axis. Theory predicts that for both optical and acoustic helical beams the ratio of the angular momentum current of the beam to the power should be given by the ratio of the beam’s topological charge to its angular frequency. This direct experimental observation that the ratio of the torque to power does convincingly match the expected value (given by the topological charge to angular frequency ratio of the beam) is a fundamental result.

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  • Received 30 January 2012

DOI:https://doi.org/10.1103/PhysRevLett.108.194301

This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

© 2012 American Physical Society

Authors & Affiliations

Christine E. M. Demore1, Zhengyi Yang1, Alexander Volovick1,2, Sandy Cochran1, Michael P. MacDonald1, and Gabriel C. Spalding3

  • 1Institute for Medical Science and Technology, University of Dundee, DD2 1FD Dundee, United Kingdom
  • 2InSightec Ltd., Tirat Carmel 39120, Israel
  • 3Department of Physics, Illinois Wesleyan University, Bloomington, Illinois 61701, USA

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Issue

Vol. 108, Iss. 19 — 11 May 2012

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