Mechanical evidence of the orbital angular momentum to energy ratio of vortex beams

Demore, C. E.M., Yang, Z., Volovick, A., Cochran, S. , MacDonald, M. P. and Spalding, G. C. (2012) Mechanical evidence of the orbital angular momentum to energy ratio of vortex beams. Physical Review Letters, 108(19), 194301. (doi: 10.1103/PhysRevLett.108.194301)

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.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Spalding, Professor Gabriel and Cochran, Professor Sandy and Demore, Dr Christine
• Authors: Demore, C. E.M., Yang, Z., Volovick, A., Cochran, S., MacDonald, M. P., and Spalding, G. C.
• College/School: College of Science and Engineering > School of Engineering; College of Science and Engineering > School of Engineering > Systems Power and Energy
• Journal Name: Physical Review Letters
• Publisher: American Physical Society
• ISSN: 0031-9007
• ISSN (Online): 1079-7114
• Published Online: 08 May 2012
• Copyright Holders: Copyright © 2012 American Physical Society
• First Published: First published in Physical Review Letters 108(19): 194301
• Publisher Policy: Reproduced under a Creative Commons License