On the natures of the spin and orbital parts of optical angular momentum

Barnett, S. M. , Allen, L., Cameron, R. P., Gilson, C. R. , Padgett, M. J. , Speirits, F. C. and Yao, A. M. (2016) On the natures of the spin and orbital parts of optical angular momentum. Journal of Optics, 18(6), 064004. (doi: 10.1088/2040-8978/18/6/064004)

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Abstract

The modern field of optical angular momentum began with the realisation by Allen et al in 1992 that, in addition to the spin associated with polarisation, light beams with helical phase fronts carry orbital angular momentum. There has been much confusion and debate, however, surrounding the intricacies of the field and, in particular, the separation of the angular momentum into its spin and orbital parts. Here we take the opportunity to state the current position as we understand it, which we present as six perspectives: (i) we start with a reprise of the 1992 paper in which it was pointed out that the Laguerre–Gaussian modes, familiar from laser physics, carry orbital angular momentum. (ii) The total angular momentum may be separated into spin and orbital parts, but neither alone is a true angular momentum. (iii) The spin and orbital parts, although not themselves true angular momenta, are distinct and physically meaningful, as has been demonstrated clearly in a range of experiments. (iv) The orbital part of the angular momentum in the direction of propagation of a beam is not simply the azimuthal component of the linear momentum. (v) The component of spin in the direction of propagation is not the helicity, although these are related quantities. (vi) Finally, the spin and orbital parts of the angular momentum correspond to distinct symmetries of the free electromagnetic field and hence are separately conserved quantities.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Padgett, Professor Miles and Gilson, Dr Claire and Speirits, Dr Fiona and Barnett, Professor Stephen and Cameron, Dr Robert and Yao, Dr Alison
Authors: Barnett, S. M., Allen, L., Cameron, R. P., Gilson, C. R., Padgett, M. J., Speirits, F. C., and Yao, A. M.
College/School:College of Science and Engineering > School of Mathematics and Statistics > Mathematics
College of Science and Engineering > School of Physics and Astronomy
Journal Name:Journal of Optics
Publisher:Institute of Physics Publishing Ltd.
ISSN:2040-8978
ISSN (Online):2040-8986
Published Online:21 April 2016
Copyright Holders:Copyright © 2016 IOP Publishing Ltd
First Published:First published in Journal of Optics 18(6):064004
Publisher Policy:Reproduced under a Creative Commons License

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
549871Challenges in Orbital Angular MomentumMiles PadgettEngineering & Physical Sciences Research Council (EPSRC)EP/I012451/1P&A - PHYSICS & ASTRONOMY
663801NOVEL MANIFESTATIONS OF OPTICAL ACTIVITYStephen BarnettEngineering & Physical Sciences Research Council (EPSRC)EP/M004694/1S&E P&A - PHYSICS & ASTRONOMY
6672319UK Quantum Technology Hub in Enhanced Quantum ImagingMiles PadgettEngineering & Physical Sciences Research Council (EPSRC)EP/M01326X/1S&E P&A - PHYSICS & ASTRONOMY