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

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 intricac...

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Bibliographic Details
Published in:Journal of optics (2010) 2016-06, Vol.18 (6), p.64004-64014
Main Authors: Barnett, Stephen M, Allen, L, Cameron, Robert P, Gilson, Claire R, Padgett, Miles J, Speirits, Fiona C, Yao, Alison M
Format: Article
Language:English
Subjects:
Angular momentum
Angular position
Confusion
Electromagnetic fields
Helical
Helicity
Laser beams
optical angular momentum
orbital angular momentum
Orbitals
spin angular momentum
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Summary: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.
ISSN:2040-8978
2040-8986
DOI:10.1088/2040-8978/18/6/064004