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

Stephen M Barnett, L Allen, Robert P Cameron, Claire R Gilson, Miles J Padgett, Fiona C Speirits, Alison M Yao

Research output: Contribution to journalArticle

46 Citations (Scopus)

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.
LanguageEnglish
Article number064004
Number of pages11
JournalJournal of Optics
Volume18
Issue number6
DOIs
Publication statusPublished - 21 Apr 2016

Fingerprint

Angular momentum
angular momentum
orbitals
propagation
confusion
Light polarization
Laser modes
light beams
Electromagnetic fields
Momentum
electromagnetic fields
Physics
momentum
physics
Lasers
symmetry
polarization

Keywords

  • optical angular momentum
  • spin parts
  • orbital parts

Cite this

Barnett, S. M., Allen, L., Cameron, R. P., Gilson, C. R., Padgett, M. J., Speirits, F. C., & Yao, A. M. (2016). On the natures of the spin and orbital parts of optical angular momentum. Journal of Optics, 18(6), [064004]. https://doi.org/10.1088/2040-8978/18/6/064004
Barnett, Stephen M ; Allen, L ; Cameron, Robert P ; Gilson, Claire R ; Padgett, Miles J ; Speirits, Fiona C ; Yao, Alison M. / On the natures of the spin and orbital parts of optical angular momentum. In: Journal of Optics. 2016 ; Vol. 18, No. 6.
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Barnett, SM, Allen, L, Cameron, RP, Gilson, CR, Padgett, MJ, Speirits, FC & Yao, AM 2016, 'On the natures of the spin and orbital parts of optical angular momentum' Journal of Optics, vol. 18, no. 6, 064004. https://doi.org/10.1088/2040-8978/18/6/064004

On the natures of the spin and orbital parts of optical angular momentum. / Barnett, Stephen M; Allen, L; Cameron, Robert P; Gilson, Claire R; Padgett, Miles J; Speirits, Fiona C; Yao, Alison M.

In: Journal of Optics, Vol. 18, No. 6, 064004, 21.04.2016.

Research output: Contribution to journalArticle

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AU - Cameron, Robert P

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AB - 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.

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Barnett SM, Allen L, Cameron RP, Gilson CR, Padgett MJ, Speirits FC et al. On the natures of the spin and orbital parts of optical angular momentum. Journal of Optics. 2016 Apr 21;18(6). 064004. https://doi.org/10.1088/2040-8978/18/6/064004