### Abstract

Language | English |
---|---|

Article number | 064004 |

Number of pages | 11 |

Journal | Journal of Optics |

Volume | 18 |

Issue number | 6 |

DOIs | |

Publication status | Published - 21 Apr 2016 |

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### Keywords

- optical angular momentum
- spin parts
- orbital parts

### Cite this

*Journal of Optics*,

*18*(6), [064004]. https://doi.org/10.1088/2040-8978/18/6/064004

}

*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.

Research output: Contribution to journal › Article

TY - JOUR

T1 - On the natures of the spin and orbital parts of optical angular momentum

AU - Barnett, Stephen M

AU - Allen, L

AU - Cameron, Robert P

AU - Gilson, Claire R

AU - Padgett, Miles J

AU - Speirits, Fiona C

AU - Yao, Alison M

PY - 2016/4/21

Y1 - 2016/4/21

N2 - 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.

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.

KW - optical angular momentum

KW - spin parts

KW - orbital parts

UR - http://iopscience.iop.org/article/10.1088/2040-8978/18/6/064004/meta

U2 - 10.1088/2040-8978/18/6/064004

DO - 10.1088/2040-8978/18/6/064004

M3 - Article

VL - 18

JO - Journal of Optics

T2 - Journal of Optics

JF - Journal of Optics

SN - 0972-8821

IS - 6

M1 - 064004

ER -