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Transverse spinning of unpolarized light

Abstract

It is well known that the spin angular momentum of light, and therefore that of photons, is directly related to their circular polarization. Naturally, for totally unpolarized light, polarization is undefined and the spin vanishes. However, for non-paraxial light, the recently discovered transverse spin component, orthogonal to the main propagation direction, is largely independent of the polarization state of the wave. Here, we demonstrate, both theoretically and experimentally, that this transverse spin survives even in non-paraxial fields (for example, focused or evanescent) generated from totally unpolarized paraxial light. This counterintuitive phenomenon is closely related to the fundamental difference between the meanings of ‘full depolarization’ for two-dimensional (2D) paraxial and 3D non-paraxial fields. Our results open an avenue for studies of spin-related phenomena and optical manipulation using unpolarized light.

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Fig. 1: Spin and polarization in paraxial and non-paraxial fields.
Fig. 2: Focused-beam experiment.
Fig. 3: Evanescent-wave experiment.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.

Code availability

The codes that support the calculations and plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

We acknowledge the help of U. Mick with the fabrication of samples for the focused-beam experiment. This work was partially supported by the European Research Council (Starting Grant ERC-2016-STG-714151-PSINFONI and iCOMM project no. 789340), EPSRC (UK), the Excellence Initiative of Aix Marseille University—A*MIDEX, a French ‘Investissements d’Avenir’ programme, NTT Research, the Army Research Office (ARO; grant no. W911NF-18-1-0358), Japan Science and Technology Agency (JST; via CREST grant no. JPMJCR1676), Japan Society for the Promotion of Science (JSPS; JSPS-RFBR grant no. 17-52-50023 and KAKENHI grant no. JP20H00134), the Foundational Questions Institute Fund (FQXi; grant no. FQXi-IAF19-06) and a donor advised fund of the Silicon Valley Community Foundation.

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Contributions

K.Y.B. conceived the idea for this research, made theoretical calculations with input from M.A.A. and prepared the manuscript with input from all the authors. Focused-beam experiment: P.B. and J.S.E. developed the idea of the experiment; J.S.E. performed the experiment; J.S.E. and P.B. performed the data processing; J.S.E. and P.B. wrote the corresponding part of the manuscript. Evanescent-wave experiment: F.J.R.-F., D.J.R., L.H.N. and A.V.Z. developed the idea of the experiment; D.J.R. and L.H.N. designed and performed the experiment; F.J.R.-F. performed theoretical modelling; D.J.R. and F.J.R.-F. performed data processing; D.J.R. fabricated the samples; F.J.R.-F., D.J.R., L.H.N. and A.V.Z. wrote the related part of the manuscript.

Corresponding authors

Correspondence to P. Banzer, A. V. Zayats or K. Y. Bliokh.

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The authors declare no competing interests.

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Peer review information Nature Photonics thanks Lorenzo Marrucci and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary text, Sections: (1) Theoretical calculations; (2) Details of the evanescent-wave experiment.

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Eismann, J.S., Nicholls, L.H., Roth, D.J. et al. Transverse spinning of unpolarized light. Nat. Photonics 15, 156–161 (2021). https://doi.org/10.1038/s41566-020-00733-3

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