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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The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.
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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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.
The authors declare no competing interests.
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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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
Light: Science & Applications (2022)
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