1.Department of Physics, King's College London, Strand, London WC2R 2LS, UK
2.London Centre for Nanotechnology, London, UK
Francisco J. Rodríguez-Fortuño (francisco.rodriguez_fortuno@kcl.ac.uk)
Published:31 August 2024,
Published Online:10 July 2024,
Received:06 November 2023,
Revised:14 March 2024,
Accepted:07 April 2024
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Vernon, A. J. et al. A decomposition of light's spin angular momentum density. Light: Science & Applications, 13, 1578-1589 (2024).
Vernon, A. J. et al. A decomposition of light's spin angular momentum density. Light: Science & Applications, 13, 1578-1589 (2024). DOI: 10.1038/s41377-024-01447-9.
Light carries intrinsic spin angular momentum (SAM) when the electric or magnetic field vector rotates over time. A familiar vector equation calculates the direction of light's SAM density using the right-hand rule with reference to the electric and magnetic polarisation ellipses. Using Maxwell's equations
this vector equation can be decomposed into a sum of two distinct terms
akin to the well-known Poynting vector decomposition into orbital and spin currents. We present the first general study of this spin decomposition
showing that the two terms
which we call canonical and Poynting spin
are chiral analogies to the canonical and spin momenta of light in its interaction with matter. Like canonical momentum
canonical spin is directly measurable. Both canonical and Poynting spin incorporate spatial variation of the electric and magnetic fields and are influenced by optical vortices. The decomposition allows us to show that a linearly polarised vortex beam
which has no total SAM
can nevertheless exert longitudinal chiral pressure due to equal and opposite canonical and Poynting spins.
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