A decomposition of light's spin angular momentum density

TL;DR

This paper introduces a novel decomposition of light's spin angular momentum density into canonical and Poynting spin components, revealing their roles in light-matter interactions and optical forces on chiral matter.

Contribution

It provides the first comprehensive analysis of light's spin decomposition, linking it to optical OAM and chiral interactions, expanding understanding of light's angular momentum.

Findings

Decomposition into canonical and Poynting spin terms.

Both terms are influenced by optical orbital angular momentum.

Linearly polarised vortex beams can exert chiral forces without spin.

Abstract

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. Both canonical and Poynting spin incorporate spatial variation of the electric and magnetic fields and are influenced by optical orbital angular momentum (OAM). The decomposition allows us to show that the OAM of a linearly polarised vortex beam can impart a first-order preferential force to chiral matter in the absence of spin.