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.

当电场或磁场矢量随时间旋转时，光携带固有自旋角动量 (SAM)。熟悉的矢量方程使用右手定则并参考电偏振和磁偏振椭圆来计算光的 SAM 密度的方向。使用麦克斯韦方程，该矢量方程可以分解为两个不同项的和，类似于众所周知的轨道流和自旋流的坡印廷矢量分解。我们提出了这种自旋分解的第一个一般性研究，表明这两个术语，我们称之为正则自旋和坡印廷自旋，是光与物质相互作用时的正则动量和自旋动量的手性类比。与正则动量一样，正则自旋也是可以直接测量的。正则自旋和坡印廷自旋都包含电场和磁场的空间变化，并受到光学涡旋的影响。分解使我们能够证明，没有总 SAM 的线偏振涡旋光束仍然可以由于相等且相反的正则自旋和坡印廷自旋而施加纵向手性压力。