Vernon Alex J, Golat Sebastian, Rigouzzo Claire, Lim Eugene A, Rodríguez-Fortuño Francisco J
Department of Physics, King's College London, Strand, London, WC2R 2LS, UK.
London Centre for Nanotechnology, London, UK.
Light Sci Appl. 2024 Jul 10;13(1):160. 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.
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）。一个常见的矢量方程使用右手定则，参照电偏振椭圆和磁偏振椭圆来计算光的SAM密度方向。利用麦克斯韦方程组，这个矢量方程可以分解为两个不同项的和，这类似于将著名的坡印廷矢量分解为轨道电流和自旋电流。我们首次对这种自旋分解进行了全面研究，结果表明，我们称之为正则自旋和坡印廷自旋的这两个项，是光与物质相互作用时其正则动量和自旋动量的手性类似物。与正则动量一样，正则自旋是可以直接测量的。正则自旋和坡印廷自旋都包含电场和磁场的空间变化，并受光学涡旋的影响。这种分解使我们能够证明，一个没有总SAM的线偏振涡旋光束，由于正则自旋和坡印廷自旋大小相等、方向相反，仍然可以施加纵向手性压力。
