A study of zeros in optical fields and the origins of spin in wave fields

当多个平面波干涉时，干涉图中有可能存在点状节点。我们所说的点状节点指的是电场中的零点，限于所有三个维度，因此电场的强度在所有向外的方向上都会上升。就像在甜甜圈光束的中心，正则动量(由电场提供)在3D电场为零的情况下消失，产生涡流状的动量循环。传统的环形光束中的涡旋是二维的，并且可以通过横向场分量的位相缠绕来表征。3D零点在很大程度上没有被研究过--我们不知道3D零点是如何排列偏振奇点(C/L--连续的、应该以某种方式穿过零点的线)的，也没有正式的方法来描述存在的不同类型的3D零点--我们正试图为这些说法提供答案。在远场辐射中，零点可以赋予近场非衍射性，当电场或磁场矢量随时间旋转时，光携带本征自旋角动量(SAM)。利用麦克斯韦方程，这个矢量方程可以分解为两个不同项的和，类似于著名的Poynting矢量分解为轨道流和自旋流。我们给出了这种自旋分解的第一个一般性研究，表明这两个项，我们称为正则自旋和Poynting自旋，是光与物质相互作用中的正则动量和自旋动量的手征类比。正则自旋和Pointing自旋都考虑了电场和磁场的空间变化，并受光学轨道角动量(OAM)的影响。这种分解表明，在没有自旋的情况下，线偏振涡旋光束的OAM可以给予手性物质一阶优势力。

When lots of plane waves interfere, it is possible for point-like nodes to exist in the interference pattern. By point-like nodes we mean zeros in the electric field, confined in all three dimensions, so that the intensity of the field rises in all outward directions. Just like in the centre of a doughnut beam, the canonical momentum (provided by the electric field) vanishes in a 3D electric field zero, creating a vortex-like circulation of momentum. A traditional vortex in a doughnut beam is two dimensional and can be characterised by the winding of the phase of the transverse field components. 3D zeros are largely unstudied - it is not known how polarisation singularities (C/L-lines which are continuous and should pass through the zero in some way) are arranged by the 3D zero, and there is no formal way to characterise the different types of 3D zero that exist - we are trying to provide answers to these statements. In far-field radiation, zeros can impart non-diffractive properties into the nearby field.Light carries intrinsic spin angular momentum (SAM) when the electric or magnetic field vector rotates over time. 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.