结构光场中的横自旋和横动量
Transverse spin and transverse momentum in structured optical fields
摘要：
最近人们认识到，除了传统的纵向角动量，结构（非均匀）光场表现出螺旋度无关的横向自旋角动量（SAM）和一个不寻常的自旋（圆偏振）相关的横向动量，所谓的贝林凡特的自旋动量。结构化光场中的动量和自旋密度的这种高度非平凡结构（例如，渐逝场）已经导致了许多基本上令人感兴趣的和复杂的现象，例如，光的量子自旋霍尔效应和表面光学模式中的光学自旋动量锁定，类似于在拓扑绝缘体中观察到的电子。本文介绍了结构光的基本概念，并对结构光中横自旋角动量和横自旋角动量的成因进行了探讨。然后，我们讨论一些说明性的例子，这些虚幻的实体可以观察到的微米和纳米光学系统。所研究的系统包括平面和球形微纳结构。我们还研究了增强难以捉摸的非凡旋转的方法和手段。特别是，我们表明，色散管理导致避免交叉以及最近发现的相干完美吸收介导的完美吸收可以积极影响横向自旋和自旋动量的共振增强。模式混合和不同的微纳米光学系统的相邻TE和TM散射模式的干涉的作用与选定的例子说明。结果表明，增强的可能性，不仅是幅度，但也横向SAM和横向动量分量的空间范围，这开辟了有趣的途径，实验检测这些虚幻的基本实体，并可能提高随后的自旋为基础的光子应用。
英文摘要：
It has been recently recognized that in addition to the conventional longitudinal angular momentum, structured (inhomogeneous) optical fields exhibit helicity-independent transverse spin angular momentum (SAM) and an unusual spin (circular polarization)- dependent transverse momentum, the so-called Belinfante's spin momentum. Such highly nontrivial structure of the momentum and the spin densities in the structured optical fields (e.g., evanescent fields) has led to a number of fundamentally interesting and intricate phenomena, e.g., the quantum spin Hall effect of light and the optical spin-momentum locking in surface optical modes similar to that observed for electrons in topological insulators. In this review, we introduce the basic concepts and look into the genesis of transverse SAM and transverse spin momentum in structured light. We then discuss few illustrative examples of micro and nano optical systems where these illusive entities can be observed. The studied systems include planar and spherical micro and nano structures. We also investigate the ways and means of enhancing the elusive extraordinary spin. In particular, we show that dispersion management leading to avoided crossing along with perfect absorption mediated by recently discovered coherent perfect absorption can positively influence the resonant enhancement of the transverse spin and spin momentum. The role of mode mixing and interference of neighboring TE and TM scattering modes of diverse micro and nano optical systems are illustrated with the selected examples. The results demonstrate possibilities for the enhancement of not only the magnitudes but also the spatial extent of transverse SAM and the transverse momentum components, which opens up interesting avenues for experimental detection of these illusive fundamental entities and may enhance the ensuing spin-based photonic applications.
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