Polarization and coherence of light in nanostructures

光的相干性在基础科学和技术中都扮演着重要的角色。光学相干性表现为电场分量之间的相关性。一个时空点场分量之间的相关性描述了光的偏振特性，偏振应该被视为一个统计学概念，它必须与时间和空间相干同等地处理，它与两个时空位置上场分量之间的相关性有关。光的偏振在许多光学现象中都是重要的，从传输、反射和散射到场景的偏振成像以及原子和分子跃迁的量子力学选择规则。直到最近，对光学偏振的研究一直局限于近轴电磁场。随着纳米光学和等离子体的出现，围绕着纳米结构的光场本质上是三维的，已经成为一个非常热门的研究领域。亚波长结构中的光场与自由空间中的光场的关键区别在于它们的3D性质。三维电磁场的相干理论是近几年来理论研究的重点之一。然而，尽管有令人兴奋的理论预测，如反常的短和长相干长度，偏振和相干热辐射，以及光的安德森局域化依赖于光场的矢量性质，但到目前为止，关于光在纳米结构中的统计性质的实验研究还很少。在拟议的项目中，我们将发展扫描探针光学偏振仪来表征三维光场的偏振特性，并将其应用于光学研究，特别是等离子体纳米结构。计划中的研究目标是：首次展示用亚波长空间分辨率映射光的全三维偏振特性的可能性，探索部分相干光如何激发等离子体纳米天线等简单孤立等离子体结构的模式，以及研究准表面上光场的相干特性。这些目标的实现将导致纳米光学领域的重大进展，并将产生影响很大的出版物。该项目的一些科学问题和里程碑是展示光的3D相干矩阵的首次测量，探索部分偏振紧聚焦光的相干特性的3D拓扑，以及研究亚表面上相干矩阵的空间分布以及是否存在局部高相干度的相干热点。该项目有可能在纳米光学领域开辟一个新的研究领域，并研究迄今尚未探索的光学领域。

The coherence properties of light play an important role in both fundamental science and technology. Optical coherence is manifested in correlations between electric field components. Correlations between field components at one space-time point describe the polarization properties of light and polarization should be viewed as a statistical concept that has to be handled on equal footing as temporal and spatial coherence, which are related to correlations between field components at two space-time positions. The polarization of light is important in a great variety of optical phenomena, ranging from transmission, reflection and scattering to polarimetric imaging of scenes and quantum-mechanical selection rules of atomic and molecular transitions. Until recently studies of optical polarization have been restricted to paraxial electromagnetic fields. With the emergence of nano-optics and plasmonics, nanostructures, around which optical fields are inherently three-dimensional (3D), have become a highly topical area of study. The critical difference between optical fields in sub-wavelength structures and in free-space is their 3D nature. The theory of coherence for 3D electromagnetic fields has been one of the key topics of theoretical research during the last few years. However, despite exciting theoretical predictions such as anomalously short and long coherence lengths, polarized and coherent thermal radiation, and the dependence of Anderson localization of light on the vectorial nature of optical fields, experimental studies on the statistical properties of light in nanostructures have been so far rare. In the proposed project we will develop scanning probe optical polarimetry to characterize the polarization properties of three-dimensional optical fields and apply it to study optical, in particular plasmonic nanostructures. The goals of the planned research are: to demonstrate the possibility to map the full 3D polarization properties of light with sub-wavelength spatial resolution for the first time, to explore how partially coherent light excites the modes of simple isolated plasmonic structures such as plasmonic nanoantennas, and to study the coherence properties of optical fields on metasurfaces. The accomplishment of these goals will lead to significant advances in the field of nano-optics and will result in high-impact publications. Some of the scientific questions and milestones of the project are to demonstrate the first measurement of the 3D coherence matrix of light, to explore the 3D topology of the coherence properties of partially polarized tightly focused light, and to study what is the spatial distribution of the coherence matrix on metasurfaces and whether there are coherence hot-spots where the degree of coherence is locally high. The project has the potential to open a new area of research in the field of nano-optics and to study so far unexplored aspects of optical fields.