This tutorial provides an overview of the local description of polarization for nonparaxial light, for which all Cartesian components of the electric field are significant. The polarization of light at each point is characterized by a $3$ component vector in the case of full polarization or by a $3\times3$ polarization matrix for partial polarization. Standard concepts for paraxial polarization like the degree of polarization, the Stokes parameters and the Poincaré sphere then have generalizations for nonparaxial light that are either not unique or not trivial. This work aims to clarify some of these discrepancies, present some new concepts, and provide a framework that highlights the similarities and differences with the description for the paraxial regimes. Particular emphasis is placed on geometric interpretations.
The 3-dimensional coherence matrix is interpreted by emphasising its invariance with respect to spatial rotations. Under these transformations, it naturally decomposes into a real symmetric positive definite matrix, interpreted as the moment of inertia of the ensemble (and the corresponding ellipsoid), and a real axial vector, corresponding to the mean angular momentum of the ensemble. This vector and tensor are related by several inequalities, and the interpretation is compared to those in which unitary invariants of the coherence matrix are studied.
It is well known that spin angular momentum of light, and therefore that of photons, is directly related to their circular polarization. Naturally, for totally unpolarized light, polarization is undefined and the spin vanishes. However, for nonparaxial light, the recently discovered transverse spin component, orthogonal to the main propagation direction, is largely independent of the polarization state of the wave. Here we demonstrate, both theoretically and experimentally, that this transverse spin survives even in nonparaxial fields (e.g., tightly focused or evanescent) generated from a tota…
We report on an experimental test of the spin selection rule for two-photon transitions in atoms. In particular, we demonstrate that the $5S_{1/2}\to 6S_{1/2}$ transition rate in a rubidium gas follows a quadratic dependency on the helicity parameter linked to the polarization of the excitation light. For excitation via a single Gaussian beam or two counterpropagating beams in a hot vapor cell, the transition rate scales as the squared degree of linear polarization. The rate reaches zero when the light is circularly polarized. In contrast, when the excitation is realized via an evanescent fiel…
We construct optical beams in free space with robust skyrmionic structures in their polarization fields, both in the electric spin vector for near-circular fields and in the polarization direction for near-linear fields, and for both Bloch (spiral) and Néel (hedgehog) textures. These structures are made possible by the spin-orbit coupling of tightly-focused nonparaxial optics as applied to higher-order Full-Poincaré beams, as well as by standing-wave configurations comprising forwards- and backwards-propagating waves. Our constructions show near-uniform circular and linear polarizations, provi…
As the size of an optical vortex knot, imprinted in a coherent light beam, is decreased, nonparaxial effects alter the structure of the knotted optical singularity. For knot structures approaching the scale of wavelength, longitudinal polarization effects become non-negligible and the electric and magnetic fields differ, leading to intertwined knotted nodal structures in the transverse and longitudinal polarization components which we call a knot bundle of polarization singularities. We analyze their structure using polynomial beam approximations, and numerical diffraction theory. The analysis…
We present an approach to fully characterize the polarization of general vector light beams. When attempting to generalize the notion of Stokes parameters to nonparaxial light beams in momentum space, we find that the Jones function that determines the Stokes parameters through the Pauli matrices is defined over a natural coordinate system that is fixed by a constant unit vector, called the Stratton vector. We further show that the Pauli matrices represent the intrinsic degree of freedom of the polarization with respect to the natural coordinate system so that the Stratton vector acts as an ad…
Geometric phases are a universal concept that underpins numerous phenomena involving multi-component wave fields. These polarization-dependent phases are inherent in interference effects, spin-orbit interaction phenomena, and topological properties of vector wave fields. Geometric phases have been thoroughly studied in two-component fields, such as two-level quantum systems or paraxial optical waves. However, their description for fields with three or more components, such as generic nonparaxial optical fields routinely used in modern nano-optics, constitutes a nontrivial problem. Here we desc…
Reformulation of conventional beam definitions into their bidirectional versions and use of Hertz potentials make beam fields exact vector solutions to Maxwell's equations. This procedure is applied to higher-order elegant Laguerre-Gaussian beams of TM and TE polarization. Their vortex and anti-vortex co-axial compositions of equal and opposite topological charges are given in a closed analytic form. Polarization components of the composed beams are specified by their radial and azimuthal indices. The longitudinal components are common for beam compositions of both types, meanwhile their trans…
Recently, it was shown that vector beams can be utilized for fast kinematic sensing via measurements of their global polarization state [Optica 2(10), 864 (2015)]. The method relies on correlations between the spatial and polarization degrees of freedom of the illuminating field which result from its nonseparable mode structure. Here, we extend the method to the nonparaxial regime. We study experimentally and theoretically the far-field polarization state generated by the scattering of a dielectric microsphere in a tightly focused vector beam as a function of the particle position. Using polar…
Polarization-controlled optical backflow in paraxial electromagnetic beams
Faculty of Mathematics and Natural Sciences, College of Sciences, Institute of Physical Sciences, Cardinal Stefan Wyszynski University in Warsaw（华沙卡尔迪纳尔·斯特凡·维什廷斯基大学理学院物理科学研究所）
Donostia International Physics Center (DIPC)（多诺西亚国际物理中心）；IKERBASQUE, Basque Foundation for Science（伊克尔巴斯克，巴斯克科学基金会）；Centre of Excellence ENSEMBLE3 Sp. z o.o.（卓越中心 ENSEMB3 有限公司）
Evolution of Geometric Phase of light since 1956: A Catalog Review
Chiba University（千叶大学）；Molecular Chirality Research Centre, Chiba University（千叶大学分子手性研究中心）；Institute for Advanced Academic Research, Chiba University（千叶大学高等学术研究所）；Quantlight and High Harmonics Lab Pvt. Ltd.（Quantlight和高谐波实验室私人有限公司）
The Lorenz-Mie scattering of a wide class of focused electromagnetic fields off spherical particles is studied. The focused fields in question are constructed through complex focal displacements, leading to closed-form expressions that can exhibit several interesting physical properties, such as orbital and/or spin angular momentum, spatially-varying polarization, and a controllable degree of focusing. These fields constitute complete bases that can be considered as nonparaxial extensions of the standard Laguerre-Gauss beams and the recently proposed polynomials-of-Gaussians beams. Their analy…
Light carries spin and orbital angular momentum. These dynamical properties are determined by the polarization and spatial degrees of freedom of light. Modern nano-optics, photonics, and plasmonics, tend to explore subwavelength scales and additional degrees of freedom of structured, i.e., spatially-inhomogeneous, optical fields. In such fields, spin and orbital properties become strongly coupled with each other. We overview the fundamental origins and important applications of the main spin-orbit interaction phenomena in optics. These include: spin-Hall effects in inhomogeneous media and at o…
It is shown that the momentum density of free electromagnetic field splits into two parts. One has no contribution to the net momentum due to the transversality condition. The other yields all the momentum. The angular momentum that is associated with the former part is spin, and the angular momentum that is associated with the latter part is orbital angular momentum. Expressions for the spin and orbital angular momentum are given in terms of the electric vector in reciprocal space. The spin and orbital angular momentum defined this way are used to investigate the angular momentum of nonparaxi…
The subject of optomechanics involves interactions between optical and mechanical degrees of freedom, and is currently of great interest as an enabler of fundamental investigations in quantum mechanics, as well as a platform for ultrasensitive measurement devices. The majority of optomechanical configurations rely on the exchange of linear momentum between light and matter. We will begin this tutorial with a brief description of such systems. Subsequently, we will introduce optomechanical systems based on angular momentum exchange. In this context, optical fields carrying polarization and orbi…
Internal flows and energy circulation in light beams
I.I. Mechnikov National University（伊·伊·梅奇尼科夫国立大学）；Applied Optics Group, School of Physics, National University of Ireland, Galway（高威爱尔兰国立大学物理学院应用光学组）；Institute of Physics, National Academy of Science of Ukraine（乌克兰国家科学院物理研究所）
This PhD tutorial is concerned with a description of the two-dimensional generalized Lorenz-Mie theory (2D-GLMT), a well-established numerical method used to compute the interaction of light with arrays of cylindrical scatterers. This theory is based on the method of separation of variables and the application of an addition theorem for cylindrical functions. The purpose of this tutorial is to assemble the practical tools necessary to implement the 2D-GLMT method for the computation of scattering by passive scatterers or of resonances in optically active media. The first part contains a deriva…
