光学自旋角动量的调控机理及研究进展(特邀)

Regulation Mechanisms and Recent Progress of Optical Spin Angular Momentum(Invited)

自旋角动量是基本粒子和场的一个基本的动力学物理量,它在光与物质相互作用中扮演着极其重要的角色。在光学研究中,光的自旋角动量与圆极化密切相关,通过研究光学自旋与物质或结构的相互作用产生了许多新颖有趣的光学现象和光学应用,并诞生了自旋光学这一新兴学科。过去的研究中,研究人员主要聚焦在与平均波矢方向平行的纵向光学自旋。近年来,科研人员通过研究限制场如聚焦波、导波和倏逝波等的自旋轨道耦合性质,发现了一类新型的光学自旋,这类自旋与平均波矢方向垂直,因此被称为光学横向自旋。横向自旋具有自旋动量绑定的性质,一经发现便受到研究人员的广泛关注。横向自旋的发现拓展了光学自旋轨道相互作用的内容,并在光学操纵、光学精密检测、手性量子光学和光学自旋拓扑态等领域具有广阔的应用前景。本文从理论、实验技术和应用3个方面详细介绍自旋光学的最新进展。自旋光学的理论概念和框架可为研究人员进一步开拓基于光学自旋在光学成像、光学探测、光通信和量子技术等领域的应用发挥巨大的作用,同时也可拓展到一般经典波场,比如流体波、声波和引力波等。

Significance To help humans explore and understand the world,researchers have been committed to exploring diverse techniques of optical field manipulation to accomplish a variety of applications since the inception of the field of optics,including imaging,detection,sensing,communications,and so on.With the rapid development of modern micro-nanofabrication techniques,there is increasing interest in manipulating multiple degrees of freedom of light flexibly.However,at the nanoscale,there are close couplings and interactions among classical degrees of freedom such as intensity,phase,and polarization,making it difficult to achieve flexible and independent control of these degrees of freedom.Whereas,momentum and angular momentum degrees of freedom of light,which are a fundamental dynamic physical quantity of elementary particles and class wave fields and play important roles in the light-matter interactions,offer extreme advantages in manipulating the light in the nanoscale.For example,through the spin-momentum equation,spin and orbit angular momentum can be individually controlled,allowing for more precise manipulation and utilization of the spin properties of photons individually.The numerous advantages of controlling the spin angular momentum of photons bring new opportunities for nanophotonics,particularly in the areas of optical manipulation,detection,information processing,chiral quantum optics,and quantum entanglement.Plenty of novel and interesting optical phenomena and applications have been proposed connecting to the interactions between optical spins and matters or nanostructures,and a new research field of spin optics has been born in recent years.Previously,most of the researchers mainly focused on the optical longitudinal spin parallel to the direction of the mean wave vector.In recent years,by studying the spin-orbit couplings of confined fields,such as focused fields,guided waves,and evanescent waves,researchers have discovered a new class of optical spins that are perpendicular to the direction of the mean wave vector,which are also known as optical transverse spins.Optical transverse spin possesses the properties of spin-momentum locking,so it has been widely studied by researchers since discovered.Moreover,the discovery of optical transverse spin expands the content of optical spin-orbit interactions,and it has potential in the applications of optical manipulation,ultrahigh-precision optical detection,chiral quantum optics,and optical spin topological states.Here,we introduce the recent progress of spin optics in detail from three aspects:theory,characterizations,and applications.These theoretical concepts and frameworks of spin optics can play a critical role in further developing applications based on optical spins in optical imaging,detection,communications,and quantum technology,and they can be flexibly expanded to other classical wave fields,such as fluid waves,sound waves,and gravitational waves.Progress In this paper,we provide a comprehensive overview and summary of the manipulating mechanisms of spin angular momentum and discuss the underlying relationship between the Abraham-Poynting momentum density,Minkowski canonical momentum density,Belinfante s spin momentum density,spin angular momentum density,and orbital angular momentum density in classical optical theory.Subsequently,starting from the longitudinal spin in the paraxial beams,we introduce the spin angular momentum in different optical fields,including transverse spin in evanescent fields and transverse spin in interference fields.Finally,to address the difficulty in simply defining transverse and longitudinal spins in structured light fields,we present a set of spin momentum equations,analogous to Maxwell s equations,to describe the dynamical properties of spin angular momentum density and momentum density.Furthermore,these spin-momentum equations extend the properties of optical spin-momentum locking from evanescent plane waves to general evanescent fields.We also comprehensively overview the measurement techniques for spin angular momentum in confined fields and free space,including scanning near-field optical microscopy,nano-particle-film structures,photoemission electron microscopy,and nonlinear optical effects.By utilizing these techniques,it is possible to effectively extract different electromagnetic field components to obtain the information of spin angular momentum carried by the optical field.The current application scenarios of spin angular momentum are also comprehensively summarized,including weak effect measurements,optical differentials,optical lateral forces,precision sensing,and magnetic domain detection.Conclusions and Prospects As a novel degree of freedom in the field of optics in addition to intensity,phase,and polarization,the spin angular momentum carried by the structured light can be applied in communication,imaging,precision detection,and other fields.In this paper,we introduce the concept,definition,classification,and physical origin of spin angular momentum and review the characterization methods of spin angular momentum developed in recent years,as well as its applications in weak effect detection,optical differentials,optical lateral forces,precision sensing,and magnetic domain detection.On the one hand,spin angular momentum is a fundamental dynamical physical quantity of basic particles such as photons and atoms,providing new perspectives for the interaction of small-scale light with matter.On the other hand,as a novel optical degree of freedom,spin angular momentum can provide new solutions for large-scale light field control,optical imaging,optical communication,and optical detection applications.In turn,it further serves to explore new mechanisms and phenomena in the interaction between light and matter,expanding the applications of spin photonics.