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