Near-field optics plays an essential role in exploring the subwavelength optical characteristics of light-matter interactions.Due to its inhomogeneous nature,the spatial and polarization degrees of freedom in optical near fields can strongly couple with each other.This gives rise to various intriguing properties and phenomena of chiral optical near fields.The interaction between the chiral optical near fields and matter provides rich physical mechanisms for state-of-the-art applications,which enable both the efficient control of near-field characteristics using optical structures/devices and the flexible optical manipulation/detection of small particles.This review introduces the recent progress in the research of chiral optical near fields from the theoretical and applied aspects.In the past few years,the study of optical near fields has been extended from evanescent fields to arbitrary inhomogeneous structured fields,revealing the intriguing physics associated with the extraordinary spin and momentum properties and promoting the development of modern optics.The property of spin-momentum locking can be employed to realize near-field optical directional coupling,non-Hermitian exception points,and near-field directional sources,with broad applications in photonic integrated circuits and quantum information processing.The transfer of transverse momentum and angular momentum between the chiral optical near fields and particles can generate the abnormal lateral optical force and torque,providing new degrees of freedom for optical trapping,chirality sorting,and optomechanical device design.Analogous to the longitudinal spin,the transverse spin of light can also give rise to geometric phases due to the curvilinear trajectory of optical near fields or spatially varying anisotropy of materials,which paves the way for the on-chip multiplexed optical manipulation.In addition,the interference of near fields can generate polarization singularities with unique properties in the real space.The interplay among the chiral near-field optics,singular optics and topological optics inspires the study of the near-field polarization structures and topological configurations,such as polarization Skyrmions and Möbius strips.The nontrivial topological properties of these near-field structures and configurations offer robust mechanisms for manipulating light's polarization and phase beyond the conventional approaches.Last but not least,chiral optical near fields have found essential applications in chiral sensing and discrimination,such as in the circular dichroism spectroscopy.The efficiency of such techniques depends on the local optical chirality.Strongly enhanced optical chirality can be achieved in the optical near fields generated by artificial structures,including the superchiral light fields associated with localized EM resonances and the fields at circular polarization singularities,which can give rise to highly efficient and sensitive chiral detection and sensing.This area of study can provide new insights into near-field optical physics and inspirations for other optical research including nano-integrated optics,non-Hermitian optics,singular optics,topological photonics,quantum optics,etc.At present,the research of chiral optical near fields is still in the early stage,with many novel mechanisms and phenomena to be explored(such as universal spin-momentum dynamics,spatiotemporal spin-orbit interactions,etc.)and key practical problems to be solved(such as the high-precision fabrication of micro-/nano-structures and integration technologies of optical devices).The research and application prospects are vast.We hope this review can contribute to the exploration of novel chiral light-matter interactions in the near fields and foster research discoveries and innovations for the next-generation optical technologies.
chiralityoptical near fieldspin-orbit couplingtransverse spinpolarization singularity
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