Spin/momentum properties of the paraxial optical beams

TL;DR

This paper explores the intrinsic spin-momentum relationships in paraxial optical beams, revealing complex spin components, topological textures, and the influence of singularities, with implications for optical focusing, imaging, and scattering.

Contribution

It provides a detailed analysis of spin-momentum relations in paraxial optics, including the discovery of transverse and longitudinal spin components and skyrmionlike spin textures.

Findings

Transverse spin arises from field inhomogeneities and vorticity.

Longitudinal spin relates to polarization ellipticity.

Skyrmionlike spin textures are maintained during propagation.

Abstract

Spin angular momentum, an elementary dynamical property of classical electromagnetic fields, plays an important role in spin-orbit and light-matter interactions, especially in near-field optics. The research on optical spins has led to the discovery of phenomena such as optical spin-momentum locking and photonic topological quasiparticles, as well as applications in high-precision detection and nanometrology. Here, we investigate spin-momentum relations in paraxial optical systems and show that the optical spin angular momentum contains transverse and longitudinal spin components simultaneously. The transverse spin originates from inhomogeneities of field and governed by the vorticity of the kinetic momentum density, whereas the longitudinal spin parallel to the local canonical momentum is proportional to the polarization ellipticity of light. Moreover, the skyrmionlike spin textures arise from the optical transverse spin can be observed in paraxial beams, and their topologies are maintained free from the influence of the Gouy phase during propagation. Interestingly, the optical singularities, including both phase and polarization singularities, can also affect the spin-momentum properties significantly. Our findings describe the intrinsic spin-momentum properties in paraxial optical systems and apply in the analysis of the properties of spin-momentum in optical focusing, imaging, and scattering systems.