分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： 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.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Chiral spin textures are researched widely in condensed matter systems and show potential for spintronics and storage applications. Along with extensive condensed-matter studies of chiral spin textures, photonic counterparts of these textures have been observed in various optical systems with broken inversion symmetry. Unfortunately, the resemblances are only phenomenological. This work proposes a theoretical framework based on the variational theorem to show that the formation of photonic chiral spin textures in an optical interface is derived from the system's symmetry and relativity. Analysis of the optical system's rotational symmetry indicates that conservation of the total angular momentum is preserved from the local variations of spin vectors. Specifically, although the integral spin momentum does not carry net energy, the local spin momentum distribution, which determines the local subluminal energy transport and minimization variation of the square of total angular momentum, results in the chiral twisting of the spin vectors. The findings here deepen the understanding of the symmetries, conservative laws and energy transportation in optical system, construct the comparability in the formation mechanisms and geometries of photonic and condensed-matter chiral spin textures, and suggest applications to optical manipulation and chiral photonics.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Electromagnetic spins, including longitudinal and transverse ones, have been playing important roles in light-matter interactions. Here, we formulate a unified equation to uncover the physical origins and topological properties of longitudinal and transverse spins in a generic electromagnetic field. The equation reveals universally that the transverse spin is locked with the kinetic momentum and originated from the transverse inhomogeneities of field, whereas the helix-dependent longitudinal spin orients parallel to the local wavevector. Remarkably, a hidden extraordinary helix-dependent transverse spin possessing helix-dependent spin-momentum locking is discovered and the number of locking states consistent with the nontrivial topological spin Chern number. Furthermore, this spin which determines the inverted helical components is related to the Berry curvature closely. The findings, which are demonstrated experimentally by measuring the three-dimensional spin components in the focusing configuration, will deepen the understanding the underlying physics of spins and open an avenue for chiral quantum optical applications.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Spin-momentum locking is an intrinsic property of surface electromagnetic fields and its study has led to the discovery of photonic spin lattices and diverse applications. Previously, dispersion was ignored in the spin-momentum locking, giving rise to abnormal phenomena contradictory to the physical realities. Here, we formulate four dispersive spin-momentum equations for surface waves, revealing universally that the transverse spin vector is locked with the momentum. The locking property obeys the right-hand rule in the dielectric but the left-hand rule in the dispersive metal/magnetic materials. In addition to the dispersion, the structural features can affect the spin-momentum locking significantly. Remarkably, an extraordinary longitudinal spin originating from the coupling polarization ellipticity is uncovered even for the purely polarized state. We further demonstrate the spin-momentum locking properties with diverse photonic topological lattices by engineering the rotating symmetry. The findings open up opportunities for designing robust nanodevices with practical importance in chiral quantum optics.
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