Spin angular momentum engineering within highly localized focal fields: from simple orientation to complex topologies

Optical skyrmions, as quasiparticles with non-trivial topological structures, have garnered significant attention in recent years. This paper proposes a method for customized spin angular momentum (SAM) distribution in highly localized focal fields, thereby enabling the generation of SAM skyrmion and bimeron topologies. The skyrmionic SAM textures can be flexibly controlled, such as polarity, vorticity, and helicity. In addition, the two-dimensional projection plane can be arbitrarily oriented within three-dimensional space. By utilizing time-reversal techniques, we obtain the required illumination fields of the 4 π -focusing system and subsequently evaluate the tightly focused field using vector Debye integral theory. Our results show that the SAM orientation within the focal field is controlled by the orientation of orthogonal dipole pairs. Using the radiation field of a multi-concentric array of orthogonal dipole pairs, the distribution of SAM orientation in the target plane can be tailored to generate SAM topological structures such as skyrmions and bimerons. Highly localized and tunable SAM engineering holds great potential for applications in optical manipulation, light–matter interactions, optical information processing, transmission, and storage.