Topological state and number transitions of optical skyrmions upon free-space beam propagation

Skyrmions are typical topological structures of vectorial texture that have emerged in quantum fields, magnetic systems, liquid crystals, and other contexts, showing great potential for applications in spintronics, data storage, and quantum computing. As the optical counterparts of topologically protected quasiparticles, optical skyrmions have a specific type of topological stability that skyrmion number is invariant upon light propagation in free space. Here, we demonstrate the topological state and skyrmion number transitions of two-dimensional (2D) optical skyrmions upon beam propagation in free space. By simultaneously tailoring the longitudinal intensity profiles of vortex and non-vortex Bessel components with orthogonal polarizations, 2D optical skyrmion transitions with controllable topological states and skyrmion numbers are observed in quasi-nondiffractive skyrmionic beams. Manipulating the field topologies may hold significant potential in applications related to information processing and transmission.