Photonic torons with 3D topology transitions and tunable spin monopoles

Topological defects and textures play crucial roles in fundamental physics and modern information science. Torons are a class of three-dimensional (3D) chiral topological structures with both skyrmionic quasiparticle textures and monopole point defects, so far only observed in liquid crystals with both polar and nonpolar symmetry. Here, we construct torons with the photonic spin of vector structured light and demonstrate the topological phase transitions among diverse 3D topological states: torons, hopfions, skyrmioniums and monopole pairs. We can also continually tune the toron's chirality and monopole's helicity. The generation of photonic torons opens a platform for studying nontrivial light-matter interaction and topological informatics.