Generation of fractional spatiotemporal optical vortices and anomalous topological conservation in the second harmonic

Spatiotemporal optical vortices (STOVs), which carry transverse orbital angular momentum on ultrafast timescales, provide a platform to probe intrinsic space–time coupling in light–matter interactions. Fractional-order STOVs (FSTOVs) introduce continuously tunable topology, giving access to controllable orientation and chirality, but their noninteger winding breaks single-valuedness and global topological protection, so their generation and nonlinear response have remained unclear. Here, we investigate the topological dynamics of FSTOVs and demonstrate anomalous angular-momentum conservation in second-harmonic generation (SHG). As the fractional charge increases, the field evolves from an open to a closed structure, with singularities migrating inward and reorganizing. Although SHG preserves global phase doubling, the singularities do not follow the simple 2ℓ rule but undergo local multiplication and reorganization into spatiotemporal interference networks. This coexistence of global conservation and local reconstruction defines a framework for nonlinear topological dynamics of structured light and enables ultrafast field control and topological information transfer.