Nonlinear Spin‐Orbit Coupling Mediated by Longitudinal Electric Fields in Tightly Focused Light

Tightly focused light exhibits strong spin‐orbit coupling (SOC) effects, yet these effects are typically confined to the focal plane and cannot be directly observed in the far field. Here, it is demonstrated that by introducing a nonlinear thin film in the focal plane, second‐harmonic generation (SHG) can effectively convert these near‐field SOC features into far‐field detectable signals. Due to the crucial role of longitudinal electric fields in the SHG process, circularly polarized pump beams generate second‐harmonic vortices of topological charges m = ±1 and m = ±3 in the two spin components, arising from cascaded SOC effects during the tight focusing of the fundamental beam and the collimation of the second‐harmonic field. In contrast, linearly polarized pumping induces spin‐dependent displacements in the second‐harmonic field, attributed to the coupling between transverse and longitudinal electric field components. Additionally, sum‐frequency generation signals are observed in Fourier space, further enriching the nonlinear optical response. These findings advance the understanding of SOC in nonlinear optical processes and provide insights for designing nonlinear spin‐photonic devices.