Nonlinear spin-orbit conversion via enhanced longitudinal fields in an epsilon-near-zero thin film.

Spin-orbit interaction (SOI) of light enables structural control of nonlinear harmonic generation by coupling spin and orbital angular momentum. However, in conventional transverse-field-driven processes, angular-momentum conversion is fundamentally constrained by symmetry. Here, we derive general selection rules showing that the longitudinal electric field of a tightly focused beam lifts this restriction without breaking rotational symmetry. By enhancing the longitudinal component at an epsilon-near-zero thin-film interface, we make nonlinear spin-orbit conversion experimentally observable and generate phase-singular vortex harmonics from a non-singular Gaussian beam at both second and third order. These results reveal the longitudinal field as a key degree of freedom for angular-momentum control in harmonic generation.