Symmetry-governed nonlinear metasurface simultaneously enhances circular dichroism, helicity purity, and Q-factor

Circularly polarized light underpins a broad range of chiroptical and spin-selective photonic technologies, yet extending resonant chirality to nonlinear harmonic emission remains challenging. Here, we experimentally demonstrate a symmetry-governed mechanism that maps the maximal linear chirality of high-Q degenerate resonances onto the polarization of the generated harmonic. Preserved rotational symmetry protects modal degeneracy and polarization-selection rules, while broken mirror symmetries enable a strong chiroptical response. The metasurface thereby filters the pump polarization and purifies the handedness of the nonlinear output, simultaneously achieving third-harmonic circular dichroism of -0.98, a degree of circular polarization of 0.83, and an effective nonlinear Q-factor of 140. Importantly, the nonlinear emission preserves its handedness even under spatially depolarized excitation, demonstrating robustness to variations in the incident polarization state. These results establish complementary symmetry preservation and breaking as a deterministic route for transferring resonant chirality from linear to nonlinear optical responses, providing a general design principle for nonlinear chiral photonics.