High-Q chiral metasurfaces with in-plane symmetric C-point engineering for nonlinear circular dichroism.

C-points in momentum space correspond to circularly polarized states, with broad applications in light-matter interactions, chiral lasing, and nonlinear optics. However, achieving a circularly polarized state at the Γ point requires relocating C-points to this position, which necessitates breaking both in-plane and out-of-plane symmetries. Besides, the chiral responses are typically confined to a narrow range of incident angles. Here, by including periodic groove perturbations on both sides of the grating slit, guided-mode resonances (GMRs) with a high quality factor (∼10) that propagate parallel to the grating slit are created, which possess two pairs of C-points off the Γ point. By adjusting the spacing between the grooves on both sides of the grating slits, one pair of C-points with the same handedness gradually evolves toward the Γ point and is eventually located on a high-symmetry axis, resulting in near-circularly polarized states over an extended momentum range along that axis, while preserving the in-plane -symmetry. Furthermore, we achieve spectrally tunable field-enhanced circular dichroism (CD) as well as nonlinear CD exceeding 50 dB. The proposed structure also exhibits strong robustness against fabrication imperfections, offering practical feasibility for chiral photonic applications.