High-Order Multipolar Excitations Governing Circular Dichroism in an All-Dielectric Chiral Metasurface.

Chiral metasurfaces provide a versatile platform for tailoring polarization-dependent light-matter interactions with strong field confinement. Here, we report an all-dielectric asymmetric gammadion metasurface that exhibits pronounced circular dichroism (CD) in the visible spectral range. By breaking the in-plane mirror symmetry of the unit cell, strong polarization-selective resonances are induced under circularly polarized illumination. Numerical simulations and experiments reveal two distinct CD resonances near 652 and 735 nm with maximum absolute CD values exceeding 50% and 40%, respectively. Electromagnetic multipole decomposition shows that the strong CD at 652 nm originates from selective excitation of high-order electric and magnetic modes under right-handed circularly polarized illumination leading to a large imbalance in multipolar scattering and strongly localized chiral near fields. In contrast, the weaker CD at 735 nm arises from a more symmetric and reduced multipolar excitation landscape. Angle-resolved analyses further demonstrate that the quadrupole-dominated resonance maintains robust CD under oblique incidence, whereas the simpler multipolar excitation at 735 nm yields a more angle-insensitive but moderate CD response. These results establish a physical link between structural asymmetry, multipolar excitation, near-field optical chirality, and angle-resolved chiroptical behavior, providing practical design guidelines for low-loss dielectric metasurfaces with robust chiroptical responses.