Description
Chiral optical responses in metasurfaces are typically engineered through structural chirality or extrinsic symmetry breaking, while nonlinear circular dichroism is commonly viewed as a resonantly enhanced manifestation of an already chiral linear response. Here, we demonstrate a different mechanism: chirality induced solely by the nonlinear susceptibility tensor of an otherwise achiral resonant metasurface. We consider a suspended membrane metasurface consisting of circular holes arranged in a square lattice and made of a cubic nonlinear material such as crystalline silicon. At normal incidence, the structure is linearly achiral and exhibits identical resonant responses to left- and right-circularly polarized light. However, when the principal axes of the cubic nonlinear tensor are misaligned with respect to the metasurface axes, the nonlinear response becomes handedness-dependent. Using quasi-normal-mode expansion and temporal coupled-mode theory extended to nonlinear frequency conversion, we show that this tensor–lattice misalignment produces unequal cross-polarized third-harmonic generation for opposite circular polarizations. We interpret the effect through near-field angular-momentum selection rules and show that nonlinear circular dichroism can emerge without geometrical chirality, linear circular dichroism, or oblique incidence. The resulting nonlinear circular dichroism follows simple angular laws: a fourfold periodicity when only fundamental-frequency resonances dominate, and an eightfold periodicity when resonances at the harmonic frequency are also involved. These results establish nonlinearity-induced chirality as a symmetry-based route to chiral optical functionality in planar, CMOS-compatible resonant metasurfaces, with implications for nonlinear polarimetry, chiral light generation, and compact nanophotonic platforms for handedness-selective frequency conversion.
| I am the presenting author | Yes |
|---|