Nonlinear chiral response governed by meta-atom rotation

Chiral photonics provides powerful tools for controlling the light handedness, yet nonlinear chiral responses are typically associated with intricate three-dimensional systems. Here, we demonstrate that strong nonlinear chirality can emerge and be precisely controlled in planar metasurfaces. We study free-standing membrane metasurfaces composed of periodic lattices of tilted elliptic holes, which preserve out-of-plane mirror symmetry while breaking all in-plane mirror symmetries through the in-plane rotation of the meta-atoms. We show that the in-plane geometric asymmetry governs the nonlinear selection rules and differential response to circularly polarized excitation, while optical resonances enhance the nonlinear interaction through strong field localization and overlap. We experimentally reveal that the interaction of meta-atom and lattice symmetries leads to the swapping of the dominant co-polarized channels for complementary meta-atom rotations. This, in turn, is manifested as interchangeable circular dichroism in the third-harmonic signal, the intensity of which is further enhanced by the resonant modes of the metasurface. Our results suggest that meta-atom rotation can be a powerful tool for engineering nonlinear chiral responses in planar metasurfaces, opening new opportunities for tunable chiral nonlinear metaphotonics devices.