Intrinsic Chirality of Dielectric Metasurfaces Unlocked by Resonant Chiral Modes.

Controlling optical chirality at the subwavelength scale is essential for applications of nanophotonic structures in polarization optics, sensing, and nonlinear photonics. Achieving a strong chiroptical response in planar dielectric metasurfaces without intrinsically chiral meta-atoms remains challenging. A recent study proposed a novel mechanism of metasurface chirality and predicted that bilayer metasurfaces with rotated C4-symmetric apertures can exhibit a chiral response originating from resonant chiral photonic modes. Here, we experimentally confirm this concept by demonstrating resonantly enhanced circular dichroism in the near-infrared. We fabricated a free-standing silicon membrane metasurface that is nominally achiral. Breaking the out-of-plane symmetry with a thin PMMA layer unlocks and activates a strong chiral response. The observed circular dichroism is governed by chiral photonic modes, interlayer coupling, and symmetry breaking, in agreement with theoretical predictions. These results establish bilayer metasurfaces as a simple and versatile platform for engineering strong mode-induced chirality in compact planar photonic metadevices.