Intrinsic Chiralityof Dielectric Metasurfaces Unlockedby 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.