Bilayer optical metasurfaces with multiple broken symmetries for nonlinear wavelength generation

Symmetry breaking along and perpendicular to the propagation direction unlocks optical properties forbidden in fully symmetric systems and enables nontrivial mode coupling with enhanced nonlinear responses. Combining concepts from photonic crystals (PC) and metasurfaces offers a powerful route to engineering light–matter interactions at the subwavelength scale. While PCs are known for producing transmission gaps, metasurfaces enable strong field enhancement and subwavelength resonance control. Therefore, when integrated into multilayer architectures, these platforms introduce new degrees of freedom for tailoring linear and nonlinear processes in compact geometries. Here, we design and experimentally demonstrate double-layer metasurfaces that achieve enhanced third-harmonic generation in the ultraviolet regime through the interplay of PC effects and coupled guided-mode resonances. We show that vertical stacking and horizontal displacement break symmetry in multiple directions, enabling nonlinear enhancements that surpass those of band-edge states alone. These results establish multilayer symmetry-broken nanophotonic structures as a versatile platform for efficient nonlinear generation in compact, integrated devices.