Monocrystalline Gold Metasurface to Control Anisotropic Second-Harmonic Generation

The tensorial nature of the nonlinear susceptibility of gold surfaces is revealed and exploited by comparing crystalline and polycrystalline thin films. The {111}-type surfaces of crystalline gold exhibit anisotropic features that are absent at the surfaces of conventionally used polycrystalline thin films. While this anisotropy does not influence the linear optical response, it becomes apparent in the second-harmonic generation, where the emission can be either co- or cross-polarized with the excitation light, depending on the crystal orientation. It is further demonstrated that nanopatterning a {111} surface enables versatile control over the anisotropic and isotropic contributions to second-harmonic generation in the vicinity of localized surface plasmon resonance condition. Focused ion beam-milling is used to pattern the surface of chemically synthesized gold flake with subwavelength-sized V-grooves, while preserving the intrinsic crystal anisotropy as demonstrated by the nonlinear microscopy. The results highlight the potential of crystalline metasurfaces to enhance, tailor and control the nonlinear optical response, paving the way for advanced nonlinear photonic devices.