Second‐Harmonic Generation by 3D Laminate Metacrystals

3D dielectric woodpile photonic crystals, which are conformally coated with locally noninversion-symmetric dielectric laminate metamaterials, are discussed. These "3D laminate metacrystals" are fabricated by 3D direct laser writing of an acrylate without photoinitiator to reduce two-photon-induced fluorescence, followed by atomic-layer deposition of alternating layers of Al2O3 and ZnO. Resonant enhancements of second-harmonic generation under normal incidence of light of up to two orders of magnitude with respect to the same laminate metamaterial on a glass substrate under oblique incidence of light are found experimentally. The nonlinear optical resonances in the range of 800-1020 nm fundamental wavelength are explained theoretically in terms of 3D photonic crystal slab guided resonances, leading to pronounced local-field-enhancement effects. Numerical calculations reproduce the salient features of the experiments and yield second-harmonic enhancement factors yet larger than the experiment due to the absence of spectral and angular averaging.