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.