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Optical materials with centrosymmetry, such as silicon and germanium, are unfortunately absent of second‐order nonlinear optical responses, hindering their developments in efficient nonlinear optical devices. Here, a design with an array of slotted nanocubes is proposed to realize remarkable second harmonic generation (SHG) from the centrosymmetric silicon, which takes advantage of enlarged surface second‐order nonlinearity, strengthened electric field over the surface of the air‐slot, as well as the resonance enhancement by the bound states in the continuum. Compared with that from the array of silicon nanocubes without air‐slots, SHG from the slotted nanocube array is improved by more than two orders of magnitude. The experimentally measured SHG efficiency of the silicon slotted nanocube array is high as 1.8 × 10−4 W−1, which is expected to be further engineered by modifying the air‐slot geometries. The result can provide a new strategy to expand nonlinear optical effects and devices of centrosymmetric materials. [ABSTRACT FROM AUTHOR]

Optical bound states in the continuum (BICs) have recently been studied in a wide range of material systems, where light is perfectly confined in the continuous spectrum of radiating modes. In this paper, we reported periodic nonlinear metasurfaces on the etchless LiNbO3 platform, realizing the enhancement of second-order generation (SHG). All-dielectric heterogeneous metasurfaces are constructed by patterning a low-refractive-index polymer on a high refractive-index LiNbO3 film without etching. Due to BICs, light is localized in the LiNbO3 film where the excellent optical second-order nonlinearity is exploited. We demonstrated that with normal incident waves, symmetry-protected BICs are formed at near-infrared wavelength showing a vanishing linewidth in the transmission spectrum. In addition, to manifest the invisible BICs to detectable supercavity resonances at normal incidence, asymmetry is introduced into the system, degrading the symmetry-protected BICs to sharp resonances with a high Q factor. Furthermore, the second harmonic generation (SHG) of the etchless lithium niobate metasurface is studied, predicting that the SHG efficiency can exceed 10−3 with 30 MW/cm2 of the pump intensity. The proposed strategy without facing the fabrication challenge of etching single-crystal LiNbO3 film opens a new avenue for the utilization of BIC in nonlinear optics of all-dielectric heterogeneous metasurfaces. [ABSTRACT FROM AUTHOR]