Second harmonic generation (SHG) is a coherent nonlinear phenomenon that plays an important role in laser color conversion. Lithium niobate (LN), which features both a large band gap and outstanding second‐order nonlinearities, acts as an important optical material for nonlinear frequency conversion covering a wide spectral range from ultraviolet to mid‐infrared. Here, LN metasurfaces with controllable SHG properties are experimentally demonstrated. Distinct enhancements for the SHG efficiency are observed at Mie‐resonances. And by changing the geometric parameters, thus the resonances of the metasurfaces, the authors manage to selectively boost the SHG efficiency at different wavelengths. The results would pave a way for developing with high flexibility the novel compact nonlinear light sources for applications such as biosensing and optical communications.

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Unprecedented data manipulation employed using a pH-stimuli responsive hydrogel was developed to achieve reversible data encryption–decryption.

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