Metasurfaces have emerged as a fascinating framework for nonlinear optics, which have advantages of a compact footprint and unprecedented flexibility in manipulating light. But their nonlinear responses are generally limited by the short interaction lengths with light. Therefore, further enhancement is highly desired for building high-efficiency nonlinear devices. Here, we experimentally demonstrate a record high second harmonic generation (SHG) efficiency of 2.0 × 10^–4 using lithium niobate (LN) membrane metasurfaces. Benefiting from the large refractive index contrast in the vertical direction and high fabrication quality, distinct spectral resonances and tight field confinements in the LN layer were achieved. Strong SHG peaks resulting from pump resonances of the metasurfaces were observed. Our nonlinear efficiency is more than 2 orders of magnitude larger than previously reported LN metasurfaces. The results inspire a way to improve the efficiency of nonlinear metasurfaces for ultracompact nonlinear light sources in applications of nonlinear holography, Li-Fi, beam shaping, etc.

中文翻译：

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膜超表面产生的巨大二次谐波

超表面已成为非线性光学的迷人框架，它具有紧凑的占地面积和操纵光的前所未有的灵活性等优点。但它们的非线性响应通常受到与光的短相互作用长度的限制。因此，构建高效非线性器件非常需要进一步的改进。^{在这里，我们通过实验证明了创纪录的 2.0 × 10 –4}的二次谐波产生 (SHG) 效率使用铌酸锂 (LN) 膜超表面。得益于垂直方向的大折射率对比度和高制造质量，在 LN 层中实现了明显的光谱共振和紧密的场限制。观察到由超曲面的泵共振引起的强 SHG 峰。我们的非线性效率比之前报道的 LN 超表面高出 2 个数量级以上。结果启发了一种方法，可以在非线性全息术、Li-Fi、光束整形等应用中提高超紧凑型非线性光源的非线性超表面的效率。