Nonlinear metasurfaces constitute a key asset in meta-optics, given their ability to scale down nonlinear optics to sub-micrometer thicknesses. To date, nonlinear metasurfaces have been mainly realized using narrow band gap semiconductors, with operation limited to the near-infrared range. Nonlinear meta-optics in the visible range can be realized using transparent materials with high refractive index, such as lithium niobate (LiNbO₃). Yet, efficient operation in this strategic spectral window has been so far prevented by the nanofabrication challenges associated with LiNbO₃, which considerably limit the aspect ratio and minimum size of the nanostructures (i.e., meta-atoms). Here we demonstrate the first monolithic nonlinear periodic metasurface based on LiNbO₃ and operating in the visible range. Realized through ion beam milling, our metasurface features a second-harmonic (SH) conversion efficiency of 2.40 × 10^–8 at a pump intensity as low as 0.5 GW/cm². By tuning the pump polarization, we demonstrate efficient steering and polarization encoding into narrow SH diffraction orders, opening novel opportunities for polarization-encoded nonlinear meta-optics.

中文翻译：

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转向和用单片的LiNbO编码第二谐波的极化在可见₃超颖表面

非线性超颖表面构成了超光学的关键资产，因为它们具有将非线性光学缩小到亚微米厚度的能力。迄今为止，主要使用窄带隙半导体来实现非线性超表面，其工作限于近红外范围。可以使用具有高折射率的透明材料（例如铌酸锂（LiNbO ₃））来实现可见光范围内的非线性亚光学。然而，到目前为止，由于与LiNbO ₃有关的纳米制造挑战，在该战略性光谱窗口中的有效操作受到了阻碍，这极大地限制了纳米结构（即，亚原子）的长宽比和最小尺寸。在这里，我们演示了基于LiNbO ₃的第一个整体式非线性周期性超颖表面并在可见范围内运行。通过离子束铣削实现，我们的超颖表面在低至0.5 GW / cm ²的泵浦强度下具有2.40×10 ^–8的二次谐波（SH）转换效率。通过调整泵浦偏振，我们证明了将有效的转向和偏振编码转换为狭窄的SH衍射级，为偏振编码的非线性元光学打开了新的机遇。