Frequency conversion processes, such as second- and third-harmonic generation, are commonly realized in nonlinear optics, offering opportunities for applications in photonics, chemistry, material science and biosensing. Given the inherently weak nonlinear response of natural materials, optically large samples and complex phase-matching techniques are typically required to realize significant nonlinear responses. To produce similar effects in much smaller volumes, current research has been devoted to the quest of synthesizing novel materials with enhanced optical nonlinearities at moderate input intensities. In particular, several approaches to engineer the nonlinear properties of artificial materials, metamaterials and metasurfaces have been introduced. Here, we review the current state of the art in the field of small-scale nonlinear optics, with special emphasis on high-harmonic generation from ultrathin metasurfaces based on plasmonic and high-index dielectric resonators, as well as semiconductor-loaded plasmonic metasurfaces. In this context, we also discuss recent advances in controlling the optical wavefront of generated nonlinear waves using metasurfaces. Finally, we compare viable approaches to enhance nonlinearities in ultrathin metasurfaces, and we offer an outlook on the future development of this exciting field of research.

中文翻译：

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非线性超表面：非线性光学的范式转变

频率转换过程，例如二次和三次谐波产生，通常在非线性光学中实现，为光子学、化学、材料科学和生物传感等领域的应用提供了机会。鉴于天然材料固有的弱非线性响应，通常需要光学大样本和复杂的相位匹配技术来实现显着的非线性响应。为了以更小的体积产生类似的效果，目前的研究一直致力于在中等输入强度下合成具有增强光学非线性的新型材料。特别是，已经引入了几种设计人造材料、超材料和超表面的非线性特性的方法。在这里，我们回顾了小尺度非线性光学领域的当前技术水平，特别强调从基于等离子体和高指数介电谐振器的超薄超表面以及加载半导体的等离子体超表面产生高谐波。在这种情况下，我们还讨论了使用超表面控制生成的非线性波的光波前的最新进展。最后，我们比较了增强超薄超表面非线性的可行方法，并对这一激动人心的研究领域的未来发展提出了展望。