White-light supercontinuum generation can be readily observed when gold nanostructures are irradiated with short pulses of light. It is believed that the nanostructures enhance the optical fields, which facilitates the supercontinuum white-light generation from the surrounding environment or the substrate. Here, we investigate the different nonlinear processes that contribute to the generation of the supercontinuum from plasmonic nanostructures themselves using a technique that isolates the different nonlinear contributions. By exciting a gold nanofilm with a pair of frequency shifted optical frequency combs, we demonstrate multiple modulation frequencies in the supercontinuum. Their dependence on the excitation intensity reveals that the supercontinuum originates from different orders of nonlinear light–matter interactions. This contrasts with the supercontinuum generation by a cascaded third-order optical nonlinear response in traditional dielectric-based white-light sources. The while-light emission from the gold nanofilm is efficient even under relatively weak excitation indicating that nonlocal effects in nanostructures may facilitate the supercontinuum generation by adding new pathways in the nonlinear interactions. The results provide experimental basis for the understanding of the collective nonlinear response of free-electrons in the metallic nanostructures and associated nonlinear processes, which are crucial in development of nonlinear metasurfaces and nanophotonic devices.

中文翻译：

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金纳米膜产生的超连续谱白光中高阶非线性的证据

当用短脉冲光照射金纳米结构时，很容易观察到白光超连续谱的产生。据信，纳米结构增强了光场，这有利于从周围环境或基底产生超连续谱白光。在这里，我们使用隔离不同非线性贡献的技术研究了有助于从等离子体纳米结构本身生成超连续谱的不同非线性过程。通过用一对频移的光学频率梳激发金纳米膜，我们证明了超连续谱中的多个调制频率。他们对激发强度的依赖性表明，超连续谱起源于不同阶的非线性光-物质相互作用。这与传统的基于电介质的白光源中级联的三阶光学非线性响应产生的超连续谱相反。即使在相对较弱的激发下，金纳米膜的白光发射也是有效的，这表明纳米结构中的非局部效应可通过在非线性相互作用中添加新的途径来促进超连续谱的产生。研究结果为理解金属纳米结构中自由电子的集体非线性响应及相关的非线性过程提供了实验基础，这对于非线性超表面和纳米光子器件的发展至关重要。即使在相对较弱的激发下，金纳米膜的白光发射也是有效的，这表明纳米结构中的非局部效应可通过在非线性相互作用中添加新的途径来促进超连续谱的产生。研究结果为理解金属纳米结构中自由电子的集体非线性响应及相关的非线性过程提供了实验基础，这对于非线性超表面和纳米光子器件的发展至关重要。即使在相对较弱的激发下，金纳米膜的白光发射也是有效的，这表明纳米结构中的非局部效应可通过在非线性相互作用中添加新的途径来促进超连续谱的产生。研究结果为理解金属纳米结构中自由电子的集体非线性响应及相关的非线性过程提供了实验基础，这对于非线性超表面和纳米光子器件的发展至关重要。