The development of on-chip nonlinear optical devices in silicon is of great importance to silicon photonics and silicon chip based quantum information processing technologies. With the aim for a viable solution to overcome the lack of second harmonic generation (SHG) in Si, which is fundamentally limited by its centrosymmetric lattice structure, our work investigates SHG behaviors from Si (100) crystals with silver nanostructures formed following deposition of an ultrathin silver film and subsequent annealing. This study is aided by additional techniques, including x-ray photoelectron spectroscopy for measuring surface band bending, secondary electron microscopy for monitoring surface morphology, and Raman scattering for assessing crystal stress. The resultant Ag nanostructures are found to strongly impact the second order nonlinear polarizations in the Si surface regions rather than the bulk. The SHG intensities are increased following the Ag deposition but reduced below the Si control levels after annealing at 600 and 700 °C, which may be due to charge transfer from Ag to SiO2/Si and/or passivation of interfacial defects. Interestingly, annealing at higher temperatures (800 and 900 °C) leads to the formation of Ag nano-shell structures embedded below the SiO2/Si interface, different from the as-deposited and low-temperature annealing cases with Ag nano-spheroid structures appearing on the surface, and concomitantly, the SHG intensities are recovered and even exceed the level for the as-deposited sample in the p-Si case. The enhanced SHG following high-temperature annealing, particularly at 800 °C, is attributed to a redshift of the localized plasmon resonance of these embedded Ag nano-shells relative to the surface-covered nano-spheroids that leads to localized surface plasmon resonance around the fundamental light wavelength. Other factors including electric-field-induced SHG and the effects of stress or oxide thickness can be ruled out. This work suggests a novel approach to enhance second-order optical nonlinearity in Si through the formation of embedded metal nanostructures.

中文翻译：

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用于硅非线性纳米光子学的具有工艺定制表面和嵌入银纳米结构的硅 (100) 晶体产生光学二次谐波

硅片上非线性光学器件的开发对硅光子学和基于硅片的量子信息处理技术具有重要意义。为了找到一种可行的解决方案来克服 Si 中缺乏二次谐波 (SHG) 的问题，该问题从根本上受到其中心对称晶格结构的限制，我们的工作研究了 Si (100) 晶体的 SHG 行为，该晶体具有在沉积银纳米结构后形成的银纳米结构。超薄银膜和随后的退火。这项研究得到了其他技术的帮助，包括用于测量表面带弯曲的 X 射线光电子能谱、用于监测表面形态的二次电子显微镜和用于评估晶体应力的拉曼散射。发现所得的 Ag 纳米结构强烈影响 Si 表面区域中的二阶非线性极化，而不是体块。SHG 强度在 Ag 沉积后增加，但在 600 和 700 °C 退火后降低到 Si 控制水平以下，这可能是由于电荷从 Ag 转移到 SiO2/Si 和/或界面缺陷的钝化。有趣的是，在较高温度（800 和 900 °C）下退火会导致形成嵌入 SiO2/Si 界面下方的 Ag 纳米壳结构，这与出现 Ag 纳米球体结构的沉积态和低温退火情况不同在表面上，同时，SHG 强度恢复，甚至超过了 p-Si 情况下沉积样品的水平。高温退火后增强的 SHG，特别是在 800 °C 时，这归因于这些嵌入的 Ag 纳米壳的局域等离子体共振相对于表面覆盖的纳米球体的红移，导致围绕基本光波长的局域表面等离子体共振。可以排除其他因素，包括电场引起的 SHG 和应力或氧化物厚度的影响。这项工作提出了一种通过形成嵌入的金属纳米结构来增强 Si 中二阶光学非线性的新方法。可以排除其他因素，包括电场引起的 SHG 和应力或氧化物厚度的影响。这项工作提出了一种通过形成嵌入的金属纳米结构来增强 Si 中二阶光学非线性的新方法。可以排除其他因素，包括电场引起的 SHG 和应力或氧化物厚度的影响。这项工作提出了一种通过形成嵌入的金属纳米结构来增强 Si 中二阶光学非线性的新方法。