While 2D materials have intriguing second‐order nonlinearities with ultrahigh coefficient and electrical tunability, their atomic layer thicknesses hinder explorations of other optical frequency conversions (OFCs) than second harmonic generation (SHG) due to inefficient light‐coupling and unachievable phase‐matching. By resonantly pumping a photonic crystal cavity integrated with a few‐layer GaSe, it is reported that it is possible to realize multiple second‐order nonlinear processes in GaSe even with microwatts continuous wave pumps, including SHGs, sum‐frequency generations (SFGs), cascaded SFGs, and their induced third harmonic generations. These OFCs arise from the significant cavity‐enhancements. The enhancement factor of an SHG process is estimated exceeding 1300. The cascaded SFGs have comparably strong intensities with those of SHGs. To the best of our knowledge, this is the first observation of cascaded second‐order nonlinear processes in 2D materials. The cavity‐assisted OFCs can provide a view to study deterministic OFCs in 2D materials with low pump power and expand their optoelectronic applications to nonlinear regime.

中文翻译：

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光子晶体腔辅助多层砷化镓中的多种光学频率转换

虽然2D材料具有极高的系数和电可调性，但是却具有令人着迷的二阶非线性特性，但由于光耦合效率低和无法实现相位匹配，因此它们的原子层厚度阻碍了除二次谐波产生（SHG）之外的其他光频率转换（OFC）的研究。据报道，通过谐振泵浦集成有几层GaSe的光子晶体腔，即使使用微瓦连续波泵，包括SHG，和频发生器（SFG），级联的SFG及其感应的三次谐波产生。这些OFC来自显着的腔增强。SHG工艺的增强因子估计超过1300。级联SFG的强度与SHG的强度相当。据我们所知，这是二维材料中级联二阶非线性过程的首次观察。腔辅助的OFC可以为研究具有低泵浦功率的二维材料中的确定性OFC提供一个视角，并将其光电应用扩展到非线性状态。