基于等离激元分子阵列的多重表面晶格共振及增强二次谐波发射特性的研究

Second harmonic generation (SHG) with noble metallic nanostructures is promising for the design of ultrafast nonlinear nanophotonic devices, where it is important to realize enhanced SHG conversion efficiency within a broad spectral range to improve the integration density. To this end, we propose in this project that multiple surface lattice resonances can be generated with plasmonic molecule arrays, where radiative damping is suppressed effectively around several spectral ranges, and the near field and nonlinear source can be strongly enhanced in this case, thereby leading to enhanced SHG within a broad spectral range. The different collective localized surface plasmon resonances (LSPRs) generated with plasmonic molecules will be used to modify the distributions of the equivalent dipoles. In this way, the coupling between the LSPRs and Rayleigh anomalies can be achieved, which results in the formation of orthogonal and parallel surface lattice resonances. The resonance coupling may occur between the two kinds of surface lattice resonances, which will be well studied in this project. Consequently, a theoretical model will be developed to investigate the physical mechanism of the coupling between the LSPRs and Rayleigh anomalies, which can help to realize the reliable tuning of the multiple surface lattice resonances generated with different plasmonic molecule arrays. Due to the excitation of the multiple surface lattice resonances, the nonlinear sources for SHG are strongly enhanced. In addition, the scattering efficiency for the SHG can be amplified by higher-order plasmon resonances of the plasmonic molecules. As a result, the mode-matching condition can be achieved to enhance the SHG conversion efficiency within a broad spectral range. The achievements of this project will be useful for the development of ultrafast nonlinear nanophotonic devices. Besides that, they can be promising for other nanodevices such as high-sensitivity bio-chemical sensors, and plasmonic nanolasers.

基于贵金属纳米结构的二次谐波发射是设计超快非线性纳光子器件的重要手段，其中实现二次谐波转换效率宽谱增强是提高器件集成度的关键。本项目提出一种基于等离激元分子阵列产生多重表面晶格共振的方法，在多个波段有效抑制辐射损耗，增大局域场强和非线性源，在较宽光谱范围内实现二次谐波的增强发射。项目将利用等离激元分子不同局域共振模式调整等效偶极矩空间分布，在阵列中实现与Rayleigh异常的耦合，同时激发垂直与平行表面晶格模式，并研究两者之间的共振耦合特性；建立理论模型阐明等离激元分子不同局域共振与Rayleigh异常的耦合机制，实现对多重表面晶格共振的可控调制；最后基于多重表面晶格共振对非线性源的增强，以及等离激元分子高阶共振对谐波散射效率的提高，利用模式匹配条件，在多个波段达到增强二次谐波转换效率的目的。本项目的实现可推动超快非线性纳光子器件的发展，并可用于高灵敏度生化传感和等离激元激光的产生等领域。

项目针对等离激元分子阵列多重表面晶格共振进行研究，可在多个波段同时抑制辐射损耗，增强纳米尺度光与物质的相互作用。项目基于分子点群理论，研究了由贵金属纳米颗粒聚合体构成等离激元分子局域共振特性，结果表明其本征模式对应于等离激元分子对称性所属不可约表示。通过降低对称性，原有的高对称性简并模式劈裂为多个非简并的不可约表示模式，故当等离激元分子构成阵列时，这些局域模式与Rayleigh异常的耦合激发起多重表面晶格共振，从而在多个波段同时抑制了辐射损耗，增大了纳米尺度光与物质的相互作用。基于局域模式的等效偶极矩空间分布，研究中结合等离激元杂化理论分析了平行表面晶格共振的激发特性；同时基于等离激元分子阵列中平行与垂直表面晶格共振非正交的特性，当二者互相逼近时会产生共振耦合效应。研究中以具有C2v对称性的金纳米盘三聚体为例，采用电子束刻蚀方法制备了等离激元分子阵列样品，测试结果表明聚合体对应不可约表示局域模式的演化与激发、阵列中平行/垂直表面晶格共振的激发及二者的共振耦合与上述理论分析很好的匹配。为增强二次谐波发射，研究中采用了贵金属/高折射率电介质/贵金属三明治型等离激元分子，通过电介质具有较强非线性极化响应的特性，以及等离激元分子共振对入射场能量的聚焦作用，相对于非杂化体系，其二次谐波强度在数百纳米波长范围内获得十倍以上的增强，非线性转换效率超过10^-5，通过非线性波段共振模式的调整，更可实现接近完美的纯磁偶极或电偶极二次谐波发射。此外，研究中也展示了等离激元分子阵列多重表面晶格共振在增强手性、窄带完美吸收体、成对相位奇点等方面的应用。这些研究可推动超快非线性纳光子器件的发展，以上述成果为基础，在项目支持下获山西省自然科学二等奖1项（排名第一），发表论文15篇，投稿论文3篇，申请国家发明专利7项，其中授权4项，培养硕士生15名，博士生3名，其中取得博士学位1名，11名取得硕士学位。