有机微纳结构的非线性光学特异性研究

Nonlinear optical (NLO) material with high-performance is crucial to the realization of all optical information processing. Inorganic crystals, as a kind of traditional NLO materials, have been employed in a wide field. Organic counterparts are causing more and more attentions due to their excellent properties, ultrafast response, low cost and so on. Organic molecules, which can be synthesized and tailored easily with rational design, exhibit distinctive self-assembling properties. Driven by various intermolecular forces during aggregation process, such as hydrogen bonds, π-stacking, and Van de Waals interactions, molecules tend to form distinct micro/nanostructures. The controllability in organic structure characteristics is essentially important for the NLO properties, especially the second order NLO parameters that are dependent significantly on the structural symmetry. As a result, organic micro/nanostructures may offer a new route to develop novel nonlinear materials and photonic devices. .In this project, we propose to finely tune the NLO properties of organic micro/nanostructures through modulating the assemble process of organic molecules. We will design and select several kinds of important NLO molecules, then construct a series of micro/nanostructures. By comparing the differences in NLO properties between molecules and corresponding micro/nano-aggregates, including nonlinear absorption, nonlinear emission as well as second harmonic generation, the relationship between the NLO properties and the microscopic structural characteristics, such as molecular orientation, packing space, dipolar angle, crystal type, etc, will be investigated to reveal the evolution of NLO properties from single molecule to micro/nano-aggregates. The "structure-effect" on NLO properties in organic aggregates will also be summarized to provide enlightments for the preparation of specific NLO materials towards future nonlinear photonic integrations.

高性能非线性光学材料是实现全光信息处理的关键。无机材料作为传统的非线性光学材料，已经在各个领域得到了广泛应用。与此同时有机材料正以其优异的性能、超快的响应、低廉的成本等优势越来越引起人们的关注。有机小分子可设计、易剪裁、便于组装，在聚集过程受氢键、π堆积、范德华力等不同驱动力作用，产物结构多样，对其非线性光学性质、特别是结构相关的二阶非线性极化率等光学参数有重要影响，有望为新型光子学材料和器件的发展带来新的契机。本项目提出利用分子聚集状态调控有机微纳结构的非线性光学性能，通过设计并筛选几类重要的非线性光学分子构筑一系列结构迥异的微纳结构，在纳米尺度上研究分子取向、排列间距、偶极夹角、晶形等结构特征与其非线性光学特异性的关联，揭示单体分子非线性光学性质在聚集过程中的传递和演化规律，从中总结出有机分子聚集体中的"结构效应"，为定向制备满足特定功能的微纳尺度非线性光子学器件奠定基础。

高性能非线性光学材料是实现全光信息处理的关键。无机材料作为传统的非线性光学材料，已经在各个领域得到了广泛应用。与此同时有机材料正以其优异的性能、超快的响应、便捷的加工、低廉的成本等优势越来越引起人们的关注。有机小分子可设计、易剪裁、便于组装，在聚集过程中受氢键、π堆积、范德华力等不同驱动作用，产物结构多样，对其非线性光学性质等光学参数具有重要影响，有望为新型光子学材料和器件的发展带来新的契机。我们系统总结了分子结构、聚集过程、结构特征和材料性能之间的关联，相关结果发表在Adv. Mater. 2013, 25, 3627-3638。. 本项目中，我们提出主客体包合自组装的方法来打破有机小分子聚集过程中的中心对称排列，成功构筑具有二阶非线性光学响应的低维超分子晶体。对硝基苯胺分子本身具有较强的二阶非线性光学响应，能够进入环糊精的空腔形成包合物，并在反溶剂扩散的条件下成为低维超分子晶体。这些超分子晶体的单元排列、尺寸等特征可以通过改变溶液的浓度、溶剂的极性、生长时间等进一步进行调控。我们得到了两种结构的超分子晶体，都表现出二阶非线性光学响应，表明对硝基苯胺单体聚集过程中的中心对称排列方式被成功打破。进一步研究表明，通道型的排列方式更加有利于二次谐波的产生。. 除了倍频，非线性激射是获得相干光学信号的另一种途径。有机小分子自组装形成的微纳结构具有规整的端面结构及光滑表面，可以作为高质量谐振腔。更重要的是，聚集体结构中分子距离变短，分子间强相互作用力能够改变分子激发态过程，抑制无辐射跃迁，从而提供显著增益，产生非线性激射。本项目中，我们以2-（2′-羟基苯基）苯并噻唑为例，利用液相自组装得到一维微纳单晶结构，利用其分子内激发态质子转移过程构建真正的四能级系统，在较低的激发功率下下实现粒子数翻转，获得微纳激光发射。调控谐振腔尺寸还能够得到单模发射。特别地，激射波长取决于材料的激发态过程。我们利用激发功率调控激发态过程，使分子在烯醇式和酮式之间切换，实现了激射波长的切换。