表面和块体材料的多极非线性光学

Under the electric-dipole approximation, second-harmonic genetation is forbidden in centrosymmetric materials. But the inversion symmetry is broken at the surfaces of materials, thus giving rise to a dipolar surface response. In fact, when the multipolar effects (magnetic-dipole and electric quadrupole interactions) are also taken into account, the second-harmonic signals detected from the surfaces of centrosymmetric materials are not pure surface electric-dipolar responses, the contributions of the bulk multipolar responses are also included.Therefore, the distiction between the effective surface contribution and the bulk contribution for centrosymmetric has been a very hot topic. The purpose of this research is to separate the surface contribution and the bulk contribution for several isotropic and centrosymmetric materials in an unambiguous and quantitative way by using a non-collinear two-beam second-harmonic generation technique, to precisely determine the second-order nonlinear susceptibility tensor components of the two contributions, and to discuss the origin of the bulk responses.The possibility for unambiguous separation of the multipolar effects also opens the door for the new nonlinear materials with strong multipolar responses, which are not limited by the rule for non-centrosymmetry.

在电偶极近似条件下，倍频效应在中心对称材料内部是禁止的。然而，表面对称性破缺导致倍频效应在材料表面是允许的。实际上，当考虑材料的多极(磁偶极和电四极)效应时，在材料表面探测到的倍频信号并不是一个纯表面电偶极响应，也包含内部多极响应的贡献。因此，如何实现各项同性中心对称材料的表面和内部非线性效应贡献的完全分离一直是人们争相研究的焦点。本项目主要是选择一些各项同性中心对称材料为研究对象，采用非共线双光束偏振倍频技术，完全分离材料的有效表面电偶极效应和内部多极效应贡献，精确确定用于表征表面贡献和内部贡献的二阶非线性极化张量元，并分析材料多极非线性效应的起源。多极效应贡献的完全分离从而也打开了研究不受中心对称限制的具有强的多极非线性响应材料的大门。

在理论上，本项目通过预先获得各向同性中心对称结构材料的非零表面电偶极和内部高阶多极张量元项，再根据单、双光束入射情况构建相应的偏振倍频理论模型，获得可直接测量的展开系数以几个表面或内部张量元线性组合形式表示的函数表达式。在实验上，本项目首先以具有面内各向同性的杆状有机分子TSe为研究对象，对搭建的单、双光束实验装置进行校验，并进一步探讨了单、双光束入射两种情况下的实验展开系数唯一性，以及它们在精确确定被测材料张量元方面的优劣势；接着，本项目以100nm金膜为实验样品，利用单、双光束探测技术，除对与两测量类型相关的实验展开系数唯一性进行研究之外，还发现纳米金的表面和内部贡献完全分离仅可通过双光束偏振倍频技术来实现；最后，本项目也对几类具有中心对称结构特征的材料（光学玻璃、硫化玻璃和纳米金悬浮液）进行了高阶多极非线性效应研究。本项目的这些研究结果将为进一步深入探讨开发不受中心对称材料结构限制且具有强多极非线性效应材料提供基础。