[论文解读] Treatment of Linear and Nonlinear Dielectric Property of Molecular Monolayer and Submonolayer with Microscopic Dipole Lattice Model: I. Second Harmonic Generation and Sum-Frequency Generation

Desheng Zheng, Yuan Wang+2 more|arXiv (Cornell University)|Aug 13, 2007

Spectroscopy and Quantum Chemical Studies参考文献 5被引用 48

一句话总结

本文提出了一种微观偶极子晶格模型，以严格描述分子单层和亚单层界面处的二次谐波产生（SHG）与和频生成（SFG）现象，取代传统的无限薄极化片模型。通过将界面视为由感应偶极子组成的离散晶格，该模型消除了对任意宏观介电常数的依赖，并利用分子极化率张量显式计算各向异性的二维局部场因子，为定量表面非线性光学分析提供了更符合物理规律的框架。

ABSTRACT

In the currently accepted models of the nonlinear optics, the nonlinear radiation was treated as the result of an infinitesimally thin polarization sheet layer, and a three layer model was generally employed. The direct consequence of this approach is that an apriori dielectric constant, which still does not have a clear definition, has to be assigned to this polarization layer. Because the Second Harmonic Generation (SHG) and the Sum-Frequency Generation vibrational Spectroscopy (SFG-VS) have been proven as the sensitive probes for interfaces with the submonolayer coverage, the treatment based on the more realistic discrete induced dipole model needs to be developed. Here we show that following the molecular optics theory approach the SHG, as well as the SFG-VS, radiation from the monolayer or submonolayer at an interface can be rigorously treated as the radiation from an induced dipole lattice at the interface. In this approach, the introduction of the polarization sheet is no longer necessary. Therefore, the ambiguity of the unaccounted dielectric constant of the polarization layer is no longer an issue. Moreover, the anisotropic two dimensional microscopic local field factors can be explicitly expressed with the linear polarizability tensors of the interfacial molecules. Based on the planewise dipole sum rule in the molecular monolayer, crucial experimental tests of this microscopic treatment with SHG and SFG-VS are discussed. Many puzzles in the literature of surface SHG and SFG spectroscopy studies can also be understood or resolved in this framework. This new treatment may provide a solid basis for the quantitative analysis in the surface SHG and SFG studies.

研究动机与目标

• 解决长期以来在非线性光学模型中分子薄界面层宏观介电常数的模糊性问题。
• 建立表面SHG与SFG-VS中各向异性二维微观局部场效应的物理解释。
• 用现实的离散感应偶极子晶格表示替代传统的无限薄极化片模型。
• 为亚单层和单层覆盖下SHG与SFG-VS数据的定量解释提供微观基础。
• 阐明平面内偶极子求和规则在连接微观偶极响应与宏观光学可观测量中的作用。

提出的方法

• 将界面表述为离散感应偶极子晶格，而非连续极化片。
• 应用分子光学理论，从偶极子晶格推导SHG与SFG过程的远场辐射场。
• 利用界面分子的线性极化率张量，显式表达各向异性的二维微观局部场因子。
• 采用平面内偶极子求和规则，将集体偶极响应与宏观光学性质关联。
• 通过麦克斯韦方程验证模型与经典极化片模型在渐近行为上的一致性。
• 评估简化有效介电常数模型在小分子集团中的有效性与局限性。

实验结果

研究问题

• RQ1 如何在非线性光学建模中一致地定义并避免使用分子单层或亚单层的宏观介电常数？
• RQ2 表面SHG与SFG-VS中各向异性二维局部场因子的正确微观表达式是什么？
• RQ3 离散偶极子晶格模型在预测远场辐射方面与传统极化片模型相比如何？
• RQ4 简化有效介电常数模型在多大程度上能准确表征界面非线性光学中的局部场效应？
• RQ5 哪些实验测试可用于验证SHG与SFG-VS测量中微观偶极子晶格模型？

主要发现

• 微观偶极子晶格模型消除了极化层中对任意宏观介电常数的依赖，解决了表面非线性光学中长期存在的模糊性问题。
• 各向异性的二维微观局部场因子通过分子极化率张量显式表达，实现了与分子结构的直接关联。
• 偶极子晶格的远场辐射场与经典极化片模型的结果渐近一致，验证了新方法的有效性。
• 平面内偶极子求和规则为连接微观偶极响应与宏观光学可观测量提供了统一框架。
• 该模型解释并解决了文献中长期存在的关于分子取向与SHG与SFG-VS光谱解析的疑难问题。
• 该方法为表面SHG与SFG-VS的定量分析提供了坚实的微观基础，尤其适用于亚单层体系。