Second–harmonic generation (SHG) is forbidden in centrosymmetric materials. However, a signal is observed from interfaces where the symmetry is broken. Whereas the effect can be phenomenologically accommodated, a qualitative and quantitative description remained elusive, preventing the exploration of questions such as how deep below the surface the second–harmonic is generated. A multi–scale approach to compute the total and layer‐dependent intensity of surface SHG from molecular crystals is thus presented. The microscopic origin of surface SHG is identified in layer‐dependent models with embedding partial charges combined with density functional theory (DFT) showing symmetry‐breaking distortions of the electron cloud as the surface layer is approached. The SHG at the molecular level is determined using time‐dependent DFT and then brought to the macroscopic scale through a rigorous self‐consistent multiple scattering formalism. The intensity of the SHG at the surface layer is two orders of magnitude larger than at the next layer below and three orders of magnitude larger than two layers below. This approach can be used for designing and optimizing optical devices containing nonlinear molecular materials, such as molecular laminates. It is shown that a basic Kretschmann‐like setup can enhance the surface SHG of centrosymmetric molecular material a thousand times.

中文翻译：

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中心对称分子晶体材料表面二次谐波产生的多尺度建模：表面有多厚？

中心对称材料中禁止产生二次谐波（SHG）。然而，从对称性被破坏的界面观察到信号。尽管这种效应可以在现象学上得到解决，但定性和定量的描述仍然难以捉摸，从而阻碍了对二次谐波产生的深度等问题的探索。因此提出了一种计算分子晶体表面二次谐波的总强度和层相关强度的多尺度方法。表面二次谐波的微观起源在层相关模型中被识别，该模型具有嵌入部分电荷并结合密度泛函理论（DFT），显示当接近表面层时电子云的对称性破缺扭曲。分子水平的倍频是使用时间相关的 DFT 确定的，然后通过严格的自洽多重散射形式将其带到宏观尺度。表层的二次谐波强度比下面一层的二次谐波强度大两个数量级，比下面两层的二次谐波强度大三个数量级。这种方法可用于设计和优化包含非线性分子材料的光学器件，例如分子层压板。结果表明，基本的类 Kretschmann 装置可以将中心对称分子材料的表面倍频增强一千倍。