Multi‐Scale Modeling of Surface Second‐Harmonic Generation in Centrosymmetric Molecular Crystalline Materials: How Thick is the Surface?

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. [ABSTRACT FROM AUTHOR]