One of the requirements for a material to exhibit a second-order nonlinear optical response is that its constituent atoms or molecules are arranged in a non-centrosymmetric manner, providing a bias that enhances or inhibits electronic polarization along specific axes. In a ferroelectric crystal such as LiNbO<sub>3</sub> or SrTiO<sub>3</sub>, non-centrosymmetric order is an inherent property of the material’s crystal structure. In amorphous materials such as guest/host polymer composites and neat dendritic materials, acentric order can be introduced by application of an external potential and further controlled by engineered intermolecular interactions. However, unlike in crystalline materials, direct measurement of acentric order in complex amorphous bulk materials has remained elusive due to the difficulty of separating the effects of molecular hyperpolarizability, local field effects, and ordering. We have developed a new technique for measuring non-centrosymmetric order using vibrational sum-frequency generation (SFG) spectroscopy at multiple polarization combinations on multiple localized vibrational modes. While SFG is typically exclusively a surface technique, the bulk response dominates in poled materials containing strong NLO chromophores, enabling measurements in micron-scale films. We further compare our experimental results with Monte Carlo simulations of ordering of complex NLO chromophores under poling conditions.

Characterization of acentric order in poled organic NLO materials using coarse-grained Monte Carlo simulations and sum-frequency generation spectroscopy

254th National Meeting (2017)

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