Octupolar Films with Significant Second‐Harmonic Generation

Abstract

Considerable efforts are being made to develop octupolar nonlinear optical materials for possible application in optical and electro-optical devices. [1–3] An advantage of such molecules in comparison to the more conventional dipolar molecules is the existence of a broader range of tensorial coefficients, providing optimum nonlinear efficiency with a polarization-independent second-harmonic response with respect to the incident light. Great progress has been made by optimization at the molecular level, as exemplified by the established structure–property relationship of two-dimensional octupoles and the development of highly efficient molecules. [3–5] Recently, we reported that 1,3,5-tricyano-2,4,6tris(p-diethylaminostyryl)benzene (TTB) produces non-centrosymmetric crystals that show very large second-harmonic generation (SHG). [1] To integrate such materials in optical devices, it is essential to prepare thin films showing large SHG and high thermal stability. As electric poling is not possible because of the lack of a ground-state dipole moment, it is not readily obvious how to align octupolar molecules non-centrosymmetrically in a thin film. Although optical poling in polymers has been proposed, it requires photoisomerizable molecular functions and resonant excitation of the molecules by intense laser pulses, which may be a limitation for industrial application. An ideal solution to this problem would be to prepare self-oriented films without applying external fields. For this purpose, we have studied thin films of TTB in a polymethylmetacrylate (PMMA) matrix prepared by using free