Molecular engineering of organic chromophores and polymers for enhanced bulk second-order optical nonlinearity

Second-order nonlinear optical (NLO) materials have been investigated for decades driven by the application requirements of electro-optic modulators for optical communication, optical switches, sensors, terahertz generation and detection. The prominent advantages of organic NLO materials are ultra-fast response and structural versatility, which gives the materials large NLO activity, good processability and other necessary properties for specific applications via suitable molecular engineering. In this review, historic and recent studies of organic dipolar chromophores in acentric alignment affording macroscopic anisotropy are overviewed. In particular, recent marked progress in the development of acentric bulky materials, including isolating chromophores to prevent centrosymmetric alignment, is focused on. We also review fascinating methods for the ordered alignment of chromophores, which include supramolecular interaction, light fields, electric fields and so on. It is hoped that better chromophore molecular structures and film construction techniques can be devised using suitable NLO films to achieve higher performances in optoelectronic devices.