Optimization strategies for comprehensive performance of organic second-order nonlinear optical materials

With the rapid development of optical communication and information processing, second-order nonlinear optical materials have shown great potential applications in optical waveguide devices, such as frequency exchange, signal processing and transmission and data storage. Recently, second-order nonlinear optical materials have also achieved great progress in the fields of biology, sensing and optoelectronics. The second-order nonlinear optical materials studied in the early stage were mostly inorganic crystal or semiconductor crystal materials, whose development is relatively mature and has been commercialized. However, the inherent shortcomings (such as high dielectric constant and half-wave voltage, more brittle crystal and difficult to grow, etc.) make their development be limited. In contrast, organic compounds have the advantages of fast response, low dielectric constant, high optical damage threshold, low cost, and easy to be processed with modified molecular structure. In the early 1980s, Meredith et al. proposed for the first time the concept of polarized polymer electro-optical materials. The materials can exhibit macroscopic second-order nonlinear optical effects due to the non-centrosymmetric arrangement of chromophores regulated by electric poling. Thereafter, poled polymers have attracted extensive attention and flourished in this field, and many organic second-order nonlinear optical materials with excellent comprehensive properties have been designed and synthesized. Unfortunately, no material system has been developed that can simultaneously satisfy the practical requirements of "sufficiently large macroscopic nonlinear optical effects, good stability and low optical loss as possible". The significant contradictions of "nonlinearity-stability" and "nonlinearity-transparency" make it very difficult for organic second-order nonlinear optical materials to meet the above three requirements simultaneously. This review focuses on the comprehensive performance optimization of poled polymer materials. Firstly, we introduce the design and optimization of chromophores, the core construction units of organic second-order nonlinear optical materials and then summarize the strategies to realize the effective transformation from microscopic second-order nonlinear optical properties of chromophores to macroscopic performance of materials. Compared with the host-guest system, side-chain and main-chain organic second-order nonlinear optical polymers were designed to stabilize the orientation of chromophores to a certain extent by using polymer chains; isolation groups are introduced to modify the shape of chromophores to reduce interaction and improve poling efficiency; dendritic polymers with high branched structure were used to improve the spatial configuration; and the design concepts of "suitable isolation group" and "isolation chromophores" are applied to improve the macroscopic second-order nonlinear optical effect and transparency of materials. Also, we describe the effective regulation ways of molecular uniting from the point of view of molecular rational design, which provide important information for thorough comprehension of the relationship between molecular structure and polymer material properties.