| In the direction of generation and detection of terahertz（THz）,nonlinear optical crystals have broad application prospects,which are due to their easy phase fitting in the terahertz band,uncomplicated growth process,more diversified designable structures,large nonlinear optical coefficients and many other advantages.The core content of this paper were designed,synthesized and grown a number of new organic nonlinear optical crystals in the benzindole system.The core research conclusions can be clarified as follows:1.Design,synthesis and characterization of new benzoindolium-based NLO crystals.Based on the benzoindolium group as the electron acceptor,the oxygen-containing groups as electron donor were introduced into different substitution sites in conjugation system,and three new iodide benzoindolium based compounds,namely,MBI(C24H24NOI,Cc)、m-OHBI(C23H22NOI,P21)and p-OHBI(C23H22NOI,P21/c)were successfully designed and synthesized.By studying the crystal structures,the contribution of oxygen-containing electron donor group in inducing non-centrosymmetric arrangement was analyzed.The optical band gaps of MBI,m-OHBI and p-OHBI are 2.08 e V,2.19 e V and 2.02 e V measured by UV-visible diffuse reflection,and the three crystals have high environmental stability.Notably,MBI and m-OHBI crystals with non-centrosymmetric structure show large nonlinear optical response,and their powder second-order harmonic generation intensity are 0.8 times and 0.6 times that of OH1(d33=120 pm/V at 1.9μm)crystals,respectively.2.Crystal growth and THz generation of new benzoindolium-based NLO crystals.By comparing the solubility of single/mixed solvent,and crystal morphology and quality,the suitable solvents system for MBI and m-OHBI crystal growth were obtained.By optimizing the growth conditions,bulk MBI crystals with high optical quality and maximum size of 20×14×8 mm3 were grown in methanol.Flake-shaped m-OHBI（8×4×1 mm3）single crystals were successfully grown in the mixed solvent of methanol and acetonitrile（1:1）.The MBI prototype device with an optical aperture of 5 mm was obtained,and the THz wave radiation with ultra-wide tuning range（2～18 THz）was realized by differential frequency generation method for the first time. |
