Exploration Of New Organic Nonlinear Optical Crystals For Terahertz-wave Generation

Organic nonlinear optical(NLO)crystals are of significant advantages in terahertzwave generation because of their large nonlinear optical coefficients,unlimited structure design possibility,and weak absorption coefficients in the terahertz region.This dissertation mainly includes design,synthesis and crystal growth of several organic nonlinear optical crystals.The main work and experimental results are as follows:1.Benzothiazolium-based organic crystalsThe hydroxyl group was introduced into the benzothiazlium system as a classic cationic electron donor and OHB-T crystal was synthesized,which belongs to the orthorhombic system,the Pca21(No.29)space group and its powder second harmonic generation(SHG)effect is 0.3 times that of OH1 crystal(d333 = 120 ± 10 pm/V@1.9 ?m)under 2.09 ?m fundamental light.For the first time,hydroxyl and methyl groups were introduced into the system as cationic electron donors,and selecting different anionic groups to regulate molecular orientation and crystal structure.Both HDB-T and HDB-CBS crystals belong to the monoclinic system,P21(No.4)space group.Their SHG effect are about 1.5 times and 0.4 times that of OH1 crystals,respectively.By studying the crystal structure of the compounds in this system,the relationship between the magnitude of the nonlinear effects and the arrangement of the molecules in the crystal was analyzed.Their IR spectroscopy,UV-vis spectroscopy and thermal properties were also characterized.Moreover,widely tunable monochromatic terahertz(0.1-20 THz)in HDB-T via difference frequency generation(DFG)were firstly realized,which proves that this crystal has potential application prospects.2.Indolium-based organic crystalsBy comparing the electron-withdrawing abilities of pyridium,quinolium,benzothiazolium,and indolium,the indolium group was introduced as a new group to construct nonlinear optical crystals.The classic hydroxyl group was used as the cationic electron donor,and different anions were selected for regulating molecular arrangement,Three indolium-based crystals of OHI-BS,OHI-CBS and OHI-T were designed and synthesized,their single crystals were grown,respectively.OHI-BS and OHI-CBS both crystallized in monoclinic system,P21/c(No.14)centrosymmetric space group,and don't exhibit nonlinear optical effect.OHI-T crystal belongs to monoclinic system,Cc(No.9)space group.The effect of different microstructures on the cation arrangement pattern was discussed.The powder SHG intensity of OHI-T is 0.7 times that of OH1 crystal.OHI-T crystal also exhibited a wide transmission range from 504 nm to 2100 nm,a higher melting temperature Tm=248 ?,and no centrosymmetric hydrate,indicating its excellent physicochemical properties.The slow evaporation method was used to grow OHI-T single crystals that meet the test size and quality.The 0.1-20 THz ultra-wide tunable single-frequency terahertz wave output was achieved by the difference frequency method,which proved that the newly discovered indolium system and OHI-T have potential application prospects in terahertz wave output.