Controlled Growth And Optical Properties Of DAST Crystals

Organic nonlinear material 4-N,N-dimethylamino-4?-N?-methyl-stilbazolium tosylate(DAST)has a high nonlinear optical coefficient and electro-optical coefficient,showing extremely strong nonlinear optical properties.DAST has been widely applied in the fields of biomedicine,optical communication and materials science.However,the small size,low crystallinity,and long growth cycle of DAST crystals limit its wide applications.Therefore,the thesis focuses on the studies of two different growths of DAST crystals.Firstly,the DAST single crystal growth was carried out by adding the oleic acid to the solution on the basis of the traditional solution cooling method.Secondly,the DAST microcrystal array was induced to grow rapidly on the ITO electrode with a square photoresist array using an external alternating electric field.In this thesis,the main research results are shown as follows:(1)The DAST was successfully synthesized using a self-made Dean-Stark apparatus,and subsequently the as-synthesized DAST was further recrystallized.The growths of DAST crystals at different concentrations were studied by solvent evaporation method.The results showed that the thickness of DAST crystals increases with the increase of the DAST concentration,showing the growth characteristics along the c-axis.(2)Two different methods for DAST crystal growth were studied,including the traditional solution cooling method and the method of adding oleic acid to the solution based on the conventional solution cooling method.The maximum DAST single crystal grown by the conventional solution cooling method has a size of 5.8×5.5×0.9mm~3 with a regular square shape.In contrast,the maximum DAST single crystal size for the addition of oleic acid growth is 11×8×1.2 mm~3with an irregular hexagon.Comparing the UV-visible spectrum,X-ray diffraction spectrum and etching analysis of the crystal surface of DAST single crystal grown by the two methods.The results show that the addition of oleic acid to the DAST methanol solution can induce the growth of DAST single crystals with high growth crystallinity,few defects,and large size.In addition,the reason for the formation of irregular hexagonal crystals was explained from the viewpoint of the influences of hydrogen bonding on the(010)crystal plane of DAST crystal.The methods and results of this experiment are valuable for future study on the growth of organic single crystals with large size,high crystallinity,and few defects.(3)The conditions for the second harmonic effect(SHG)were theoretically derived.A new method was proposed for inducing the growth of DAST microcrystal array on the ITO surface with square photoresist pattern by using an external alternating electric field.The crystallinity and optical properties of the microcrystalline material on the surface of ITO electrode were characterized and measured.The results show that the introduction of the alternating electric field does not lead to the change of the chemical structures of the DAST molecules,but causes significantly change of the crystallinity of the DAST crystallites.Besides,the reasons for the formation of the DAST microcrystal array were explained.Based on the DAST microcrystalline array induced by electric field,the relationships between SHG and annealing temperature of the microcrystalline array were investigated by SHG measurements.The results show that as the annealing temperature increases,the intensity of the second harmonic of the microcrystal array increases first and then decreases.Particularly,this phenomenon was also explained by the effects of the annealing temperature on the grain sizes and surface defects of the crystallites.Related results will provide a feasible experimental solution for the rapid growth of organic microcrystal arrays.