Study On The Growth And Mechanism Of DAST Crystal

The research of nonlinear optical materials has attracted much attention since the invention of laser.It is of great importance in basic research and radio communication,safety detection,optical computing and other fields which have the value of potential application.Especially in recent years,terahertz(THz)technology has received widespread international attention in the detection for its important role and broad application prospects in imaging,biomedical science,safety monitoring,military,drug testing,biological,scientific research and other fields.Therefore,it is very important to find a material with a high nonlinear coefficient to realize THz radiationThe organic nonlinear crystals are received great attention because of its high nonlinear coefficient and photoelectric coefficient,low dielectric constant,dispersion and fast response,width application band,easy cutting,low cost and other advantages.Due to the unique chemical structure of their molecules,molecular design flexibility and functional diversity,Organic pyridine salt crystals not only has large molecular hyperpolarizability,the diversity of molecular assembly also greatly enriched the nonlinear optics.The type of material makes it an important place in the field of nonlinear optics.In 1989,Marder et al,"Science" journals reported a DAST crystal with second order nonlinear optical properties for the first time.As an organic pyridine salt,DAST crystal formed by strong Coulomb force,the cation has a conjugated large ? bond,no symmetric center.DAST crystal has a large second-order nonlinear optical coefficient and electro-optic coefficient.The second-order nonlinear coefficient of DAST crystals at 1542 nm is 840 pm/v.The electro-optic coefficient is 75 pm/v at 820nm,which is 1-2 orders of magnitude larger than the corresponding value of ZnTe,and its multiplier effect is 1000 times of carbamide,which is the ideal material for terahertz radiation.In 1999,Kawase et al.produced terahertz waves by DAST crystals using a nonlinear difference frequency method.The main growth methods of DAST crystal are laser induced nucleation method,inclined plate method,double zone method,seed crystal method and so on.Foreign scholars for DAST crystal research mainly focused on improving the growth rate and improve the crystal quality.Domestic research on DAST crystals started earlier,but the development of slow,whether the crystal quality or crystal size are relatively backward.The main reason is that it is difficult to get high-purity powder materials.Another reason is that DAST crystal growth process and nucleation difficult to control,it is difficult to grow large crystals.This topic will start from the three aspects to start research the DAST crystal:Powder material synthesis,crystal growth and growth mechanism.The synthesized materials were detected by X-ray powder diffraction?1HNMR and 13CNMR.The results show that the powder material is DAST.The thermal stability of raw materials was studied and TG-DTA curve reflects that the phase transition temperature of DAST raw material is 263.76? which is consistent with the literature.The purity was 97%by elemental analysis and could be used for crystal growth.The raw material is recrystallized and purified,and after a recrystallization,the purity is about 98.6%,but the purity is slightly decreased after two times of purification.The crystals can be grown by volatile solvent method and cooling method respectively.The crystal integrity,transmittance,crystal morphology and growth mechanism of DAST crystals were investigated under different conditions.It is found that:?the supersaturation a required for crystal growth is close to the boundary of the metastable zone,which is the main reason for the difficult growth of DAST crystals.?The regularity of the growth of the DAST crystals is obvious anisotropy,in the order of(001)>(010)>(111).? The crystal quality of the growth method is lower than that of the crystal.The higher the cooling rate,the better the crystal quality and transmittance.The crystal quality of the purified raw material is obviously improved and the transmittance is higher.? cooling method to grow the crystal,the saturation point of 45?,0.05 ?/day cooling rate of the most suitable for crystal growth.The morphologies of the crystals of different growth methods were studied,and the anisotropy of crystal(001)growth rate was explained by PBC theory.The(001)plane of growth crystal was observed by AFM,and the dislocation grrowth mechanism and two-dimensional nuclear growth mechanism were found.