| Second-order nonlinear optical(NLO)crystals as a kind of important functional material have been widely applied in many fields,including the laser frequency conversion,remote communication and signal processing,information storage and national defense military,etc.Up to now,the existing second-order NLO crystal material is mainly used in visible and ultraviolet,and for infrared nonlinear optical crystal materials are still relatively lack.Therefore,finding the NLO materials in the infrared wavelengths with a better performance is currently one of the areas of research focus in the materials science.It is well known that only a noncentrosymmetric structure may possess second-order NLO effects,and correspondingly,major efforts have been paid to the construction of the noncentrosymmetric structures.Therefore,it is necessary to find effective ways to predict the mode of packing pattern of those asymmetric building blocks,as well as the associated NLO properties prior to the experiments.In this work,based on the results of density functional theory calculations,a theoretical method to design the inorganic NLO crystal for the second harmonic generation(SHG)is presented.In this method,a specialized genetic algorithm is developed to search the stable structures of the inorganic crystal with known compositions,and then for the noncentrosymmetric stable structures,the second order nonlinear optical properties can be studied by calculating the corresponding SHG coefficients.Using above method,the crystal structures and SHG effects of two ternary compounds,AgGaS2 and LiAsX2(X= S,Se)have been explored,as well as the quaternary compounds Ba-Ga-X-S.For the AgGaS2 crystal,three kinds of structures with similar energy are predicted,which are corresponding to Pmc21,Pna21 and I-42d space groups,respectively.Among them,the I-42d phase with the so-called chalcopyrite structure is most energetically favorable,and this result is consistent with the experimental finding.According to the calculated results of the linear and NLO properties of three structures,it is clear that the configuration and the packing mode of the[GaS4]5-blocks have obvious effects on the optical properties of AgGaS2,including the transparency range,the phase matchability and the strength of the SHG response.Especially,for the I-42d phase,the static SHG predicted in this work(17.47 pm/V)is in good agreement with the experimental result(1812.7 pm/V).For the LiAsX2(X = S,Se)systems that contain[ASX3]3-building blocks,besides the experimental structure in the Cc phase,two other stable structures with Pm and Pmc21 space groups are found by our method.It is noted that although the conformations of the ?1[AsSe2-]1D chains are observed in three crystal structures,the orientations and the packings of asymmetric[AsSe3]3-units are different.The results of the linear optical properties show that this kind of compound exhibits very strong optical anisotropy,and consequently,the corresponding birefringence is very large.As for the NLO properties,the Pm structure shows the strongest SHG response.This is caused by the fact that in the Pm structure,the dipole moments of the[AsSe3]3-groups align in the same direction,to give a polar structure without cancellation of the individual moments,and thus the maximum magnitude of the total microscopic dipole can be achieved.We also try to investigate the structures and optical properties of two quaternary chalcogenide compounds,Ba4GaSbS7 and Ba3GaBS6.Our results indicate two compounds are crystallized in P1 space group,and they have good transparency in the infrared region(IR).The main feature of Ba4GaSbS7 is the large band gap,suggesting the large laser-induced damage threshold of this crystal.Additionally,the birefringence of Ba4GaSbS7 is also suitable and the corresponding SHG intensity is close to AgGaS2.For Ba3GaBS6 crystal,it exhibits remarkable SHG response in the IR region,and the maximum SHG coefficient is predicted to be about 200 pm/V,which is about 10 times of AgGaS2 crystal. |
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