| Infrared lasers have important applications in environment monitoring,medical treatment,civil and military fields.Herein,infrared nonlinear optical(IR NLO)crystals are fundamental optical devices.Specially,IR NLO crystals can be used to produce wide band tunable infrared laser output by optical parametric amplification and optical parametric oscillation technology in IR All-solid-state lasers.At present,commercially applied IR NLO materials are Ag Ga S2,Ag Ga Se2 and Zn Ge P2 crystals.They have excellent optical properties such as large nonlinear optical coefficients and wide medium infrared transmission ranges.However,with the expansion and development of laser applications,the requirements for IR NLO optical crystals are becoming higher and higher.Some defects such as low laser damage threshold of Ag Ga S2 crystals,severe two-photon absorption of about 1?m wavelength lasers for Ag Ga Se2 and Zn Ge P2crystals hints their further applications.To avoid these problems,new IR NLO candidates should obtain wider bandgap,besides,the SHG responses should stay at the level of Ag Ga S2.Up to now,few candidates for next-generation IR NLO crystal materials are discovered and developed by the domestic researchers independently.Therefore,it is of great research value to search for new materials with promising application as IR NLO crystals.In this dissertation,chalcogenides instead of oxides are chosen as the target system to investigate new IR NLO crystals because oxides have intrinsic absorption and can not penetrate at longer wavelengths in mid-IR while chalcogenides have no such problems and are suitable for the exploration of new materials suited for IR transmission.Moreover,it has been found that introducing the alkali metals and the alkaline earth metals into the chalcogenide system could make the obtained compound wide bandgap,which further could realize high laser-induced damage threshold.Therefore,this dissertation aimed at the following systems,namely,the single alkali or alkaline earth metal sulfides,the di-alkali metals sulfides and the alkali-alkaline earth metals mixed selenides to search for new IR NLO materials.The preparation method of crystals is high temperature flux-spontaneous crystallization in vacuum and sealed-silicon tubes.The characterization includes UV–vis–NIR diffuse reflection,IR spectrum,Raman spectrum,second-harmonic generation(SHG)at 2?m and theoretical calculations such as the electronic structure,density of states,SHG coefficients and SHG density et.al.During the study,two lithium sulfides Li2Zn Si S4 and Li2Cd Si S4,one lithium barium selenide Li2Ba Si Se4 and one lithium cesium sulfide with non-centrosymmetric structures are obtained.Besides,two new compounds Li3Ge3Se6 and Ba4(BS3S)2S4 with centrosymmetric structures are obtained.Herein,four materials Li2Zn Si S4,Li2Cd Si S4,Li2Ba Si Se4,and Li2Cs4Ge2S5(S2)Cl2have been proved experimentally and theoretically to show obvious NLO properties and free of absorption in mid-IR region.In addition,Li2Zn Si S4,Li2Cd Si S4 and Li2Ba Si Se4all present wider bandgap than commercial materials and achieve a performance balance between wide band gap and large second harmonic generation under preliminary evaluations,showing great application prospect.Since the obtained Li2Zn Si S4,Li2Cd Si S4 and Li2Ba Si Se4 compounds belong to the A2IBIICIVD4VI(A=Li,Na,Cu,Ag;B=Sr,Ba,Zn,Cd,Hg;C=Si,Ge,Sn;D=S,Se)family,we summarized and compared the structures and properties of 36 compounds in this system,and concluded that MQ4(M=Si,Ge,Sn;Q=S,Se)is an important SHG contributor in the structure,and Si Q4 and Ge Q4 are more preferred because a larger optical band gap can be obtained from Si/Ge Q4 but Sn Q4,and the SHG effect can remain at a level of 1×Ag Ga S2.This is in line with the performance requirements of new IR NLO optical crystals.On the other hand,three compounds Li2Cs4Ge2S5(S2)Cl2,Li3Ge3Se6 and Ba4(BS3S)2S4 exhibit novel structures.The[Ge2S5S2]2-unit in Li2Cs4Ge2S5(S2)Cl2,?[Ge Se2]n chain formed by ethylene-like[Ge2Se6]units in Li3Ge3Se6 and the[BS3S]3-unit in Ba4(BS3S)2S4 are all discovered for the first time,which enlarged the structural diversity of sulfur matrix compounds.Notably,[Ge2S5S2]2-and[BS3S]3-motifs have intrinsic non-centrosymmetric structural types.First-principles calculations show that they have very large second-order hyperpolarizabilities,which provides an optimal basis for the design of new materials in the field of NLO materials.The research in this paper not only provides alternative materials for the next generation of new IR NLO crystals,but also provides new functional units for the design of materials in this field.Moreover,the work provides ideas for the selection of elements in the design of new materials.For the development of this field,this work has brought certain progress. |
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