Novel Multinary Alkali Metal Chalcogenides Derived From AgGaQ₂(Q=S,Se) And Their Applications In High-Power Laser Technology

The frequency conversion of Nonlinear optics(NLO)crystal materials can be realized by the nonlinear processes of frequency doubling,sum frequency,difference frequency,optical parametric amplification and multi-photon absorption.This makes them widely used in civilian,medical,exploratory and military applications.At present,commercialized and relatively successful infrared NLO crystals in the mid-infrared(MIR)band are chalcopyritetype AgGaS2(AGS),AgGaSe2(AGSe)and ZnGeP2(ZGP).They can generate large secondharmonic generation(SHG)responses,but their low laser-induced damage thresholds(LIDTs)and harmful two-photon absorption limit their applications,so they cannot meet the requirements of high-power laser technology.Therefore,in view of the performance shortcomings of these commercial MIR NLO crystals,some strategies should be taken to enhance their LIDTs while retaining large SHG responses,so as to achieve balanced NLO performances to meet the application requirements of high-power lasers.By investigating the literature,we found that introducing alkali/alkaline earth metal ions with strong ionic properties into the structure can widen the band gaps of the materials,and thus improve the LIDTs.In this context,we proposed the following design strategies:based on the structures of AgGaQ2(Q=S,Se)with large NLO coefficients and monocline Ga2Q3 with excellent LIDT,the derivative chalcogenides,namely general formula m[AgGaQ2]·n[Ga2Q3],were designed,and alkali/alkaline earth metals were introduced into the structure to occupy part of Ag sites,for the purpose to obtain the balanced properties of NLO materials.The main research contents and results are as follows:(1)Three quaternary sulfide AAgGa6S10(A=K,Rb,Cs)crystal materials with novel structures were obtained by the high temperature solid state boron-sulfide method,namely,KAgGa6S10(1),RbAgGa6S10(2),and CsAgGa6S10(3).They crystallize in the monoclinic space group Cc(No.9)and are closely related to the structures of AgGaQ2 and Ga2Q3.They can be thought to be derived from a 2:2 AgGaS2 and Ga2S3 interpenetrating network,i.e.,2[AgGaQ2]·2[Ga2Q3],where one Ag position is occupied by an alkali metal A.This series of compounds have the largest band gaps in Ag-containing NLO crystal sulfides.Their powder crystal samples show the SHG responses of 0.5-0.7 times that of AGS(2.1?m).Although phase matching was not achieved,their LIDTs were significantly improved,reaching 1.5,3 and 4 times that of AGS,respectively.Finally,we calculated their electronic structures and optical properties.The band gaps of 1-3 are mainly determined by the S-3p,Ag-4d,and Ga4p orbitals.The Ag-4d orbital of 1-3 contributes relatively little to the valence band due to the introduction of alkali metal cations.(2)Two novel quaternary sulfides KAg3Ga8Se14(4)and KAg3Ga8S14(5)were synthesized by the high temperature solid-state boron-sulfide method.They are crystallized in the monoclinic space group Cm(No.8),and their three-dimensional structures are constructed by two basic tetrahedral units[AgQ4]and[GaQ4].The structure is also closely related to the structures of AgGaQ2 and Ga2Q3,which can be considered as the derivatives of the latter two,through the 4:2 interpenetrating network of AgGaS2 and Ga2S3,namely 4[AgGaQ2]·2[Ga2Q3].in which one Ag position is occupied by alkali metal K.The NLO effect of KAg3Ga8Se14 is 0.6 times that of AGS,and phase matching is also achieved.In addition,its LIDT is two times that of AGS.Differently,KAg3Ga8S14 shows non-phase-matching behavior and exhibits the largest NLO response at 75-110?m particle sizes,approximately 0.8 times larger than that of AGS(2.1 ?m).This work is the first report of the NLO effect in ordered chalcogenides in the A-Ag-Ga-Q system.Finally,the electronic structure and optical properties of 4 and 5 are studied by theoretical calculations,which are consistent with the experimental results.(3)K2Ba0.5Ga9O2S13(6,P6,No.174)was synthesized by a high temperature solid-state method.There are two NLO active units[GaS4]and[GaOS3]in its structure,forming two windmill clusters[Ga(2)3S10]and[Ga(1)3OS9],respectively.Its structure is also closely related to the structures of AgGaS2 and Ga2S3,which can be considered to be derived from the 3:3 AgGaS2 and Ga2S3 interpenetrating network,i.e.3[AgGaQ2]·3[Ga2Q3].In which,two Ag positions are occupied by alkali metal K and alkaline earth metal Ba,respectively,and part of S was replaced by O element.The maximum SHG responses of K2Ba0.5Ga9O2S13 under 1.064?m and 2.1 ?m laser are 2 times of KH2PO4(KDP)and 0.5 times of AGS,respectively.In particular,its LIDT is significantly improved,up to 17 times that of AGS.Finally,we make a systematic theoretical calculation of 6,and illustrate the main NLO contribution units through dipole moment calculation.The results show that the windmill cluster[Ga(2)3S10]contributes more to the SHG response.