Stannous Chalcogenide-type Infrared Nonlinear Optical Crystals

Mid-infrared nonlinear optical(NLO)materials inspire strong research interest attributable to their capability of generating coherent infrared light sources with tunable frequencies,which can be widely applied in many fields,such as infrared remote sensing,environmental monitoring,industrial process controls and minimally invasive medical surgery.Up to now,chalcopyrite-type AgGaS2(AGS),AgGaSe2(AGSe)and ZnGeP2(ZGP)have become the extensively commercialized infrared NLO materials,but they cannot meet the growing market demand due to their inherent defects.Chalcogenides have large second-harmonic generation(SHG)responses and wide transmission ranges,providing a broad platform for exploration of infrared NLO crystals with excellent performance.Introduction of stereochemically active lone pair tin(?)into structure may be favorable for constructing asymmetric structure units with great contribution to microscopic second-order polarizability,and different coordinations of tin(?)and chalcogens lead to rich structures.Guided by ideas abovementioned,we successfully synthesized a series of noncentrosymmetric stannous chalcogenides with strong SHG responses by high temperature solid-state methods.Then,their NLO performance is systematically studied experimentally and theoretically.1.A ternary sulfide Sn2Ga2S via combination of tin(?)and[GaS₄]tetrahedron crystallizes in the orthorhombic Pna2I space group.Its three-dimensional(3D)structure is built by[SnS3]triangle pyramid and[GaS₄]tetrahedron units,which can be deemed as the 1:2 combination of Cc Ga2S3 and Pnma SnS.Sn2Ga2S5 demonstrates obvious NLO activity,around a maximum of 1.6 times that of AGS and a strong laser-induced damage threshold(LEDT)of 9.7 times that of AGS.It represents the first NLO compound in the M?n2-M?2-Q5(M?=divalent Ca,Sr,Ba,Pb,Sn,and Eu;M?=B,Al,Ga,and In;Q=S and Se)family.Theoretical calculations show that Sn2Ga2Ss cannot achieve phase matchability due to excessive birefringence and the NLO coefficient is 1.1 times that of AGS,which are in accordance with the experimental results.The non-phase matchable behavior of Sn2Ga2S5 excludes its applicable potential.Next,we can tune the arrangement of NLO-active units to adjust birefringence to a suitable value,thus achieving phase matchability.2.Continuing our efforts to investigate phase-matchable stannous chalcogenides infrared NLO materials,we successfully synthesized three thiophosphates SnPS3,SnPS2.86Se0.14 and SnPSe3.They all crystallize in the monoclinic noncentrosymmetric space group Pn.The 3D network is constructed by[SnQ8](Q=S,Se)polyhedra and[P2Q6](Q=S,Se)bicapped trigonal prisms,among which[Sn(1)Q8]hendecahedra and[Sn(2)Q8]dodecahedra connect with each other by sharing vertices and edges.The isolated[P2Q6]polyanions are filled in the cavities in the 3D network.SnPS3 and SnPS2.86Se0.14 exhibit wide transparencies with optical band gaps of 2.35 and 2.13 eV,respectively.At present,as the Pn structure of SnPSe3 can only be obtained under low temperature,and its room-temperature structure is centrosymmetric,we did not evaluate its NLO performance.The powder SHG measurements indicate that SnPS3 and SnPS2.86Se0.14 exhibit phase-matchable SHG responses of approximately 1.1 and 1.2 times that of AGS,respectively,which are consistent with the theoretical calculation results.Dipole moment calculation result suggests that[SnQ8]polyhedra make main contribution to their excellent NLO performance.3.Based on the results of 1 and 2,it is necessary to further improve the NLO performance of Sn(?)-based chalcgonegieds.After hard efforts,a chalcohalide Sn7Br10S2 was obtained.It crystallizes in the hexagonal P63 space group,and there are two types of[SnL8](L=Br,Br/S)hendecahedra interlaced to form cavities along with 63 helical axis,which are occupied by the Sn(Br/S)7 octahedra built chains.Compared with the former two types of Sn(?)-based chalcogenide NLO compounds,Sn7Br10S2 shows an improved phase-matchable SHG response of 1.5 times that of AGS and an enlarged band gap of 2.59 eV,as well as a wide transpancy of 0.48-25?m.Besides,its structure takes Pb7Br10S2 as the prototype,which can be derivated by replacing lead(?)with tin(?)to realize structure transformation from centrosymmetry to non-centrosymmetry.This type of strategy to build noncentrosymmetric structure can provide ideas for designing more potential NLO-active chalcohalides.