Design,Syntheses And Characterizations Of Some New Mid-Far IR Nonlinear Optical Crystals

Nonlinear optical（NLO）crystals play a pivotal role in wavelength-tunable laser systems that have widespread applications in military and civilian fields.The development of NLO crystals in the ultraviolet（UV）and visible regions has reached a mature stage,and the focus has shifted towards deep-UV and mid-far infrared（IR）NLO crystals,which are considered the current research hotspots in the field.Therefore,this paper focuses on designing and synthesizing novel mid-far IR NLO crystals,and conducting the following innovative research.（1）The design,synthesis,large size crystals growth and characterizations of mid-IR oxide Bi₃₂Cd₃P₁₀O₇₆The new non-centrosymmetric（NCS）and polar metal phosphate,Bi₃₂Cd₃P₁₀O₇₆（BCPO）,was synthesized by coalescing Bi³⁺and Cd²⁺cations in phosphate system.The incorporation of Bi³⁺with stereochemically active lone pairs（SCALP）and Cd²⁺with d¹⁰ electron configurations into structures enhances the generation of large second harmonic generation（SHG）responses.Furthermore,these relatively heavy cations are beneficial for expanding the IR transmission region.As a result,BCPO exhibits a large SHG response（4×KDP）and wide transmission range（0.36-4.90μm）.In addition,large size single crystals were successfully grown,and evaluated the physical properties.The comprehensive analysis revealed that BCPO is a multifunctional crystal material.（2）The design,syntheses and characterizations of far-IR NLO crystalsThe commercially available IR NLO materials,AGS and AGSe,contain the transition metal Ag in their structures,which hinders the attainment of wide band gaps resulting in low laser damage thresholds.To address this issue,two new compounds,AE₂Sn Se₄（AE=Sr,Ba）,were synthesized by introducing alkaline earth metals into a novel ternary system.Despite the similarity in composition and stoichiometric ratio,they crystallize in different space groups.Notably,the structural difference between them arises from the variation in the coordination environment of AE cations and the different connection modes of AE-Se polyhedra.In addition,it is an effective approach to controlling the balance between SHG and band-gap by incorporating multiple types of anions into the structures.Inspired by this,we selected Pb Br₂ as the parent template and partially substituted Se^2-for Br^-to synthesize a new NCS lead chalcohalide,Pb₄Se Br₆,which displays a significant SHG response（1.3×AGS）,wide IR cutoff edge（～25μm）,and high laser damage thresholds（10×AGS）.Moreover,a new salt-inclusion chalcogenide-halide[Ba₄Br₂][Zn Ga₄Se₁₀]was successfully synthesized in the Ba-Zn-Ga-Se-Br system.This compound features an alkaline earth metal Ba²⁺and halogen Br^-that support wide band gaps,while Zn Se₄ and Ga Se₄ tetrahedral groups contribute significantly to large SHG responses.As a result,[Ba₄Br₂][Zn Ga₄Se₁₀]achieves ideal performances balance between SHG and band-gap.（3）The design,syntheses and characterizations of new oxy-chalcogenidesIt is an effective approach to controlling the performances of material by introducing oxygen ions with highly electronegativity.A CS compound Ba₃[Ge S₄][S]featuring a single anionic group was chosen as the structural template.Employing an isovalent ion substitution strategy,three compounds comprising mixed-anionic groups were designed and synthesized,Ba₃[CO₃][Sn Q₄]（Q=S,Se）and Sr₃[Sn OSe₃][CO₃].Due to the contribution of highly distorted[Sn OSe₃]groups and planesπ-conjugated[CO₃],Sr₃[Sn OSe₃][CO₃]displays a significant SHG response（～1×AGS）,ample birefringence（0.12@1064 nm）,wide band-gap（3.46 e V）,and high laser damage threshold（～10×AGS）.（4）The crystal growth and 266 nm laser output of UV NLO crystal Rb₃Ba₃Li₂Al₄B₆O₂₀FIn addition to the above work,by optimizing the flux selection,a high-quality and large-size single crystal of Rb₃Ba₃Li₂Al₄B₆O₂₀F（RBLABF）was successfully grown.The crystal devices were cut and polished to evaluate the physical properties.The results indicate that RBLABF exhibits excellent comprehensive performances,including short PM wavelength（262 nm）,large NLO coefficient(0.46 pm V^-1),small walk-off angle（0.741@532 nm）,and large acceptance angle（10.56 mrad·mm@532nm）.Besides,the RBLABF crystal device achieved the 266 nm laser output by direct FHG of the Nd³⁺:YAG laser（1064 nm）.