Exploration Of New Mid-infrared Nonlinear Optical Materials Containing Transition Metals

The development of infrared?IR?technology has promoted the development of IR optical materials.The study of IR nonlinear optical?IR NLO?materials is at the forefront of these infrared optical materials.Generally,a NLO crystal with excellent performance in addition to having a noncentrosymmetric?NCS?structure must meet the following basic requirements:large NLO efficiency,wide transparency region,high laser-induced damage threshold?LIDT?,moderate birefringence for phase matching,good chemical stability,and easy-to-grow large crystals.For decades,numerous IR NLO materials have been found,however,only a few of which,namely,AgGaS₂?AGS?,AgGaSe₂?AGSe?,and ZnGeP₂?ZGP?,are commercially available due to their wide IR transparency.These materials suffer from intrinsic drawbacks,such as low laser damage threshold,which significantly hinder their high-power applications.Therefore,the discovery of superior mid-IR NLO metal chalcogenides is still a big challenge mainly due to the difficulty of achieving a good balance between the NLO effect and LIDT.In this thesis,we introduce a tetrahedral structural unit of MQ₄?M=Ge,Sn;Q=S,Se?with large distortion capability into the compound by high-temperature solid-state reaction in evacuated closed silica tubes,and obtain five novel chalcogenides,namely,SrCdSnS₄?1?,SrCdSnSe₄?2?,Sr₂CdGe₂S₆?3?,Ba₃CdGe₃S₈O₂?4?,Ba₃MnGe₃S₈O₂?5?.The synthesis,crystal structure and optical properties of these compounds have been studied and discussed.The main achievements as follows:1.Two new infrared?IR?nonlinear optical?NLO?chalcogenides,SrCdSnQ₄?Q=S,Se??1,2?,were obtained by mixing NLO-active metal chalcogenide and d¹⁰ metal tetrahedral motifs in the Alkali metal-containing system.The SHG intensities of compounds 1 and 2 are approximately 1.3 and 1.5 times higher,respectively,than those of AGS at the same particle sizes under the 2040 nm laser with type-I phase-matchable behavior.Furthermore,The LIDT of these two compounds are about10 and 5 times that of the benchmark AGS,combining with the congruent melting behavior and wide IR transparency,indicating that they are suitable for high power applications in the infrared region.In addition,the strong NLO efficiency of these two compounds can be ascribed to SnQ₄ tetrahedra with the synergetic enhancement from the large distortion of CdQ₄ tetrahedra through the analysis of crystal structure,shedding light on the exploration of new IR NLO materials with mixed NLO-active and synergetic distorted motifs.2.Based on the Chen's Anionic Group Theory,which stated that the nonlinear coefficient is due to the geometric sum of the microscopic SHG coefficients of the NLO-active anion groups,we introduced oxygen into the chalcogenides.Due to the mixing of sulfur and oxygen anions,their physical and chemical properties are significantly different from those containg sulfur or oxygen anion.In addition,due to the significantly difference between the radius of sulfur and oxygen,compounds having large structural distortion can be relatively easily obtained,which is beneficial to performance enhancement of nonlinear optical material.We obtained three novel oxysulfides 3,4 and 5,which have wide band gaps of 3.15 eV,3.09 eV and 3.23 eV,respectively.Among them,the SHG signal intensity of compound 3 is 0.3 times that of the reference standard AGS,and compounds 4 and 5 exhibit interesting near-infrared luminescence properties.This work indicate that the strategy of mixing different anions is an effective method to obtain novel NLO compounds.