Exploration Of Some New Mid-IR Second-Order Nonlinear Optical Crystal Materials

Nonlinear optical (NLO) crystals have played a very important role in laser technology, and have been widely used in laser frequency conversion, optical information production, processing and transmission, national defence and so on. Nowadays, the current mid-infrared (mid-IR) NLO crystals are mainly chalcopyrite compounds that suffered from low laser damage thresholds (LDT), and their applications are therefore heavily hindered. Many factors may cause laser damage threshold, and it is believed that small band gaps of the semiconductor chalcogenides are intrinsic reason for their low LDT. Therefore, the search for new mid-IR NLO crystals with large band gaps has become one of the great challenges in this field.One of our lab's research interests is to explore the new IR NLO crystal materials with high LDT. The main candidate compounds are halides and oxysalts. There are four halogen elements for choice, so halides usually have many different compositions, variable structures and properties, especially they tend to have the relatively larger band gaps which are beneficial to improve the LDT. Many halides can be dissolved in water and some common organic solvents, so we can grow their crystals in solution. Oxysalts also have various types of structural distortions. They usually have a higher thermal stability and wider band gap. If the mass of the selected central element is relatively heavier, they will have a wider transparent window in mid-IR region.In this thesis, with the guidance of the Anionic Group Theory, we have pursued research to explore the new mid-IR NLO crystals from halides and oxysalts. We choose our target compounds mainly by molecular design, and then synthesize the target compounds, obtain their single crystal structures and measure their NLO properties and the other related properties. As the results, we have obtained some new potential mid-IR NLO crystals with good comprehensive properties. In addition, we have deepened our understanding on the relationship of composition, crystal structure and various properties.The thesis is divided into seven chapters and the main contents of each chapter are as follows:In chapter1, some basic definitions, measurement methods and the main theory are introduced, and the research progress of NLO crystals, especially the mid-IR NLO crystal materials are reviewed. And then, the main idea of this thesis is provided.In chapter2, the d10transition metal cation Hg2+is chosen as the central element. A new potential mid-IR NLO crystal material, HgaBrsI, is synthesized. Its crystal structure and composition are confirmed by single crystal diffraction and EDX analysis. Its NLO property and some other related properties are tested. The relationship between the crystal structure and NLO property is also discussed. Its powder SHG effect is about0.7times that of KTiOPO4(KTP). The optical band gap is2.76eV and the transparent range of the powders is0.45-38um. Its thermal decomposition temperature is about180?. The first-principles method is used to study electronic structure and NLO property of the compound.In chapter3, in order to obtain the material with the larger second harmonic generation (SHG) response while maintaining the advantages wide band gap of Cs2HgCl2I2, the atoms Cl of Cs2HgCl2I2are replaced with the atoms Br. On the other hand, the research is not limited to Cs-Hg-Br-I system, it is extended to Rb-Hg-Br-I system. As a result, six new mixed halides Rb2Hg2Br2I4·2H2O, A2Hg2Br2I4·H2O (A=Rb, Cs), Cs2HgBr2I2and AHg5Br9I2(A=Rb, Cs) are synthesized. Rb2Hg2Br2l4-2H2O and A2Hg2Br2I4·H2O (A=Rb, Cs) are all noncentrosymmetric (NCS) structures with the same space groups. The powder SHG effect of Rb2Hg2Br2I4-2H2O is about4times that of KH2PO4(KDP). The optical band gap is2.65eV and the transparent range of the powders is0.47-2.7,2.8-6.0and6.5-44?m. Rb2Hg2Br2I4H2O and Cs2Hg2Br2I4-H2O have the same structures, the powder SHG effect of Cs2Hg2Br2I4·H2O is about6times that of KDP. Their optical band gaps are2.71eV and2.72eV, respectively. The transparent ranges of their powders are0.46-2.7,2.8-6.0and6.5-45?m and0.46-2.7,2.8-6.0and6.5-44?m, respectively. All of these three compounds'thermal decomposition temperatures are about100?. The other three new mixed halides (Cs2HgBr2l2, RbHgsBr9I2and CsHgsBr9I2) don't contain crystalline water. They are all of centrosymmetric (CS) structure. Although they don't possess the NLO property, their optical band gaps, IR transmission ranges and thermal stabilities are still tested, and the relationship between structure and properties is also discussed for the consideration of systematic study about the series.In chapter4, two compounds with similar composition and structure, namely KHgBr3-H2O and KHgClBr2·H2O, are deeply studied. The former has been reported before by our group. It was re-synthesized by using different solvent in this chapter. The latter is a new compound. Crystal structure analysis showed that they have similar crystal structure with the same space group Cmc21and the anionic groups are almost in the same arrangement. The difference is their elemental composition. The powder SHG effect of KHgBr3·H2O is as strong as KTP. Its optical band gap is3.31eV, and it has a wide transparent range (0.38-2.7,2.9-6.0and6.5-25?m). The powder SHG effect of KHgClBr2·H2O is about0.6times that of KTP. Its optical band gap is3.37eV, and it also has a wide transparent range (0.37-2.7,2.9-6.0and6.5-24?m). Their thermal decomposition temperatures are about110?. By comparing their related properties, the influence of different halogen atoms on the properties is further elucidated as that when the halogen atom with larger electronegativity is replaced (or partially replaced) by the halogen atom of smaller electronegativity, the NLO effects are generally weakened, but the band gap and LDT will be improved.In chapter5, the NLO property of K2SbF2Cl3is studied for the first time. It is synthesized by hydrothermal reaction. Its crystal structure is retested by X-ray single crystal diffraction. Its powder SHG effect is about4times that of KDP. The relationship between the crystal structure and NLO property is discussed. The optical band gap is4.01eV and the transparent range of powder is0.31-14?m. Its thermal decomposition temperature is about180?. The reason for the different NLO coefficients between two compounds Na2SbF5and K2SbF2Cl3that have the similar composition and structure is analyzed with theoretical calculation.In chapter6, two cations W6+and Se+, that are prone to second-order Jahn-Teller (SOJT) distortion are selected as central ions. A new NCS oxysalt, K2W3Se0i2, is synthesized by hydrothermal reaction. Its crystal structure and NLO property are measured. Its powder SHG effect is about9times that of KDP. The optical band gap is3.24eV and the transparent range of its powders is0.39-10?m. It has a good thermal stability with the thermal decomposition temperature of500?.In chapter7, two new NCS compounds, K2BiI5O15and Rb2BiI5O15, are synthesized by hydrothermal reactions. A novel [I3O9]3-bridge is found in iodates for the first time. Their NLO properties and some other related properties are tested. The relationship between the crystal structure and NLO property is discussed. Their powder SHG effects are about3times that of KDP. The transparent ranges of their powders can reach up to12?m. Their optical band gaps are3.50and3.53eV, respectively. The LDT of their powders are84and72MW/cm2(1064nm,5ns), respectively, that are about15times that of AgGaS2at the same conditions. Both compounds have good thermal stability and thermal decomposition temperatures are450?. The relationship between the electronic structure and properties is discussed with theoretical calculation.