Study On New Second-Order Nonlinear Optical Crystal Materials

Nonlinear optical (NLO) crystal materials are one of key materials in high technology. These NLO crystal materials have been widely used in laser frequency conversion, optical communication, optical storage, infrared monitoring, and national defense. With the increasingly demand of applications, the search for new second-order NLO crystal materials with good comprehensive properties is of great importance and has become one of the great challenges in the field of materials science.Halides and oxides (including oxysalt) usually show comparatively large band gaps evidenced by their insulating characteristics, and possess high laser damage threshold and good evirvonmental stability. They are important alternative compounds in exploring new NLO crystal materials. Based on the Anion Group Theory and from the perspective of molecule design and crystal structure selection, this thesis chooses suitable halides and oxysalt as the research targets, synthesize those compounds and investigate the NLO property, band gap, transparent region and thermal stability of those compounds. The electronic structures of those compounds are calculated by using first-principles method. The relationship between the composition, crystal structure of those compounds and their NLO property is also discussed. We hope to obtain new second-order NLO crystal materials with good comprehensive properties, and to provide new information for exploration of new NLO crystal materials in future.The thesis contains seven chapters and the content of each chapter is as follows:Chapter One introduces the basic definitions and the main theory and methods for the study of inorganic NLO crystal materials, and reviews the research progress of the inorganic NLO crystal materials, especially the NLO crystal materials in the IR region. The main ideas of the thesis are also outlined.Chapter Two investigates the compound Hg2Brl3as a new potential second-order NLO crystal material in the IR region. To build new noncentrosymmetric molecule, the single halogen in the anionic group MXn is replaced by mixed halogens (X and Y). The synthesis, crystal structure and other properties are reported. Crystallographic analysis reveals that what we obtained is Hg2BrI3which belongs to the orthorhombic system with Cmc21(No.36) space group. The compound shows phase-matchable second harmonic generation (SHG) of about1.2times as strong as that of KTiOPO4(KTP) based on the powder SHG measurement. The relationship between the crystal structure of the compound and NLO property is also discussed. It exhibits a wide transparency (from0.5to30μm) and a good thermal stability (up to200℃). The band gap derived from UV-Vis absorption spectrum is2.6eV. The first-principles method is used to study electronic structure of the compound.Chapter Three chooses the compound KlO2F2as the research target. The synthesis, crystal structure and second-order NLO property of the compound are reported. The compound shows phase-matchable second harmonic generation (SHG) of about4times as strong as that of KH2PO4(KDP) based on the powder SHG measurement. Its crystal is transparent in the region from0.3to11.6μm and is thermally.stable up to400℃. The experimental band gap is4.2eV. The electronic structure, the birefringence and the SHG coefficients are also calculated by using first-principles method. The relationship between the structure and the NLO property is also discussed.Chapter Four reports two new potential second-order NLO crystal materials, A4CdV5O15Cl (A=K、Rb). With guidance of the second-order Jahn-Teller (SOJT) distortion theory, the d10transition metal cation Cd2+and the d0transition metal cation V5+are selected to create the distorted polyhedron and benefit to produce macroscopic NLO effect. The compounds A4CdV5O15Cl are synthesized by solid state reaction. K4CdV5O15Cl shows non-phase matchable second harmonic generation (SHG) of about0.8times as strong as that of KTP based on the powder SHG measurement. Their powder are transparent in the region from0.6to10μm and are thermally stable up to700℃. Both of their band gaps derived from UV-Vis absorption spectrum are2.3eV. The electronic structure and the SHG coefficients are calculated by using first-principles method.Chapter Five investigates two new potential second-order NLO crystal materials, Li2MTeO6(M=Ti、Sn). The Ti4+of d0electron configuration or Sn4+of d10electron configuration is mixed into Te6+oxides system. These cations and oxygen atoms are easy to form distorted polyhedron. The compounds Li2MTeO6are synthesized by solid state reaction. The powder SHG of Li2TiTeO6and Li2SnTeO6are about5and3times as strong as that of KDP, respectively. The relationship between the crystal structure of those compounds and NLO property is discussed. The powder of Li2TiTeO6and Li2SnTeO6are transparent in the region from0.4to11.6μm and0.4to12.5μm, respectively. They also exhibit a good thermal stability (>780℃). The band gaps derived from UV-Vis absorption spectrum are both3.6eV. Band structure, densities of states and SHG coefficients are also calculated by using first-principles method.Chapter Six investigates two chlorates with large band gaps as new potential second-order NLO crystal materials. The anion group ClO3-is of a trigonal pyramidal structure and they are piled up along the same direction in crystal structure creating macroscopic second-order NLO effect. Two chlorates of RbC103and CSClO3are synthesized. They show phase-matchable second harmonic generation (SHG) of about4times as strong as that of KH2PO4(KDP) based on the powder SHG measurement. The relationship between the crystal structure of those compounds and NLO property is also discussed. Their powders are transparent in the region from0.3to9μm and exhibit a good thermal stability (>400℃). The band gaps derived from the UV-Vis absorption spectrum are5.4eV and5.5eV, respectively. Band structure, densities of states and SHG coefficients are also calculated and analyzed.Chapter Seven makes a systematic investigation on second-order NLO effect of two compounds with different A-site cations, A2Nb4O11(A=Ag、Na). In A2Nb4O11, the d0transition metal cations Nb5+and oxygen atoms form the distorted NbOn polyhedron. are synthesized by solid state methods. Powder SHG of Ag2Nb4O11are about0.8times as strong as that of KTP (or about8times as strong as that of KDP), and Na2Nb4O11are about3times that of KDP. The relationship between the crystal structure of those compounds and NLO property is discussed. These powders are transparent in the region from0.4to10.5μm and exhibit a good thermal stability (>800℃). The band gaps derived from the UV-vis absorption spectrum are3.2eV and3.5eV, respectively. Band structure, densities of states and SHG coefficients are also calculated by using first-principles method. The relationship between the structure of those compounds and NLO properties is discussed, and the effect of the different A-site cations on NLO property is also analyzed.