The Research Of Nonlinear Optical Properties In Iodate System And Fluorine Influence Mechanism

Nonlinear optical(NLO)crystal is a core device for laser frequency conversion in solid-state lasers,and plays a very important role in many fields,such as laser precision machining,medical treatment,national defense and so on.Iodate is also an important branch of NLO materials owing to its large second harmonic generation(SHG),high laser damage threshold(LDT)and wide range of light transmission.In this work,we focus on the open questions about the origin of SHG coefficient in iodate and the role of fluorine in the SHG.Therefore,the theoretical study of a series of iodates on the structure and optical properties were carried out by using the first principles.And the contribution of cations and anion groups for the compounds was quantitatively analyzed by using the SHG density analysis and the real space atom cutting(RSAC)method.The main research contents are as follows.1.The microscopic mechanism of optical effect of the iodates AIO₃(A=K,Na,Rb,Cs)and the fluoroiodates AIO₂F₂(A=K,Na,Rb,Cs)is explored.In this work,the typical iodates AIO₃(A=K,Na,Rb,Cs)and the new fluoroiodates AIO₂F₂(A=K,Na,Rb,Cs)are discussed respectively.The NLO effect and birefringence are calculated by the first principles method.The results show that the[IO₃]^-/[IO₂F₂]^-anionic groups with lone pair electrons play a significant role.In addition,for the birefringence,the asymmetric arrangement of electronic distribution in anionic groups can enhance the optical anisotropy,which can render the enhancement of the birefringence.Furthermore,it is found that the stereochemically active lone pair effect can be modulated,the balance among band gap,SHG and birefringence could be achieved accordingly.2.The influence mechanism of anionic groups with different polymerization on iodates is explored.In order to explore the influence mechanism of anionic groups with different degrees of polymerization on optical properties,the[I₃O₈]^-anionic group and isolated[IO₃]^-anionic group in K₂Na(IO₃)₂(I₃O₈)are studied by means of electron density analysis,dipole moment calculation,polarizability and hyperpolarizability and SHG density analysis,the results show that compared with the isolated[IO₃]^-anions,the polymerized anions are more likely to produce large SHG effect.By comparing the compounds containing[I₃O₈]^-polymerized anions,it is shown that the torsion angle of[I₃O₈]^-in the non-centrosymmetric compounds is smaller than that of centrosymmetric ones,which is also the reason for the difference between their symmetry and properties.