Exploration And Properties Of Novel Nonlinear Optical Crystals With Tetrahedral And Trigonal Conical Coordination

With the rapid development of laser science,nonlinear optical crystals,which are the core materials of all-solid-state lasers,have always been the research focus of researchers.Therefore,it is of great scientific significance to explore new nonlinear optical crystals with excellent performances.For decades,thanks to the hard work of scientific researchers,the exploration experience and theory in the field of nonlinear optical crystals have been greatly developed.This paper reviews the origin of nonlinear optics,and focuses on the research status of nonlinear optical crystals in recent years.Based on the intrinsic relationship between the crystal microstructure and its properties,we choose(PO4)3-,(SO4)2-,(HPO3)2-,and(SeO3)2-anion groups with tetrahedral and pyramidal coordination as the research object,and design and explore a series of novel nonlinear optical crystals.The specific research work of this study is as follows:(1)Strong second harmonic response and large birefringence of nonlinear optical(NLO)materials have important scientific and technical significance for tunable lasers.The second harmonic generation(SHG)response in NLO crystals has been generally enhanced by improving the polarization of bridged oxygen atoms or local structural distortions,while the synergy of the two contributions to enhance the SHG response is rarely explored.To this end,new crystals with traditional RTP-structure,Rb1-xTi1xSbxOPO4(x=0.01,0.03,and 0.06),containing novel functional SbO6 octahedra and Rb vacancies are designed through a one-site aliovalent substitution and grown in this work.Rb0.94Ti0.94Sb0.06OPO4 exhibits an enhanced SHG response,nearly 1.48 times that of the RbTiOPO4(RTP),companying with the largest birefringence in phosphates(exp.0.202 at 546 nm,cal.0.156 at 1064 nm).Theoretical calculations and crystal structure analysis confirmed that the higher polarizability of bridged oxygen and the local structural distortions in Rb1-xTi1-xSbxOPO4 strengthened the NLO activity of neighboring units,thus significantly prompt the SHG effect and optical anisotropy.This study provides a new paradigm to deep improve the NLO performance of crystal materials and benefits for expanding their practical applications.(2)Wide bandgap and phase-matching nonlinear optical(NLO)response are vital for deep-ultraviolet(DUV)NLO materials.However,due to the small hyperpolarizability of[AO4](A=S,P,B,Si)unit,related DUV NLO oxides generally exhibit small optical anisotropy.Currently,rational design of non-π-conjugated anion units with enhanced hyperpolarizability has attracted a lot of attention,design and molecular stacking modification of polar cation have long been overlooked.Herein,we propose a strategy that the alkali-metal cations could be substituted with unsymmetric non-π-Conjugated cations to enhance optical anisotropy of NLO materials while keeping moderate NLO response.LiN2H5SO4(LNSO)exhibits strong phase-matching NLO response of 0.6 times that of the benchmark KH2PO4 and the widest band gap(7.69 eV,corresponding to an absorption edge of 161 nm)among all reported NLO sulfates.It indicates that LNSO is a promising DUV NLO materials with excellent performances.The first-principles calculations demonstrate that the large band gap and phase-matching ability of LNSO originate from the introduction and packing arrangement of novel NLO-active unit:N2H5+.This work provides an innovative perspective for the targeted design and synthesis of DUV NLO materials with larger optical anisotropy.(3)The linear and nonlinear optical properties of the crystal materials can be significantly improved by introducing the low-symmetry non-π-conjugated anion group into the nonlinear optical crystals,but the(HPO3)2-group with the same low symmetry has been ignored for a long time.We have successfully designed and synthesized two phosphite nonlinear optical crystals:Mg(H2O)6(HPO3)and Mg2(H2O)2(HPO3)2.The basic units of the two crystals are identical,but the crystal structure and optical properties are quite different.The results of powder frequency doubling tests show that the quadratic frequency doubling effects of Mg(H2O)6(HPO3)and Mg2(H2O)2(HPO3)2 crystals are 0.3 and 0.2 times of KDP,respectively,and both meet the phase matching.In addition,they all have a wide UV transmittance window,which can meet the requirements of ultraviolet applications.The discovery of this kind of crystal proves that(HPO3)2-group is also a nonlinear active anion group which is suitable for ultraviolet transmittance range,has important leading value in the exploration of phosphite system nonlinear optical crystals,and provides ideas for the future exploration of excellent phosphite nonlinear optical crystals.(4)Large nonlinear optical response and wide infrared transmittance range are important optical properties of mid-infrared nonlinear optical crystals,synthesis of midinfrared nonlinear optical crystals with excellent comprehensive performance has always been an important research focus.We have successfully synthesized a millimeter-level mid-infrared nonlinear optical crystal Cs2(VO2)2(SeO3)2 by hydrothermal method.The crystal has a strong phase-matching NLO response(3.0×KDP)and a wide transmission range(0.41-7.1 μm),and the transmittance range of Cs2(VO2)2(SeO3)2 includes an "atmospheric window" of 3-5 μm.First-principles calculations show that the NLO response of Cs2(VO2)2(SeO3)2 is mainly derived from the the novel VO5 group and[SeO3]2-group with stereochemically active lone electron pairs,and the contribution of VO5 group to the macroscopic NLO response is greater than that of the[SeO3]2-group.Compared with the more common VO6 octahedron coordination and VO4 tetrahedron coordination,the VO5 polyhedron exhibit stronger structural flexibility and NLO activity,which provides more possibilities for molecular design and molecular stacking configuration of novel nonlinear optical crystals.