Explorations Of Novel Langasite Crystals And Studies Of Their Mid-Infrared Nonlinear Optical Properties

In the 1960s,Maiman made the first laser.Later,P.A.Franken et al.from the University of Michigan in the United States first applied ruby laser directly to quartz crystals,and observed double-frequency radiation.Bloembergen et al.analyzed the principle of nonlinear optical parametric generation theoretically.Since then,nonlinear optics has been born and played an important role in human society.Excellent nonlinear optical materials is of great significance for laser frequency conversion and high power laser generation.At present,the most widely used nonlinear optical crystal materials in mid-infrared band are semiconductor crystals represented by ZnGeP2(ZGP).However,their laser damage thresholds are low,which limits the application range.Therefore,one of the key and difficulties in the field of nonlinear optical materials at present is to explore new mid-infrared nonlinear optical crystal materials with excellent performance applied to high-power laser.Langasite materials are famous for their multifunctionality in optoelectronic applications,such as piezoelectric convertors,electro-optic Q switched laser generation,and surface acoustic wave(SAW)devices.The structure without central symmetry determines that the crystal has a series of effects such as electro-optic,piezoelectric and nonlinear optics,and the laser damage threshold is high because it belongs to oxide,and its composition determines that the phonon energy of the crystal is small and its composition determines that the phonon energy of the crystal is small and the transmission range is wide,which may be have important applications in the mid-infrared high-power laser.In this study,we focus on langasite materials,we first investigated the mechanism of the NLO effect in known langasites using first-principle calculations.A structure-composition-property map was then constructed using a real-space atom-cutting technique to quantify the respective contribution of each anionic species to the overall SHG coefficients.Based on the map,a special atomic substitution(octahedral Ga3+/Nb5+ site replaced by Sn4+)was proposed based on rational design and the bond valence model,in an attempt to extend the IR absorption edge and improve the SHG effect.Accordingly,a new compound,La3SnGa5O14(LGSn),was theoretically predicted,and this compound was successfully experimentally synthesized via a high-temperature solid-state reaction.Furthermore,centimetersized LGSn single crystals that exhibit excellent mid-IR properties were grown.These crystals exhibited a broad transparent region,high LDT,and the strongest NLO effect among known langasites.The main work is as follows:?.IntroductionIt mainly introduces the basic knowledge about second-order nonlinear optical materials,and summarizes the latest research and development of nonlinear optical materials applied from ultraviolet to infrared in recent years.The characteristics of langasite family compounds and the key theories of nonlinear optics are introduced.The main idea and contents of this thesis are then provided based on these,and the main contents of each chapter are briefly described.?.Theoretical calculationThe first theoretical principle is used to calculate the possible theoretical multiplication factor of the material after elemental regulation.Then,according to the anion group theory proposed by Academician Chen Chuangtian,the contribution of anionic groups to optical properties is obtained by real-world atomic cutting technique.Finally,the flexibility factor model is used to measure the contribution of specific groups to nonlinear optical effects under external disturbances.Based on the crystal structure and theoretical calculation of the gallium silicate bismuth system,Nd and Pr are respectively introduced into the LGN to change the elemental occupancy in the dodecahedron in the crystal structure,and the atomic radius is larger and the atomic mass is heavier.Sn is introduced into LGS to replace its octahedral structure.The compound Nd3Ga5.5Nb0.5O14,Pr3Ga5.5Nb0.5O14,La3GasSnO14 was designed and synthesized.?.Synthesis and crystal growth of polycrystalline materials1.The pure polycrystalline material Nd3Ga5.5Nb0.5O14,Pr3Ga5.5Nb0.5O14,La3Ga5SnO14 controlled by three elements were obtained by high temperature solid phase reaction technology.2.LGSn crystals were grown and then a piece of LGSn crystal flakes having a size of 6 × 10 × 0.02 mm3 was obtained by cutting and polishing.IV.The basic physical properties1.The three materials Nd3Ga5.5Nb0.5O14,Pr3Ga5.5Nb0.5O14 and La3Ga5SnO14 X-ray diffraction spectrum were analyzed,the position of the diffraction peak is consistent with the position of the standard card diffraction peak,indicating the three sample is pure.2.Taking AgGaS2 and LGN as reference,we make the SHG test on three samples of Nd3Ga5.5Nb0.5O14,Pr3Ga5.5Nb0.5O14 and La3Ga5SnO14.The experiment shows that the LGSn in the 1064nm band is non-phase matched,but the intensity increases with the increase of particle size under 2090nm laser,indicating that LGSn has phase matching in this band.3.The transmission range of La3GasSnO14 crystal flakes was tested at room temperature.It was confirmed that the La3Ga5SnO14 has the widest transmission region(0.27-11.0 ?m)in the oxide.