Growth And Properties Of A New Laser Crystal LiY（MoO₄）₂

Artificial crystal is an indispensable key material in modern national defense technology and high technology.The rapid development of nano,information,new energy,and biology has created new opportunities and challenges for the research of artificial crystals.Crystal materials provide material support for the rapid development of photoelectric technology.The development of photoelectric technology has put forward broader and higher requirements for crystal materials.Laser technology has been widely used in the military and civilian fields.Among them,laser crystal is the core component of the laser,which can directly determine the properties and application range of laser.Long-range order is a typical characteristic that distinguishes crystals from other materials.Different from the cubic crystal system,although the middle and low symmetry crystals maintain long-range order in structure,some of their physical properties still show anisotropy.The generation of laser depends on the energy level transition of the activated ion in the lattice field.In the middle and lower symmetry crystals,some crystals,especially those with disordered structures,may provide anisotropic lattice field for the active ions,so that the energy level splitting of active ions shows anisotropy in the crystal.In this paper,LiY（MoO₄）₂ is selected as the research object for the special material requirements of tunable orthogonal polarization multi-wavelength laser,aiming at exploiting the disorder of the crystal structure and anisotropy of the physical properties and exploring the application of the crystal in dualwavelength laser.At the same time,most molybdate is a potential material of stimulated Raman scattering.To explore the anisotropy of stimulated Raman scattering of crystals,it is expected to realize dual-wavelength laser output with stimulated Raman scattering as the substrate.Based on the above analysis,this paper carried out research on the growth and performance of LiY（MoO₄）₂ crystal.Based on breaking through the key technology of growth of large-size and high-quality crystal,the physical properties of crystal were systematically studied,and the structure-activity relationship between structure and performance was discussed.The crystal lattice field and stimulated Raman scattering anisotropy were used to achieve the controllable output of the crystal laser.This paper consists of five chapters,the first chapter is the introduction,the fifth chapter is the summary,and the second to the fourth chapter of the experimental research content is as follows:Ⅰ.Optimal growth of high quality LiY（MoO₄）₂ crystalsThe quality of flux growth crystals is relatively high,and the phase region of LiY（MoO₄）₂ crystals has been explored for the first time.There are some problems in the initial growth of crystals,such as small size,uneven mass,internal inclusion and crack,and polycrystal.To improve the utilization rate and meet the quality and size requirements of the crystal measurement,a series of controls were carried out from the perspectives of seed crystal direction and mass,temperature field design,cooling rate,etc,and the high-quality LiY（MoO₄）₂ crystal with the size of 23×20×18 mm3 and weight of about 20 g was successfully grown along the[001]seed crystal direction.The Laue diffraction pattern shows that the spots are symmetrical and clear,and the half-peak widths of the（100）and（001）directional wafers measured by high-resolution X-ray are 29 "and 43",respectively,indicating that the crystal quality is good and can meet the requirements of physical properties characterization.Ⅱ.Crystal structure and basic properties of LiY（MoO₄）₂LiY（MoO₄）₂ belongs to the tetragonal system,I41/a（No.88）space group,and the cell parameters a=5.149（3）?,c=1.209（2）?,Z=2.The polyhedral and crystal skeleton of LiY（MoO₄）₂ consists of tetrahedrons[MoO4]and mixed occupying dodecahedrons[YO8]and[LiO8].The hardness test shows that the LiY（MoO₄）₂ crystal has a moderate hardness of 4.9 Mohs hardness.The thermal properties of LiY（MoO₄）₂ crystal have been comprehensively studied.The specific heat of LiY（MoO₄）₂ increases linearly from 0.716 J（g·K）at 25℃ to 0.919 J/（g·K）at 300℃.The average thermal expansion coefficients of a and c axes at 25～500℃ were 1.49×105 K-1 and 2.01×10-5 K-1,respectively,and showed a small anisotropy of only 1.35.The thermal diffusion coefficient increases from 0.364 mm2/s to 0.419 mm2/s along a-axis and from 0.375 mm2/s to 0.429 mm2/s along c-axis at 30-500℃.The thermal conductivity coefficient of the aaxis changes from 1.027 W/（m·K）to 1.402 W/（m·K）,and the thermal conductivity coefficient of c-axis changes from 1.135 W/（m·K）to 1.654 W/（m·K）.The optical properties of LiY（MoO₄）₂ crystals were systematically characterized.The calculated band gap of LiY（MoO₄）₂ crystal is 3.52 eV,the ultraviolet cut-off edge of LiY（MoO₄）₂ is 344 nm,the infrared cut-off edge is 5493 nm,and the transmittance of LiY（MoO₄）₂ is 80%in the wavelength range of 360-5400 nm.The laser damage threshold of LiY（MoO₄）₂ crystal is 1095 MW/cm2 in the direction of（100）and 1064 MW/cm2 in the direction of（001）.Ⅲ.Study on laser properties of Nd:LiY（MoO₄）₂ crystalAccording to the growth experience of LiY（MoO₄）₂ crystal,Nd:LiY（MoO₄）₂ crystal with size of 26×22×28 mm3 and mass of about 24 g without cracks were grown,and the doping concentration of Nd3+ was 0.95%.The half peak width of the swing curve of（100）and（001）crystal plane reaches 32 "and 36",respectively.The thermal properties of the crystal were measured,and the specific heat changed slowly from 0.733 J/（g·K）to 0.645 J/（g·K）.The thermal diffusion coefficient of a-axis increases from 0.348 mm2/s to 0.425 mm2/s,and the thermal diffusion coefficient of c-axis increases from 0.34 mm2/s to 0.435 mm2/s at 50～500℃.The thermal conductivity coefficient of a-axis changes from 1.07 W/（m.K）to 1.548 W/（m.K）,and the thermal conductivity coefficient of c-axis changes from 0.962 W/（m·K）to 1.282 W/（m·K）at 50～500 ℃.The optical properties of Nd:LiY（MoO₄）₂ crystals were characterized.The absorption spectra show that the material has an obvious absorption peak at 808 nm,which verifies the existence of Nd3+.The emission spectra of Nd:LiY（MoO₄）₂ change obviously between 1061 nm and 1068 nm with the change of pump light polarization.It is verified that Nd:LiY（MoO₄）₂ crystal can realize dual-wavelength laser output by crystal tangent regulation.The laser performance of Nd:LiY（MoO₄）₂ crystal shows that the emission peak of a-cut crystal is around 1067.6 nm,the pump threshold is 606 mW,and the photo-optical conversion efficiency is 17.18%.The emission peak of the c-cut crystal is around 1060.5 nm,the pumping threshold is 320 mW,and the optical conversion efficiency is 26.45%.The emission peak of 45° tangential crystals from axis a to c axes is around 1067.4 nm,the pumping threshold of 45° tangential crystals from axis a to c axes is 437 mW,and the photoconversion efficiency is 24.82%.