| Photoelectric functional crystalline materials were thought to be crystals that can realize photoelectric conversion functions,and they usually exhibit excellent photoelectric properties,thermal properties,and structural stability.Among them,borate crystals have rich structure types,possessing a high melting point,high hardness,high chemical stability and good optical properties.It is widely used in laser technology,optoelectronics,wireless communication,infrared optics,optical imaging and so on.This paper takes rare earth calcium oxyborate crystals as the research object,aiming to explore its application potential in optoelectronics and laser technology.Rare earth calcium oxyborate crystals have become one of the research hotspots because of their excellent laser and nonlinear properties.This study will analyze the crystal structure and physicochemical properties of REECOB,and provide new ideas and solutions for their application in new optoelectronic devices and laser technology.In this paper,Sm:GdCOB and HoCOB crystals were grown by the Czochralski method for the first time.The problems in the process of growing were analyzed and summarized.A method to obtain REECOB crystals was proposed.The grown bulk crystals are of excellent optical quality without any macroscopic defects,and the crystalline quality is good.The structure and doping method of Sm:GdCOB were determined.The Sm3+-ion doping concentration was determined to be 3.0 at.%by ICP method,and the segregation coefficient was up to 90%.The density of Sm:GdCOB crystals was determined to be 3.72 g/cm3 by Archimedes method.The relationships between the crystallographic axis,the optical axis and the physical axis were determined by a polarizing microscope:(a,N2)=26.9°,(c,Nx)=15.6°,(a,x)=11.3°.Based on the above basic physical characteristics,the thermal and optical properties of Sm:GdCOB were studied,respectively.(1)Thermally,the largest thermal expansion coefficient was along the NX direction:αNX=3.76×10-6 K-1,αNY=6.73×10-6 K-1,αNZ=9.44×10-6 K-1.And the crystal density will decrease with the increase in temperature.At 500℃,the density decreased to 3.64 g/cm3,and the rate was only 2.2%,which indicated that the crystal could be used in a variable temperature environment.Through the thermogravimetric-differential thermal curve,the melting point is determined to be 1366℃.There is no phase transition before melting.At room temperature(25℃).the specific heat of Sm:GdCOB is 0.605 J·g-1·K-1,which theoretically has a high resistance to the photodamage threshold.The laser induced damage threshold of Sm:GdCOB was measured,the value of 1064 nm reached 155.0 GW/cm2,and the damage threshold of 532 nm was 38.8 GW/cm2.(2)Optically,the linear absorption spectra of NX,NY and NZ polarization directions were systematically studied under J-O theory,including absorption cross-section σabs,spectral line intensity parameter Ω,spontaneous emission transition probability A,fluorescence branching ratio β,radiation fluorescence lifetime τrad,polarization emission cross section σem and fluorescence lifetime τ,etc.The energy level distribution and cross-relaxation phenomenon of Sm3+ and Gd3+ions in Sm:GdCOB were analyzed.These results show that Sm:GdCOB crystals are the potential laser and nonlinear optical materials.A new type of intra-cavity polarization element was designed based on Sm:GdCOB crystals.The absorption of Sm:GdCOB crystals shows strong anisotropy around 1 μm,and NY polarization is the weakest.The large absorption-loss difference would suppress the laser oscillation in the Nx and Nz polarization.The laser in the NY polarization will oscillate firstly,achieving intra-cavity polarization selection.The performance has been verified in Nd3+-doped solid-state lasers.For the Nd:YAG laser,the polarized laser output with a maximum polarization degree of 114:1 is obtained.Due to the influence of the orientation distribution of Nd3+ ions,different output wavelengths are obtained in different polarization directions.The emission spectrum of Nd:LGMO crystal has anisotropy.Laser outputs with different wavelengths are obtained in different polarization directions.The maximum polarization degree is 135:1.For the Nd:LYSO laser,the maximum polarization degree is located in the Y tangential direction,which is 233:1.The experimental results show that the dual regulation of polarization and wavelength can be realized using Sm:GdCOB crystal.Bulk crystals of HoCOB with a size of 25×25×50 mm3 were grown for the first time by the Czochralski method.The crystal structure was analyzed by single crystal/powder X-ray diffraction patterns,and the HoCOB crystals belonged to the monoclinic system and the Cm space group.It has Ho/Ca co-occupancy phenomenon,that is,there are Ca and Ho distributions on the ideal sites of Ho and Ca,respectively.So the actual chemical formula of the crystal is Ho0.937Ca3.063O(BO3)3.The basic physical properties of the crystal were measured,including transmittance and density(3.85 g/cm3),etc.With the polarizing microscope and the crystal structure,the positional relationship of axes was determined as(a,NZ)=26.7°,(c,NX)=15.4°,(a,X)=11.3°.Through the study of HoCOB crystals,the understanding of REECOB crystal structure,properties and formation mechanism has been studied in depth.(1)The X-ray photoelectron spectrum of HoCOB was measured and analyzed,and the high-resolution spectrum curves of Ho,Ca,B.O were fitted by the Gaussian-Lorenzian equation.The research results found that the Ols binding energy(531.4 eV)of HoCOB is higher than that of YCOB(530.6 eV)and GdCOB(530.35 eV)crystals.(2)Taking the KDP crystal as the standard sample,the powder frequency doubling conversion efficiency of HoCOB was measured by Kurtz method.When the particle size of the sample is 250 μm,the intensity is 0.8 times larger than KDP.We worked out the dipole moment intensity of the BO3 group in HoCOB crystal to analyze the reason for the low-frequency doubling intensity and compared it with YCOB,GdCOB and TmCOB crystals.The research results show that the BO3 group located at the mirror position in the REECOB crystal contributes weakly to the dipole moment intensity,and the total dipole moment intensity(4.9680 D)of HoCOB is weaker than that of YCOB(6.0432 D)and GdCOB(5.9951 D).It is the main reason for the weaker frequency doubling intensity.And Ho3+ ions can absorb light around 532 nm,which further reduces the conversion efficiency.By analyzing the calculation results of dipole moment,it is also found that when the REE ions is lanthanide,the dipole moment intensity of REECOB crystal tends to decrease with the decrease of rare earth ion diameter.(3)The thermal properties of the HoCOB crystal were measured,including thermal expansion coefficient,thermal diffusion,specific heat,and thermogravimetric-differential heat curve.The melting temperature of HoCOB was found to be 1358℃,and the chemical properties were stable before melting.The maximum thermal expansion coefficient was located on c axis(α33=15.15×10-6 K-1),and the main axis of thermal expansion of HoCOB is determined.The thermal conductivity in different tangential directions is measured.As the temperature increases,the thermal conductivity gradually decreases,and the thermal conductivity in the c-axis direction is the largest,which is 2.6 W·m-T·K-1 at room temperature(300 K).(4)The dielectric constant and dielectric loss of different REECOB crystals were measured from room temperature to 900℃,and the relationship between the dielectric constant and the electronegativity of rare earth ions was discussed.HoCOB crystals still have high dielectric activity at high temperatures.The dielectric constant is up to 16 at 900℃,which is higher than other REECOB crystals,proving a piezoelectric material with excellent performance. |
