Fabrication And Properties Of Yb³⁺doped CaF₂ Transparent Ceramics

Yb:CaF₂ material possess excellent properties such as broad range of transmittance,lower phonon energy and low refraction index than garnets and sesquioxides;in these hosts Yb³⁺features a very broad absorption and emission bands?useful for the generation and amplification of ultrashort laser pulses?and long fluorescence lifetime that ensures efficient energy storage for the laser operation.In addition,Yb:CaF₂ has a negative thermo-optical coefficient which is suitable for the high output power in lasers due to mitigation of the thermal lens effect.Due to these characteristics,it has great application value in an ultra-short pulse laser and amplifier.Issues related with single crystals such as the existence of cleavage surface?111?which results in lower laser damage threshold,poor doping uniformity and inferior mechanical properties still limit the performance and wide dissemination of Yb:CaF₂.In contract,Yb:CaF₂ ceramics can easily solve the issues.Meanwhile,they have the advantages of low sintering temperature,short preparation time and large size,which make Yb:CaF₂transparent ceramics have good application prospects.At present,many researchers have begun to study the fabrication and properties of Yb:CaF₂ transparent ceramics.Some progress has been made.Therefore,the main goal of this paper is to prepare Yb:CaF₂ transparent laser ceramics with high optical quality and achieve high-performance laser output,so as to shorten the gap between domestic and foreign.Yb:CaF₂ nano-powders were synthesized by the co-precipitation method.Yb:CaF₂ transparent ceramics were fabricated by various sintering methods.The related properties of the nano-powders and transparent ceramics were systematically studied.In addition,Yb³⁺,La³⁺::CaF₂ transparent ceramics with different doping concentration were prepared,which laid a foundation for further study on the effect of La³⁺ions on the spectral and laser properties of the ceramics.The main conclusions of this paper are as follows:?1?5 at.%Yb:CaF₂ nano-powders were synthesized by the co-precipitation method.The powder was pure cubic CaF₂ phase and no secondary phase can be detected.The average particle size was about 45 nm.Meanwhile,the powders are well-distributed.The shape of the small particles is close to cubic.?2?5 at.%Yb:CaF₂ transparent ceramics were fabricated by vacuum sintering and hot pressing post-treatment.The effect of sintering pressure on the optical properties and microstructure of the ceramics was studied.When the sintering pressure is 15 MPa,the ceramic has the best optical quality.The in-line transmittance of the sample with the thickness of 1.5 mm reaches 92.0%at the wavelength of 1200 nm,which is close to the theoretical value.However,the transmittance in visible range decreases rapidly,which can be attributed to the residual nanoscale pores in the ceramics.At the same time,the optical properties of Yb:CaF₂ ceramics prepared at pressure of 15 MPa were systematically studied.The 5 at.%Yb:CaF₂ transparent ceramics have broad absorption and emission band.The strongest absorption and emission peaks are both at 977 nm.Meanwhile,the absorption cross section at 977 nm and the emission cross section at the1030 nm of the 5 at.%Yb:CaF₂ ceramics are 0.39×10^-20 cm² and 0.26×10^-20 cm²,respectively.The lifetime of the ⁵F_5/2 level of the Yb³⁺is 1.95 ms.In the laser experiment,the maximum slope efficiency of 23.6%and the maximum output power of 0.9 W under QCW pump conditions were obtained from the ceramics.?3?5 at.%Yb:CaF₂ transparent ceramics were fabricated by vacuum sintering and HIP post-treatment.The effects of vacuum pre-sintering temperature on the phase,densification,microstructure and optical quality of the ceramics were studied.The results show that all the ceramics pre-sintered in vacuum at different temperatures are pure cubic phase,no second phase has been detected,and the density of the ceramics is not high.After HIP treatment,the ceramics pre-sintered at 650 ^oC under vacuum have the best optical quality and the transmittance reaches 87%at 1200 nm.Meanwhile,the effect of HIP temperature on the microstructure and optical quality of ceramics was also studied.The results show that when HIP temperature is 700 ^oC,the transmittance of the ceramics decreases obviously.The vacuum pre-sintered 650 ^oC ceramics with the thickness of 3 mm still have the highest transmittance,which is only 81.1%at 1200 nm.?4?5 at.%Yb:CaF₂ transparent ceramics were fabricated by air sintering and HIP post-treatment.The effects of pre-sintering temperature on the phase,densification,microstructure and optical quality of the ceramics before and after HIP were studied.Laser properties of ceramics were also studied.The results show that the ceramics pre-sintered in air at different temperatures are pure cubic CaF₂ phase.After HIP treatment of 650 ^oC pre-sintered ceramics with the thickness of 3 mm,almost no pore was found,and the transmittance was the best,reaching 91%at 1200 nm.In laser experiments,when Toc=7.2%,the maximum output power is 0.81 W,the corresponding optical efficiency?_O=19.2%,and the maximum slope efficiency?_S=22.6%.?5?Yb³⁺,La³⁺:CaF₂ nano-powders with different doping concentration were synthesized by the co-precipitation method.The powder was pure cubic CaF₂ phase.The ceramics with different doping concentration were prepared by air pre-sintering and HIP post-treatment.Among them,3 at.%La,5 at.%Yb:CaF₂ transparent ceramics with the thickness of 3 mm have the highest optical quality,with a transmittance of 90%at 1200 nm.Meanwhile,the transmittance of the ceramics increases with the increase of Yb³⁺doping concentration.