Investigation On Spectral Properties And Visible Laser Performance Of Re (Pr,Eu,Dy) Doped Calcium Fluoride Crystals

The spectroscopic properties depend strongly on the lattice structure of the rare-earth ions.Researchers initially put the idea of the design and regulation on the lattice structure of the rare-earth active ions to alkline earth fluohalide crystals in effect.Through the process of co-doping R³⁺（R=Y,La,Gd,Sc）into Nd:CaF₂ crystals,the emission cross-section as well as the quantum efficiency have been enhanced due to the change of the lattice stucture of Nd³⁺ions.Based on this theory,lattice structure and luminescent properties of Pr,Eu and Dy ions in CaF₂ crystals were studied.Pr,Eu and Dy rare-earth ions are most studied for its’emissions in the visible range,which cover the range from blue to deep red.Y³⁺or Gd³⁺ions were co-doped in the Re（Pr,Eu,Dy）:CaF₂ crystals.Through X-ray diffraction test,absorption and emission test,luminescence decay test,as well as quantum efficiency test,it was found that Y³⁺or Gd³⁺ions could change the lattice structure of the rare-earth ions.As a result,the quantum efficiency of Pr³⁺ions was enhanced and a red laser was obtained in Pr:CaF₂ crystal.Besides,by changing the concentration of Eu ions or the excitation wavelength,tunable emissions（from bluish violet to reddish orange）were obtained in the Eu:CaF₂ crystals.And,tunable white lights were obtained in Dy,Gd:CaF₂ crystals by changing the concentration of Gd³⁺ions.The main results are as follows:Single crystals of 0.6%Pr,x%R:CaF₂（x=0.6,1.2,3.0,6.0,R=Y,Gd）were grown by Bridgman method and temperature gradient technique.The experimental results show that co-doping 0.6%Pr:CaF₂ crystals with Y³⁺and Gd³⁺ions could significantly improve the spectral properties of the crystals in the visible wavelength region.While the modulating function was different.Y³⁺ions have a more outstanding influence on the blue and orange emissions.When x was increased to 6.0,the blue intensity increased to be 36 times of the0.6%Pr:CaF₂ crystal.The second one is the orange emission band at 598 nm with 156 times.While Gd³⁺ions are more focused on the enhancement of red emissions.When x was increased to 6.0,the red intensity increased to be 16 times of the 0.6%Pr:CaF₂ crystal.Y³⁺and Gd³⁺ions were proved to be modulating ions because they can not only enhance the luminescence intensity of the crystals,but also have effect on the local structure of the luminescent centers.Finally,a 642 nm red laser of the 0.6%Pr,1.2%Gd:CaF₂ crystal has been realized for the first time.Single crystals of x%Eu:CaF₂（x=0.6,1.2,3.0,6.0）and 3.0%Eu,x%Gd:CaF₂（x=3.0,6.0,12.0）were grown by Bridgman method.With the increase of Eu ions,the proportion of Eu²⁺ions decreased,while the proportion of Eu³⁺ions increased.As for the Eu:CaF₂crystals,Eu²⁺and Eu³⁺ions possess broad bands of absorption and emission.Tunable colors could be obtained in a large range by varying the concentration of Eu ions.With the increase of Eu ions,the color changed from blue to orange.Besides,tunable colors could also be obtained by changing the excitation wavelengths（from 320 nm to 330 nm）.The effect of Gd ions concentration on the lattice structure of Eu³⁺ions was analyzed.It was found that the symmetry of Eu³⁺ions was decreased with the increase of Gd³⁺concentration.The Gd³⁺ions co-doping process significantly enhanced the luminescence intensity of Eu³⁺ions.When the concentration of Gd³⁺ions increased to 6.0%,the emission intensity increased to be 1.5 times of the 3.0%Eu:CaF₂ crystal.However,the emission intensity of Eu²⁺ions decreased.Single crystals of x%Dy:CaF₂（x=0.6,1.0,3.0,6.0）and 0.6%Dy,x%Gd:CaF₂（x=0.3,0.6,1.2,3.0）were grown by Bridgman method and TGT method.Through the spectroscopic tests,we found that luminescence quenching effect of Dy³⁺ions occurred when the concentration of Dy³⁺was 0.6%.Co-doping Dy:CaF₂ crystals with Gd³⁺ions could significantly enhance the lunminescence intensity of the crystals.When the concentration of Gd³⁺ions increased to be 3.0%,the 573 nm emission intensity increased to be 28 times of the 0.6%Dy:CaF₂ crystal.Under the excitation of 273 nm,Gd³⁺→Dy³⁺energy transfer process was analyzed.Continuously variable white lights were obtained in the Dy,Gd:CaF₂ crystals by adjusting the proportion of Gd³⁺ions.When the concentration of Gd³⁺increased to 3.0%,the coordinate moved from（0.54,0.40）to（0.38,0.42）.In a conclusion,the Y³⁺and Gd³⁺ions,were therefore worked as local structure regulators.By finely manipulating the local structures of the Pr,Eu and Dy ions,the spectral and laser properties of rare-earth-doped fluorite crystals could be designed artificially.