The Upconversion Luminescence Properties Research Of Rare Earth （Yb/Er/Tm）doped CaF₂ Single Crystal

Rare reath-doped(RE³⁺)upconversion luminescent materials have been rapidly developed in recent years mainly due to their unique optical features such as large anti-Stokes shifts,narrow emission bands,and high resistance to photobleachin.They are capable of converting tissue-penetrable near-infrared（NIR）photonic irradiation into ultraviolet,visible,and NIR emissions and have a significant potential in bioimaging,diagnosis,and therapy,as well as in photovoltaic systems and optical data storage.Herein,CaF₂ single crystal is selected as the carrier of RE³⁺ions because its phonon energy(about 350 cm^-1)is low,which reduces the non-radiative relaxation process and can obtain higher upconversion efficiency.The main results of this paper are as follows:（1）A series of Er:CaF₂ single crystals with different concentrations of Er³⁺ions were prepared by temperature gradient technique（TGT）method.It was found that Er³⁺exhibited strong red and green upconversion luminescence（UCL）,and as the concentration of Er³⁺increased,a large range of regulation from green to red light was observed in the Commission Internationale de l’Eclairage（CIE）diagram.And the upconversion process existing in the Er:CaF₂ single crystal are ground state absorption（GSA）,excited state absorption（ESA）and cross-relaxation（CR）process.（2）A series of Yb,Er:CaF₂ single crystals with varous contents of Yb³⁺ions were prepared by TGT method.It was found that the luminescence intensity was much higher than that of undoped after co-doping Yb³⁺ions.As the concentration of Yb³⁺ions increased（0 at%¹0at%）,the luminescence intensity first increased and then decreased.When the concentration of Yb³⁺ions reached 5 at%,the luminescence intensity reached the maximum.From the CIE diagram,a large range of regulation from red to green light can be observed.The upconversion process existing in Yb,Er:CaF₂ single crystal are energy transfer（ET）,ESA,CR and back energy transfer（BET）process.（3）A series of Yb,Er,Tm:CaF₂ with different concentrations of Tm³⁺ions were prepared by TGT method.It was found that Tm³⁺ions exhibited strong blue and red UCL,and as the Tm³⁺ion concentration increased（0.05 at%⁰.5 at%）,a large range of regulation from yellow,white,and blue light were observed in the CIE diagram.Upconversion white light emission was achieved when the Tm³⁺ion concentration reached 0.05 at%.The results show that the Yb,Er,Tm:CaF₂ single crystal has great application prospects in white LED.