Study On The Upconversion Luminescence Of Yb: Ho: PbWO₄ Crystals

Because of its lower light field , the lead tungstate (PbWO4, PWO) just applied in few fields. To improve its luminescence properties, doping Re³⁺ to the crystals have been extensively studied. In this paper, Czochralski Growth of doped PWO crystal technology were studied. and growth of the good quality of doped crystals. These doped PWO crystals structure and optical properties were studied too.The structure of Ho:PWO and Yb:Ho:PWO crystals were studied by the means of XRD. The XRD testing results showed that all crystals are in perfect scheelite structure comparing with standard card of pure PWO, cell parameters became smaller.Crystal Raman spectra show that the doped PWO crystals, compared with pure PWO crystal,the characteristic peaks of the (WO4)^2- group is not obvious displace- ment, Doped ions had no significant effectgroup structure of (WO4)2 -.UV absorption spectras show that the absorption of the doped crystal, compared with pure PWO crystal, intensity increases, and the characteristic absorption peak of Ho³⁺ is emerged, Ho³⁺ and Yb³⁺ prefers to occupy the sites of Pb²⁺. the absorption edge shift to blue and then to red.By a wavelength of 980nm laser excited upconversion emission spectra, we found single-doped Ho³⁺ in PWO no upconversion, Ho³⁺ and Yb³⁺ double-doped PWO achi- eved 540nm, 650nm and 756nm upconversion. The Ho³⁺ and Yb³⁺ energy level diagram can be seen: Yb³⁺ energy level structure is very simple, 540nm, 650nm and 756nm luminescence corresponding to Ho³⁺ in the ⁵F₄,⁵S₂→⁵I₈,⁵F₅→⁵I₈ and ⁵F₄,⁵S₂→⁵I₇ radiative transition. When [Yb]/[Ho] is 20, the maximum conversion green stre- ngth emerged, [Yb]/[Ho] is 4, the maximum conversion red strength emerged.From fluore- scence intensity and the pump power curve testing and analysis,we can find that Yb³⁺(0.2mol%):Ho³⁺(0.1mol%):PWO,Yb³⁺(0.4mol%): Ho³⁺(0.1mol%):PWO,Yb³⁺(1.0mol%):Ho³⁺(0.1mol%):PWO,Yb³⁺(2.0mol%): Ho³⁺(0.1mol%):PWO,Yb³⁺(3.0mol%):Ho³⁺(0.1mol%):PWO crystals, the power curve slope of green light upconversion are 1.936, 2.110, 2.339, 1.693, 1.186, the power curve slope of red light upconversion are 1.627, 1.703, 1.746, 1.620, 1.302. We can find that both red light and green light upconversion include not only single-photon emission process, but also two-photon process. And high concentration in the Yb³⁺ doped PWO, the crystal is relatively small slope of the power curve, which indicated the occurrence of cooperative sensitization upconversion mechanism.