Luminescence properties of Tb3+-doped zinc phosphate glasses for green laser application

Tb3+-doped zinc phosphate glasses of composition in mol%: (100.0 − x)Zn(PO3)2 - xTb2O3, x = 0.6, 1.0, 2.0 and 5.0, were prepared by conventional melt quenching technique and characterized by photoluminescence and decay time spectroscopy. The integrated intensities of the 5D4 → 7F5 (green at 541 nm) and 5D3 → 7F4 (blue at 435 nm) emissions and their intensity ratios IG/IB upon 350 nm excitation have been evaluated as function of Tb3+ concentration. The CIE1931 color of the glasses excited at 350 nm varies from turquoise to green by increasing the Tb3+ content. The increased IG/IB ratio up to a factor of 364 for the phosphor with the highest Tb3+ content (ZP5Tb) is consistent with the observed shift toward the green region in the CIE coordinates, so that the ZP5Tb glass exhibits a green color purity of 66.9% with chromaticity coordinates (0.290, 0.581), being very close to those (0.29, 0.60) of European Broadcasting Union illuminant green. This interesting feature of the ZP5Tb phosphor, together with an experimental branching ratio larger than 60% of the 5D4 → 7F5 green emission, highlights its capability as solid state green laser pumped by AlGaN (350 nm) LEDs. The decay time profiles of the 5D3 level resulted to be non-exponential for all the studied concentrations due to energy transfer between Tb3+ ions through cross-relaxation. Such decay profiles were well fitted to the Inokuti-Hirayama model for S = 6, which indicates that an electric dipole-dipole interaction might be the dominant mechanism in the cross-relaxation energy transfer occurring in Tb3+ ion clusters.