Multi-phonon-assisted relaxation and Yb3+ sensitized bright red-dominant upconversion luminescence of Ho3+ in YF3-BaF2-Ba(PO3)2 glass

Unusual bright red-dominant upconversion light was observed in Ho³⁺/Yb³⁺ co-doped YF₃-BaF₂-Ba(PO₃)₂ glasses excited by the 980-nm laser diode at room temperature. The integral intensity ratios of the red upconversion emission to the green one reached about 10:1 in optimized 0.125Ho³⁺-15Yb³⁺ co-doped sample. In order to find out its behind-the-scene mechanism, the optical properties and the phonon-assisted relaxations on the excited levels of Ho³⁺ in our samples were investigated. Additionally, the effects of the concentrations of the doping ions, excitation pump power, and temperature on the upconversion emissions were also systematically studied. These results revealed that the proper phonon frequency of fluorophosphate glasses, the efficient phonon-assisted relaxations from ⁵I₆ to ⁵I₇ levels (4,960 s^?1), and the long lifetime of the ⁵I₇ (about 2.8 ms) levels should be responsible for bright red upconversion emission at a much greater concentration ratio of C _Yb ³⁺ /C _Ho ³⁺ .