Ultraviolet and visible up-conversion luminescence of Er3+/Yb3+ co-doped CaF2 nanocrystals in sol-gel derived glass-ceramics

Glass-ceramics containing Er3+/Yb3+ codoped CaF2 nanocrystals have been prepared by using the controlled crystallization above 650 °C of the (Yb3+/Er3+) codoped CaF2-SiO2 xerogels. X-ray diffraction and scanning electron microscopy data have revealed the formation of CaF2 nanocrystals of about 16 nm size. For pumping at 973 nm, this glass-ceramics shows, besides green and red up-conversion luminescence, violet (at ∼408 nm) and near-ultraviolet emission (at ∼380 nm). The mechanisms responsible for the upconversion luminescence are presented and discussed. The fluorescence lifetimes of the metastable levels responsible for violet, green and red luminescence are measured and their quantum efficiencies are estimated.