Intense cooperative upconversion emission in Yb/Er: TeO2-Li2O-WO3 oxyfluoride glass ceramics

Novel oxyfluoride glass ceramics based on Yb3+-doped TeO2-Li2O-WO3-ErF3 (TWLE) were prepared using the two-stage heat-treatment method. The structural characterisation of these materials with X-ray diffraction indicated that crystalline phases of LiYbErF4 nanocrystals grew in the glass ceramics. Upon excitation with a 980 nm laser diode the Yb3+ doped (2 mol%) TWLE glass-ceramic produced intense blue emission at 487 nm, weak green emission at 525 nm and 550 nm, and red emission at 650 nm at room temperature. The mechanism of the blue emission, which is attributed to cooperative upconversion from pairs of excited Yb3+ ions to ground-state Er3+ ions, and the weak red and green emissions, which are attributed to energy transfer from Yb3+ to Er3+ ions, were investigated. The results of a pump-power dependence measurement suggested that the blue cooperative emission is a two-photon process. A rate equation was used to describe the cooperative energy-transfer process among the donor and acceptor ions. The observed results were compared to the behavior of another glass materials that exhibits blue cooperative emission, revealing the considerable advantage of TWLE:2Yb glass ceramics for optical devices.