Cooperative energy transfer in Tb3+/Yb3+- and Nd3+/Yb3+/Tb3+-codoped oxyfluoride glasses

This paper reports on cooperative energy transfer and upconversion luminescence properties of Tb3+/Yb3+- and Nd3+/Yb3+/Tb3+-codoped oxyfluoride glasses. Upon excitation with a 980 nm laser diode, an intense green upconversion luminescence along with weak ultraviolet (UV)–visible emissions has been observed in Yb3+/Tb3+-codoped oxyfluoride glasses. Power dependence of UV–visible upconversion luminescence intensity has been examined, revealing that a cooperative energy transfer mechanism from Yb3+ ions is responsible for the excitation of Tb3+ ions. Meanwhile, it is noticed that Tb3+ upconversion emission bands have also been clearly detected at 487, 542, 587 and 620 nm in Nd3+/Yb3+/Tb3+-codoped oxyfluoride glasses upon excitation with a 808 nm laser diode. The quadratic dependence of the upconversion luminescence on the pump-laser power indicates two-photon process for the population of Tb3+:5D4 state via Nd3+→Yb3+→Tb3+ energy transfer. However, no emission has been observed in the oxyfluoride glasses codoped with Yb3+/Tb3+ or Nd3+/Tb3+, respectively, upon excited at a 808 nm laser diode. A proposed upconversion mechanism involving energy transfer from Nd3+ to Yb3+, and then a cooperative energy transfer process from two excited Yb3+ to Tb3+ has been presented.