Thermally enhanced cooperative energy-transfer frequency upconversion in terbium and ytterbium doped tellurite glass

Infrared-to-visible frequency upconversion through cooperative energy-transfer and thermal effects in Tb3+/Yb3+-codoped tellurite glasses excited at 1.064 μm is investigated. Bright luminescence emission around 485, 550, 590, 625 and 655 nm, identified as due to the 5D4→7FJ (J=6, 5, 4, 3, and 2) transitions of the terbium ions, respectively, was recorded. The excitation of the 5D4 emitting level of the Tb3+ ions is assigned to cooperative energy-transfer from pairs of ytterbium ions. The effect of temperature on the upconversion process was examined and the results revealed a fourfold upconversion enhancement in the 300–500 K interval. The enhancement of the upconversion process is due to the temperature dependence of the Yb3+-sensitizer absorption cross-section under anti-Stokes excitation. A rate-equation model using multiphonon-assisted absorption for the ytterbium excitation combined with the energy migration effect between Yb–Yb pair and Tb3+ ground-state depopulation via multiphonon excitation of the 7FJ excited states describes quite well the experimental results.