Decay behavior of Tb3+ green fluorescence in borate glasses

In 70B2O3–30CaO–xTb2O3 glasses (x = 0.10–20.0 (mole fraction)) prepared by the melt-quenching method, the decay behavior of the green fluorescence due to the 5D4 → 7F5 transition of Tb3+ ions was investigated by measuring the fluorescence spectra and time-resolved fluorescence spectra and then by analysis using the integrated intensity ratio of fluorescence bands due to the 5D3 → 7FJ (J = 5, 4, and 3) and 5D4 → 7FJ (J = 6, 5, 4, 3, 2, 1, and 0) transitions of Tb3+ ions, and the rate equations. In a 70B2O3–30CaO glass with high phonon energy, it was found that the 98.5% of all the excited electrons of Tb3+ ions were relaxed from the 5D3 to 5D4 level due to the multiphonon relaxation (MR) process. In addition, for x > 6.0, the green fluorescence rapidly decayed with increasing x, because the cross-relaxation process, (5D4 → 7F0) → 3(7F0 ← 7F6) among the excited Tb3+ ion at 5D4 and its three neighboring Tb3+ ions at 7F6 easily occurred. For x = 6, the cross-relaxation process, (5D3 → 5D4) → (7F0 ← 7F6) between Tb3+ ions was fully promoted and the cross-relaxation process, (5D4 → 7F0) → 3(7F0 ← 7F6) was repressed, and almost 100% of excited electrons in Tb3+ ions were available for the fluorescence due to the 5D4 → 7FJ transitions. As a result, it was found that the 70B2O3–30CaO–6Tb2O3 glass had a potential of the green fiber laser.

► Tb3+ fluorescence decay behavior in 70B2O3–30CaO–xTb2O3 glasses was investigated.
► The decay curves of the 5D4 initial level were single exponential functions.
► The lifetime of the 5D4 initial level was constant and 2.58 ms for x = 0.1–6.0.
► For x = 7.0–20.0, the lifetime decreased because of the cross-relaxation process.
► The 70B2O3–30CaO–6Tb2O3 glass had a potential of the green fiber laser.