Tb3+ Fluorescence and Nonradiative Energy Transfer from Gd3+ to Tb3+ in Borate Glass

The fluorescence behavior of Gd3+ and Tb3+ has been studied with the ions dissolved together in a Calibo-1 glass host. The two ions form a coupled system so that energy absorbed by Gd3+ is transferred to Tb3+ by a nonradiative mechanism and is then emitted as Tb3+ fluorescence. When Tb3+ is incorporated alone into this glass, concentration quenching effects are observable. These have been found to be due to the shortening of the Tb3+ 5D4 lifetime, which in turn is due to the Tb3+ ions being packed closer together as the concentration of Tb3+ is increased. It is proposed that this gives rise to ``chains'' of Tb3+ ions along which energy can migrate, by a nonradiative transfer mechanism, to sinks in the structure. The rate of internal conversion from Tb3+ 5D3 to Tb3+ 5D4 has also been studied as a function of concentration. The results have been interpreted as being due to the formation of Tb3+-Tb3+ resonance-coupled pairs, which causes the rate of internal conversion to increase as the concentration of Tb3+ increases. The effects of changes in temperature and base glass composition on the Tb3+ 5D4 lifetime have also been investigated and are reported.