Energy transfer and fluorescence characteristics of erbium-doped ytterbium aluminum garnet

Abstract

We have examined the fluorescence characteristics of the garnet-type crystal Yb 3Al 5O 12 : Er 3+ (YbAlG : Er 3+) and studied the energy transfer process between the two rare earth ions over a temperature range 78–297 K. Certain data were compared with those of YAlG : Er 3+. In YbAlG : Er 3+, Yb fluorescence is observed at ≉1.03 μm (corresponding to the 2F 5/2 → 2F 7/2 transition); Er fluorescence occurs at ≉8500 Å ( 4S 3/2 → 4I 13/2 transition) and ≉1.6 μm ( 4I 13/2 → 4I 15/2 transition). In YAlG : Er 3+, the same Er lines are observed with the addition of a band at ≉1 μ ( 4I 11/2→ 4I 15/2 transition). In YbAlG : Er 3+, the decay pattern of the Yb emission is purely exponential at all the temperatures examined; the fluorescence lifetime ranges from 36 μ s (at 78 K) to 74 μs(at 269 K). The lifetime of the Er 4I 13/2 level in the same sample increases from 5.4 ms (at 78 K) to 6.85 ms (at 294 K). The lifetime of this Er level in YAlG : Er 3+ is weakly temperature dependent over the same range with a value of ≉12 ms. Excitation spectra were obtained for the Er 1.53 μm fluorescence in YbAlG : Er 3+ in order to verify the presence of Yb → Er energy transfer in this sample. The presence of the Yb absorption band (≉1 μm) in these spectra provides direct evidence of this energy transfer. The relative enhancement of this Yb band with respect to the Er bands in going from 78 K to 175 K is an indication of a more efficient transfer at the higher temperature. Excitation spectra obtained for the Yb 1.03 μm fluorescence in YbAlG : Er 3+ revealed the presence of Er → Yb energy transfer as well in this sample. The existence of both Yb → Er and Er → Yb transfer is expected, due to the resonance between the 4I 11/2 → 4I 15/2 transition of Er and the 2F 5/2 → 2F 7/2 transition of Yb. The above results are explained in terms of a rate equation model in which transfer in both directions is treated in the following manner: Yb → Er transfer is considered to be much more probable than decay processes originating at the Yb 2F 5/2 level; Er → Yb transfer is treated as much more probable than decay processes originating at the Er 4I 11/2 level.