Role of Eu ions and defect centers in formation of high-temperature persistent spectral holes in glass

The mechanism of room temperature persistent spectral hole burning was investigated using Eu-doped sodium borate glass (35Na2O–65B2O3). The concentrations dependence of defect centers on the extent of hole burning was investigated for several Eu concentrations because the mechanism proposed so far are of two types: electron transfer to Eu3+ from Eu2+ and from defect centers in the glass. When the defect concentration increased due to X-ray irradiation, a high-temperature hole burning was observed at a Eu concentration (0.3 mol%); where the phenomenon is not observed in a normal sample without X-ray irradiation. When the Eu concentration was very low (0.1 mol%), a spectral hole was observed only at low temperatures even when the concentration of defect centers became saturated due to X-ray irradiation. Furthermore, a concentration quenching of fluorescence from Eu3+ became obvious at Eu concentrations of more than ca. 1 mol%. Hole burning became obvious at the same concentration. These findings indicate that the mechanism is an energy transfer from selectively excited Eu3+ to adjacent Eu2+ and subsequent electron transfer from Eu2+ to Eu3+. Defect centers in the glass reduce the barrier height of this electron transfer.