Room-temperature photochemical hole burning of Eu3+ in sodium borate glasses

Persistent spectral hole burning (PSHB) has been observed at 77 K to room temperature for Eu3+ in sodium borate glasses with compositions of xNa2O.(100 - x)B2O3 (x = 10, 25, 35). Glasses melted in air do not show the phenomenon of PSHB, while room-temperature PSHB occurs in the glasses prepared by melting in reducing conditions. Eu-151 Mossbauer spectra reveal that europium ions are present as both Eu3+ and Eu2+ in glasses melted in reducing conditions. The relative hole area increases with an increase in the molar ratio of Eu2+ to EU3+ when the content of EU2+ is low, whereas the relative hole is almost independent of the molar fraction of Eu2+ when the content of Eu2+ is high. Furthermore, a comparison of hole burning among sodium borate glasses with the molar fraction of Eu2+ being almost the same as one another shows that the hole area grows larger as the content of Na2O becomes higher. These observations are interpreted in terms of the photoinduced redistribution of electric charges between Eu3+ and Eu2+. The effect of the glass composition on hole widths is also discussed based on the structural change in sodium borate glasses.