Optical properties and valence change of europium ions in a sol-gel Al2O3-B2O3-SiO2 glass by femtosecond laser pulses

The Al2O3-B2O3-SiO2 glass containing europium ions was prepared by a sol-gel method. Fluorescence line-narrowing spectra (FLN) indicate two different environments of the Eu 3+ ions. The calculated second crystal-field parameters exhibit the opposite behaviors of the two different environments. The FLN excitation and emission spectra before and after irradiation show that the change of the emission mainly comes from the Eu 3+ ions at site I, revealing that the concentration ratio of the Eu 3+ ions at site I to site II was decreased. The emission spectra confirmed that some Eu 3+ ions were reduced into Eu 2+ ions. The excitation spectra indicate that the Eu 3+ ions at the sites with higher covalence degree can be easily reduced, implying that the Eu 3+ ions are more easily reduced at site I than at site II. The absorption spectra before and after irradiation exhibit that the absorption of Eu 2+ ions increases and that the positive hole centers appear. These results suggest a mechanism of the formation of the Eu 2+ ions by femtosecond laser irradiation.