Upconversion luminescence properties of europium in ZnO–SiO2 glasses by femtosecond laser excitation

Red upconversion luminescence (UCL) of Eu3+ in SiO2 and 10ZnO–90SiO2 glasses was observed at femtosecond (fs) laser excitation and their UCL properties were analyzed and compared. The results of the dependence of UCL intensity on pump power indicated that the two-photon excitation of Eu3+ in SiO2 glass and three-photon excitation in ZnO–SiO2 glass occurred. The mechanism of UCL of Eu3+ was two-photon simultaneous absorption in SiO2 glass and three-phonon simultaneous excitation in ZnO–SiO2 glass. According to the excitation spectra, this can be attributed to the remarkable changes of the proportion of f–f transitions to charge transfer (CT) absorption of Eu3+ in different glasses.