Broadband down-conversion through the co-contribution of simultaneous energy transfer from Eu3+/2+ to Yb3+ and CTS absorption of Yb3+

• Partial reduction of Eu3+ to Eu2+ in silicate glass in air was obtained.
• Simultaneous energy transfer from Eu3+/2+ to Yb3+ was proved.
• Contribution of CTS absorption of Yb3+ to down-conversion was confirmed.
• The distribution of Eu3+/2+ in glass matrix at different sites was discussed.
• The influence of local non-uniform of glass on quantum efficiency was discussed.

The europium (Eu) and ytterbium (Yb) co-doped SiO2–Al2O3–MF2 (M = Ca, Sr) oxyfluoride glasses were prepared by the melting-quenching method, in which the partial reduction of Eu3+ ions during high temperature fabrication in air atmosphere was explained by optical basicity model. In this system, the near-infrared emission of Yb3+ at 974 nm can be observed upon the excitation of both Eu3+ and Eu2+, as well as charge transfer state absorption of Yb3+. The steady and time-resolved luminescence spectra measurements were carried out to further prove the energy transfer from Eu3+/2+ ions to Yb3+. Based on the measured luminescence decay of Eu3+ and Eu2+, the energy transfer efficiency and the quantum efficiency are much higher in the glass containing SrF2 due to the efficient energy transfer from both Eu3+ and Eu2+ to Yb3+.