Broadband down-conversion through the co-contribution of simultaneous energy transfer from Eu^3+/2+ to Yb³⁺ and CTS absorption of Yb³⁺

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 Eu^3+/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⁺. - Highlights: • Partial reduction of Eu³⁺ to Eu²⁺ in silicate glass in air was obtained. • Simultaneous energy transfer from Eu^3+/2+ to Yb³⁺ was proved. • Contribution of CTS absorption of Yb³⁺ to down-conversion was confirmed. • The distribution of Eu^3+/2+ in glass matrix at different sites was discussed. • The influence of local non-uniform of glass on quantum efficiency was discussed