Cooperative upconversion and energy transfer of new high Er³⁺- and Yb³⁺-Er³⁺-doped phosphate glasses

Systematic studies of cooperative upconversion and Yb³⁺-Er ³⁺ energy transfer in newly developed phosphate glasses were performed by a rate-equation formalism. The cooperative-upconversion coefficients of the ⁴I_13/12 level for different Er ³⁺ concentrations were determined from the luminescence-decay curves for high pump intensities. A small cooperative-upconversion coefficient of 1.1 × 10^-18cm³/s was obtained for a high Er ³⁺ concentration of 4 × 10²⁰ ions/cm³. Yb³⁺-Er³⁺ energy-transfer coefficients for an Er ³⁺ concentration of 2 × 10²⁰ ions/cm³ codoped with different Yb³⁺ concentrations were calculated from the lifetime measurements of the ²F_5/2 level of Yb ³⁺ ions. For Er³⁺ codoped with an Yb³⁺ concentration of 6 × 10²⁰ ions/cm³, an energy-transfer coefficient of 1.1 × 10^-16 cm³/s and an energy-transfer efficiency higher than 95% were determined from our measurements under weak excitation. The cooperative-upconversion coefficients of Yb³⁺-Er³⁺-doped samples were found to be consistent with that of an Er³⁺-doped sample with the same Er³⁺ concentration. The weak cooperative-upconversion effect of high Er³⁺ concentrations and efficient Yb³⁺-Er³⁺ energy transfer indicate that these newly developed Er³⁺- and Yb³⁺-Er ³⁺-doped phosphate glasses are excellent for active device applications.