Excited state absorption and energy-transfer mechanisms of up-conversion luminescence in Er-doped oxyfluoride glass ceramics at different temperatures

Oxyfluoride silicate glass SiO₂-Al₂O₃-Na₂CO₃-NaF-LaF₃-ErF₃ was synthesized. The glass transition and crystallization temperatures were determined by differential thermal analysis. Glass ceramics containing LaF₃:Er³⁺ crystallites of size ∼20 nm were formed in the glass matrix after the heat treatment of the precursor glass in the vicinity of the crystallization temperature. Up-conversion luminescence, excitation spectra as well as time-resolved up-conversion luminescence of the glass and glass ceramics were studied at different temperatures. The up-conversion transients showed that at room temperature the dominant mechanism of the up-conversion luminescence in the glass ceramics is excited state absorption while at lower temperatures energy-transfer mechanism prevails. The origins of these differences are discussed in terms of the transitions between Stark manifolds of ⁴I_15/2, ⁴I_11/2 and ⁴F_7/2 states in Er³⁺ ions.