Spectroscopy and frequency upconversion of Er ions in fluorotellurite glasses

In this work we report the optical properties and upconversion luminescence of Er³⁺ ions in TeO₂–WO₃–PbO–BaF₂ and TeO₂–TiO₂–Nb₂O₅–BaF₂ fluorotellurite glasses and their comparison with those of TeO₂–TiO₂–Nb₂O₅ glass. The optical properties of Er³⁺ ions have been established in terms of absorption and emission spectra and lifetime measurements. The 1.5 μm emission cross-section has been determined from the line shape of the emission spectrum and the calculated emission probability for the ⁴I_13/2 level. The highest emission cross section (6.9 × 10⁻²¹ cm²) corresponds to the TeO₂–TiO₂–Nb₂O₅–BaF₂ glass with a figure of merit for the bandwidth of 524.4 cm² nm. Upconversion emissions at 530, 548 nm, and 660 nm have been obtained under infrared excitation at 800 nm in the ⁴I_9/2 level and compared with those obtained under one photon excitation. The green emission corresponding to the ⁴S_3/2 → ⁴I_15/2 transition is dominant in all glasses. The excitation wavelength dependence of the upconverted luminescence together with its time evolution after infrared pulsed excitation suggest that energy transfer upconversion processes are responsible for the green upconversion luminescence.