Ultraviolet Upconversion Luminescence in a Highly Transparent Triply-Doped Gd3+–Tm3+–Yb3+ Fluoride–Phosphate Glasses

We report near-infrared to ultraviolet (UV) upconversion emissions in triply-doped Gd3+–Tm3+–Yb3+ fluoride–phosphate glasses. Emission at 310 nm, originated from the Gd3+:6P7/2 → 8S7/2 transition, was observed for the first time in glasses. The high-purity glasses prepared exhibit extended transparency in the UV down to 200–250 nm. The mixed fluoride–phosphate environment of the rare-earth ions was characterized by means of NMR techniques using scandium as a diamagnetic mimic for the luminescent species, for which the ligand distribution was quantified by 45Sc{31P} rotational echo double-resonance NMR. Both the intensity of the Gd3+ emission as well as those of the UV emissions at 290, 347, and 363 nm increase with increasing ratio of fluoride to phosphate ligands coordinating to the rare-earth ion.