Optical transitions and upconversion fluorescence in Ho3+∕Yb3+ doped bismuth tellurite glasses

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10 mol % of bismuth oxide was designed to add in making novel Ho3+ doped heavy metal tellurite glasses (LKBBT) with large refractive indices and glass densities. Based on optical absorption, Judd-Ofelt parameters Ω2, Ω4, and Ω6 have been derived to be 4.373×10−20, 1.906×10−20, and 1.451×10−20cm2, respectively, which indicate higher asymmetry and stronger covalent environment in LKBBT glasses compared with ZnF2–CdF2, oxyfluorobotate, and zirconium-barium-lanthanum-aluminum-sodium-fluoride glasses. Efficient visible upconversion emission bands peaked at 544, 657, and 753nm in Ho3+∕Yb3+ codoped LKBBT glass system have confirmed that two photons contribute to the upconversion processes under the pumping of a 974nm diode laser. Owing to large absorption and emission cross sections, Yb3+ ion is considered to be a preferable sensitizer for catching enough pumping energy and transferring considerable energy to Ho3+ in LKBBT glasses. Low maximum phonon energy and high spontaneous transition probabilities of Ho3+ are beneficial in obtaining efficient upconversion and IR emission in the glass system.