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

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 2, 4, and 6 have been derived to be 4.373× 10-20, 1.906× 10-20, and 1.451× 10-20 cm2, respectively, which indicate higher asymmetry and stronger covalent environment in LKBBT glasses compared with Zn F2 -Cd F2, oxyfluorobotate, and zirconium-barium-lanthanum-aluminum-sodium-fluoride glasses. Efficient visible upconversion emission bands peaked at 544, 657, and 753 nm in Ho3+ Yb3+ codoped LKBBT glass system have confirmed that two photons contribute to the upconversion processes under the pumping of a 974 nm 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.