IRRS, UV-Vis-NIR absorption and photoluminescence upconversion in Ho-doped oxyfluorophosphate glasses

Infrared reflection spectroscopic (IRRS), ultraviolet-visible-near infrared (UV-Vis-NIR) absorption and photoluminescence upconversion properties with special emphasis on the spectrochemistry of the oxyfluorophosphate (oxide incorporated fluorophosphates) glasses of the Ba(PO₃)₂-AlF₃-CaF₂-SrF₂-MgF₂-Ho₂O₃ system have been studied with different concentrations (0.1, 0.3 and 1.0 mol%) of Ho₂O₃. IRRS spectral band position and intensity of Ho³⁺ ion doped oxyfluorophosphate glasses have been discussed in terms of reduced mass and force constant. UV-Vis-NIR absorption band position has been justified with quantitative calculation of nephelauxetic parameter and covalent bonding characteristics of the host. NIR to visible upconversion has been investigated by exciting at 892 nm at room temperature. Three upconverted bands originated from the ⁵F₃→⁵I₈, (⁵S₂, ⁵F₄)→⁵I₈ and ⁵F₅→⁵I₈ transitions have found to be centered at 491 nm (blue, medium), 543 nm (green, very strong) and 658 nm (red, weak), respectively. These bands have been justified from the evaluation of the absorption, normal (down conversion) fluorescence and excitation spectra. The upconversion processes have been explained by the excited state absorption (ESA), energy transfer (ET) and cross relaxation (CR) mechanisms involving population of the metastable (storage) energy levels by multiphonon deexcitation effect. It is evident from the IRRS study that the upconversion phenomena are expedited by the low multiphonon relaxation rate in oxyfluorophosphate glasses owing to their high intense low phonon energy (∼600 cm⁻¹) which is very close to that of fluoride glasses (500-600 cm⁻¹).