Blue, green and red upconversions in Ho2O3-doped fluorophosphate glasses

Near infrared (NIR) to visible upconversions of a fluorophosphate glass of composition (mol%) 7Ba(PO3)2–32AlF3–30CaF2–18SrF2–13MgF2 doped with various concentrations (0.1, 0.3 and 1.0 mol%) of Ho2O3 have 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 been found to center at 491 nm (blue), 543 nm (green) and 658 nm (red), respectively. These bands have been justified from the evaluation of the absorption, normal (down conversion) fluorescence and excitation spectra. The upconversion processes have been interpreted 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 infrared reflection spectral (IRRS) analysis that the upconversion phenomena are expedited by the low multiphonon relaxation rate in fluorophophate glasses owing to their high intense low phonon energy of ∼600 cm−1 which is very close to that of fluoride glasses (500–600 cm−1).