Upconversion luminescence and energy transfer in the fluorophosphate glasses with low phosphate content doped with Tm3+ and Yb3+ions

Abstract We report an experimental observation of the upconversion in fluorophosphate glasses doped with different concentrations of Tm3+ ions (from 1020 to 1015 ion/cm3) and a constant content of Yb3+ ions (1021 ion/cm3) when pumped by a diode laser with a wavelength of 975 nm with a change in the pumping power density from 2.0 to 14.5 kW/cm2. Possible mechanisms of population of the upper 1D2, 2G4 and 3F2,3 states of Tm3+ ions were discussed depending on their concentration. The role of cross-relaxation processes in the population of the upper excited levels is considered. A change in the values of n-parameter of the luminescence at 475 nm (1G4) from 3 to 2 and high upconversion at low Tm3+ ions concentrations (about 1018-1015 ion/cm3) were attributed to the cooperative energy transfer from Yb-Yb* dimers. It was found that Tm3+/Yb3+ co-doped fluorophosphate glasses could be promising as up-converting materials.