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 Tm 3+ ions (from 10 20 to 10 15 ion/cm 3 ) and a constant content of Yb 3+ ions (10 21 ion/cm 3 ) 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/cm 2 . Possible mechanisms of population of the upper 1 D 2 , 2 G 4 and 3 F 2,3 states of Tm 3+ 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 ( 1 G 4 ) from 3 to 2 and high upconversion at low Tm 3+ ions concentrations (about 10 18 -10 15 ion/cm 3 ) were attributed to the cooperative energy transfer from Yb-Yb* dimers. It was found that Tm 3+ /Yb 3+ co-doped fluorophosphate glasses could be promising as up-converting materials. • The concentration dependence of the UC luminescence of Tm 3+ /Yb 3+ co-doped fluorophosphate glasses was studied. • Mechanisms of population of the state 2 D 1, 3 F 2,3 , 1 G 4 and 1 P 0 of Tm 3 + ions were discussed depending on their concentration. • The cooperative energy transfer from Yb-Yb* dimmers led to the high upconversion at low Tm 3+ ions concentrations.