Absolute quantum yield for understanding upconversion and downshift luminescence in PbF 2 :Er 3+ ,Yb 3+ crystals.

The search for new materials capable of efficient upconversion continues to attract attention. In this work, a comprehensive study of the upconversion luminescence in PbF 2 :Er 3+ ,Yb 3+ crystals with different concentrations of Yb 3+ ions in the range of 2 to 7.5 mol% (Er 3+ concentration was fixed at 2 mol%) was carried out. The highest value of upconversion quantum yield ( ϕ UC ) 5.9% (at 350 W cm -2 ) was found in the PbF 2 crystal doped with 2 mol% Er 3+ and 3 mol% Yb 3+ . Since it is not always easy to directly measure ϕ UC and estimate the related key figure of merit parameter, saturated photoluminescence quantum yield ( ϕ UCsat ), a method to reliably predict ϕ UCsat can be useful. Judd-Ofelt theory provides a convenient way to determine the radiative lifetimes of the excited states of rare-earth ions based on absorption measurements. When the luminescence decay times after direct excitation of a level are also measured, ϕ UCsat for that level can be calculated. This approach is tested on a series of PbF 2 :Er 3+ ,Yb 3+ crystals. Good agreement between the estimates obtained as above and the directly experimentally measured ϕ UCsat values is demonstrated. In addition, three methods of Judd-Ofelt calculations on powder samples were tested and the results were compared with Judd-Ofelt calculations on single crystals, which served as the source of the powder samples. Taken together, the results presented in our work for PbF 2 :Er 3+ ,Yb 3+ crystals contribute to a better understanding of the UC phenomena and provide a reference data set for the use of UC materials in practical applications.