Absolute quantum yield for understanding upconversion and downshift luminescence in PbF2:Er3+,Yb3+ 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 PbF2:Er3+,Yb3+ crystals with different concentrations of Yb3+ ions in the range of 2 to 7.5 mol% (Er3+ concentration was fixed at 2 mol%) was carried out. The highest value of upconversion quantum yield (phi(UC)) 5.9% (at 350 W cm(-2)) was found in the PbF2 crystal doped with 2 mol% Er3+ and 3 mol% Yb3+. Since it is not always easy to directly measure phi(UC) and estimate the related key figure of merit parameter, saturated photoluminescence quantum yield (phi(UCsat)), a method to reliably predict phi(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, phi(UCsat) for that level can be calculated. This approach is tested on a series of PbF2:Er3+,Yb3+ crystals. Good agreement between the estimates obtained as above and the directly experimentally measured phi(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 PbF2:Er3+,Yb3+ 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.