In the presented work, we investigated the optical and thermal stability of upconversion nanoparticles based on the three widely used matrices (NaYF4, Y2O3, LaF3). Analysis of the upconversion emission as a function of pump power density in a wide range revealed a multi-stage functional dependence. The stages of linear growing, saturation and degradation with both reversible and irreversible characters were discovered. For matrices of nanoparticles with low-temperature stability (NaYF4), the dependence proves to be irreversible that could cause by a change in the structure and chemical composition of the matrix. Reversible dependence occurs in matrices with high-temperature stability (Y2O3 and LaF3) and is caused by multiphonon nonradiative relaxation, which can be temperature-stimulated because of self-heating and low air-cooling of the crystal matrixes with low thermal conductivity.
A. Fedotov, D. Pominova, E. O. Orlovskaya, Yu. V. Orlovskii, T. Niemi, and R. Gumenyuk, "Analysis of upconversion nanoparticles as an active medium for upconversion light sources," Proc. SPIE 10914, Optical Components and Materials XVI, 109140R (Presented at SPIE OPTO: February 05, 2019; Published: 1 March 2019); https://doi.org/10.1117/12.2507599.
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