Energy transfer upconversion and avalanche mechanisms in systems doped with Ho3+, Pr3+ and Yb3+ ions

This work analyzes the mechanisms that enable the upconverted green emission (5S2→5I8) of the Ho3+ ion under infrared excitation (700 nm-900 nm) in several crystalline hosts (YAlO3, YLiF4, BaY2F8) with a special attention to the avalanche excitation mechanism. The possibility to obtain upconverted green emission (5S2→5I8) of the Ho3+ ion due to avalanche excitation mechanism in Ho3+, Yb3+:YAlO3 and Ho3+, Yb3+:YLiF4 under excitation in the spectral range 750-840 nm were also studied. Parameters involved in the upconversion such as excited state absorption (ESA) cross section and cross-relaxation rates were determined from spectroscopic measurements. A system of differential equations (rate equations) was used to describe the upconversion mechanism and was numerically solved. The results were compared with experimental data. A reduction of this system to a 3-level ''simplified system'' was presented The simplified system includes only the ground state level, the emitting level, and the intermediate level. The differences between the photonavalanche mechanism and looping mechanism were discussed and analyzed according to this simplified system. (orig.)