Nonradiative dynamics of avalanche upconversion in Tm:LiYF4

In this paper we examine nonradiative processes (cross relaxation and energy migration) vital for the generation of blue avalanche emission in Tm:LiYF4 crystals subjected to radiation near 648 nm. Time-domain and temperature-dependent measurements indicate that two sequential cross-relaxation processes dominate the nonradiative decays of the (1)G(4) and H-3(4) excited states in this crystal. The occurrence of a sharp peak in the H-3(4) fluorescence decay constant versus temperature is reported and explained. Similar behavior is observed in (1)G(4) emission. Our analysis indicates that individual thermally activated Stack levels dominate cross-relaxation dynamics, revealing unexpected specificity for off-resonant, dark processes in this material. Energy migration in the lower level of the avalanche absorption transition is also sufficiently rapid to explain the magnitude and comparative efficiency of avalanche emission in this material vis-a-vis Tm:YAG. Based on these findings, we predict and experimentally verify the temperature dependence of the threshold pumping intensity for avalanche upconversion in a quantitative manner.