Nonradiative dynamics of avalanche upconversion in Tm : LiYF4

In this paper we examine nonradiative processes (cross relaxation and energ y migration) vital for the generation of blue avalanche emission in Tm:LiYF 4 crystals subjected to radiation near 648 nm. Time-domain and temperature- dependent measurements indicate that two sequential cross-relaxation proces ses dominate the nonradiative decays of the (1)G(4) and H-3(4) excited stat es in this crystal. The occurrence of a sharp peak in the H-3(4) fluorescen ce decay constant versus temperature is reported and explained. Similar beh avior is observed in (1)G(4) emission. Our analysis indicates that individu al thermally activated Stack levels dominate cross-relaxation dynamics, rev ealing unexpected specificity for off-resonant, dark processes in this mate rial. Energy migration in the lower level of the avalanche absorption trans ition is also sufficiently rapid to explain the magnitude and comparative e fficiency of avalanche emission in this material vis-a-vis Tm:YAG. Based on these findings, we predict and experimentally verify the temperature depen dence of the threshold pumping intensity for avalanche upconversion in a qu antitative manner.