Mode-mismatched thermal lens (TL) measurements in Cr<sup>3+</sup> -doped fluoride crystals (LiSrAlF<sub>6</sub> and LiSrGaF<sub>6</sub>) are reported. A nonlinear increase of the TL signal, and decrease of quantum efficiency, with increasing excitation power was observed and attributed to energy-transfer upconversion (ETU). Assuming an upconversion rate that is proportional to the excited-state population, <i>W</i><sub>up</sub> = λ <i>N</i><sub>e</sub> , the theoretical model developed fits the experimental data well. The ETU parameters (λ) were determined with unprecedented accuracy for low Cr concentrations. Thermo-optical parameters (<i>K</i>, d<i>s</i>/d<i>T</i>) were also obtained.
© 2003 Optical Society of America
(190.7220) Nonlinear optics : Upconversion
(260.2160) Physical optics : Energy transfer
(350.5340) Other areas of optics : Photothermal effects
(350.6830) Other areas of optics : Thermal lensing
Viviane Pilla, Tomaz Catunda, Hans P. Jenssen, and Arlete Cassanho, "Fluorescence quantum efficiency measurements in the presence of Auger upconversion by the thermal lens method," Opt. Lett. 28, 239-241 (2003)