Nonexponential decay of the <sup>4</sup><i>F</i><sub>3/2</sub> upper laser state in Nd:YLF was observed with time-resolved fluorescence spectroscopy and small-signal gain probes in low-doped (1.16-at. %) samples when intensely pumped to high population inversions. A rapid initial decay is observed after <i>Q</i>-switched laser pumping, followed by a longer nonexponential decay that is clearly identified as a phase of migration-assisted energy-transfer upconversion (ETU) in the hopping regime. During this phase, a rate-equation treatment is valid and the macroscopic ETU coefficient, α<sub>2</sub> = (0.98 ∓ 0.03) × 10<sup>−16</sup> cm<sup>3</sup>/s, is directly evaluated from the decay kinetics. The ETU and the migration microparameters are also estimated to be <i>C<sub>da</sub></i>* ≈ 200 × 10<sup>−40</sup> cm<sup>6</sup>/s and <i>C<sub>dd</sub></i> ≈ 1000 × 10<sup>−40</sup> cm<sup>6</sup>/s, respectively, and are compared with theoretical values found in the literature. On the basis of these values, rate-equation treatments of intensely pumped Nd:YLF are not strictly valid.
© 1999 Optical Society of America
(140.3530) Lasers and laser optics : Lasers, neodymium
(140.3580) Lasers and laser optics : Lasers, solid-state
(190.7220) Nonlinear optics : Upconversion
(260.3800) Physical optics : Luminescence
(300.6500) Spectroscopy : Spectroscopy, time-resolved
Jonathan D. Zuegel and Wolf Seka, "Upconversion and Reduced 4F3 /2 Upper-State Lifetime in Intensely Pumped Nd:YLF," Appl. Opt. 38, 2714-2723 (1999)