Upconversion and reduced 4F3/2 upper-state lifetime in intensely pumped Nd:YLF

Nonexponential decay of the F-4(3/2) 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 Q-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, alpha(2) = (0.98 +/- 0.03) x 10(-16) cm(3)/s, is directly evaluated from the decay kinetics. The ETU and the migration microparameters are also estimated to be C-da* approximate to 200 X 10(-40) cm(6)/s and C-dd approximate to 1000 X 10(-40) cm(6)/s, respectively, and are compared with theoretical values On the basis of these values, rate-equation treatments of intensely pumped 200 X 10(-40) cm(6)/s and C-dd approximate to 1000 X 10(-40) cm(6)/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. (C) 1999 Optical Society of America.