Upconversion and reduced ^4F_3/2 upper-state lifetime in intensely pumped Nd:YLF

Nonexponential decay of the 4F(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, alpha2 = (0.98 +/- 0.03) x 10(-16) cm3/s, is directly evaluated from the decay kinetics. The ETU and the migration microparameters are also estimated to be C(da)* approximately 200 x 10(-40) cm6/s and C(dd) approximately 1000 x 10(-40) cm6/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.