Measurement of upper level lifetime in presence of radiation trapping: A revisitation of the pinhole method

In this work we expose a recently developed theoretical model for the analysis of the measurements obtained with the so-called pinhole method, for the determination of the upper level lifetime in materials where the superimposition between fluorescence and absorption spectra determines significant radiation trapping effect. Under fairly general conditions the fluorescence decay curve after a pulsed excitation of the sample obtained with this experimental set-up can be conveniently described by a double exponential functional form. The faster of the two decay times is close to the intrinsic fluorescence lifetime, with a difference that can be calculated with geometrical considerations. The theoretical results were tested with a suitably designed experiment, where the upper level lifetime of the laser transition of some Yb doped samples were investigated, obtaining results that are consistent with literature data and in good agreement with the theoretical results as for the temporal dependence of the fluorescence decay.