Fluorescence quenching and spectral diffusion inLa1−xP5O14:Ndx3+

Donor-donor and donor-acceptor spectral dynamics are studied in the laser material ${\mathrm{La}}_{1-x}{\mathrm{P}}_5{\mathrm{O}}_{14}$:$\mathrm{Nd}_x^{}{}_{}{}^{3+}$ using time-resolved fluorescence line narrowing. Direct excitation in the ${}_{}{}^4{}_{}{}^{}F_{\frac{3}{2}}^{}{}_{}{}^{}\mathrm{}\left(1\right)\mathrm{}$ state in $x=0.2,0.75, \mathrm{and} 1.0$ samples shows exponential fluorescence decay and linear concentration quenching of the line-narrowed component between 4.5 and 20 K. No spectral diffusion across the inhomogeneously broadened line is observed in this temperature region. This unusual exponential decay and linear quenching in the absence of fast donor-donor transfer is explained with a generalized Inokuti-Hirayama model. An onset of spectral migration is observed in $x=0.2\mathrm{} \mathrm{and} 0.75$ at 20 K, indicating the presence of phonon-assisted energy transfer. The temperature dependence of the phonon-assisted donor-donor transfer rate is measured for $x=0.2\mathrm{}$ between 17 and 24 K. This temperature dependence identifies the one-site resonant process, with an activation energy corresponding to the ${}_{}{}^4{}_{}{}^{}I_{\frac{9}{2}}^{}{}_{}{}^{}\mathrm{}\left(2\right)\mathrm{}-{}_{}{}^4{}_{}{}^{}I_{\frac{9}{2}}^{}{}_{}{}^{}\mathrm{}\left(1\right)\mathrm{}$ splitting, as the probable donor-donor interaction mechanism. The low-temperature results and the spectral diffusion measurements are described consistently in the studied temperature range by phonon-assisted energy transfer models, in which the donor excitation is transferred between single ions in an incoherent manner.