Intensity-dependent photon-echo relaxation in rare-earth-doped crystals

Photon-echo-relaxation measurements made on the ³H₄{{{-}}}³P₀ transition of 0.01 at. % Pr³⁺:YAG (where YAG represents yttrium aluminum garnet), ³H₄{{{-}}}¹D₂ transition in 0.1 at. % Pr³⁺:YAlO₃, and ⁷F₀{{{-}}}⁵D₀ transition in 0.25 at. % Eu³⁺:YAlO₃ show that the photon-echo relaxation rate increases when the intensities of the excitation pulses are increased. Although a part of the relaxation-rate increase in Pr³⁺:YAG may be attributed to an instantaneous spectral diffusion (ISD) in which the presence of excited neighboring Pr³⁺ ions change the local field and the absorption frequency of the rare-earth ions, our data deviate significantly from the ISD-model predictions. An additional intensity-dependent relaxation mechanism is required to explain the results.