Two- and three-photon excitation of Gd in CaAl12O19

We have employed two-photon excitation to study the higher energy levels of Gd³⁺ ions in CaAl₁₂O₁₉ and we compare the results with those obtained using conventional UV excitation techniques. Under two-photon excitation, the luminescence intensity exhibits an unusual temporal behavior, a very long build-up followed by a decrease by orders of magnitude, ascribed to a recombination-assisted luminescence excitation mechanism assuming photo-ionization of Gd³⁺ ions and trapping of free electrons on deep traps.

We also find that the two-photon excitation spectra contain an additional broadening contribution which can be attributed to homogeneous broadening of excitation levels caused by excited state absorption into the conduction band. We believe that this may be a general phenomenon whenever participating photons produce ionization of impurity ions from metastable excited states. The phenomenon can manifest itself also in two-photon ionization spectral hole burning and in up-conversion processes (in the latter case, the homogeneous broadening can be caused by an intra-ion excited-state absorption).