Two- and three-photon excitation of Gd3+ 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.