Direct imaging of rare-earth ion clusters in Yb:CaF2

The existence and the identification of only one or several coparticipating luminescent Yb3+ centers in the heavily doped Yb:CaF2 laser crystals which are considered in the development of several high intensity laser chains have been examined first by using two complementary and original experimental approaches, i.e., registration of low temperature site-selective laser excitation spectra related to near-infrared and visible cooperative emission processes, on the one hand, and direct imaging at the atomic scale of isolated ions and clusters using a high-resolution scanning transmission electron microscope in the high angle annular dark-field mode, on the other hand, and then correlating the data with simple crystal field calculations. As a consequence, and although all the experimental details could not be accounted for quantitatively, a good overall correlation was found between the experimental and the theoretical data. The results show that at the investigated dopant concentrations, Yb:CaF2 should be considered as a multisite system whose luminescent and lasing properties are dominated by a series of Yb3+ clusters ranging from dimers to tetramers. Hexameric luminescent centers may be dominant at really high dopant concentrations (likely above 20 at. %), as was originally proposed, but certainly not at the intermediate dopant concentrations which are considered for the laser application, i.e., between about 0.5 and 10 at. %.