Selective excitation study of Yb3+ in GdCa4O(BO3)3 and YCa4O(BO3)3

The Yb3+-doped non-linear rare-earth calcium oxoborate crystals GdCa4O (BO3)3 (GdCOB) and YCa4O(BO3)3 (YCOB) were investigated by low-temperature absorption and selective excitation spectroscopy. The selective excitation and emission spectra revealed the spectral features of the main Yb3+ centre and the principal minority one in both crystals. The energy level schemes for these centres in both hosts, as well as structural assignments, were proposed. It was established that the minority centre, which represents about 10% of the total intensity and was assigned to Yb3+ in a Ca2+ site, is not a trap but an energy donor for the main centre. Common features of and differences between the vibronic structure of the two centres in GdCOB and YCOB are discussed. The existence of additional features in YCOB:Yb spectra is pointed out. (author)