Site-selective luminescence spectroscopy of Ho~(3+) ions in CaF_2 and CsCdBr_3 crystals

摘要

The technique of selective laser excitation is used to document the centres present in CaF_2 and CsCdBr_3 crystals containing Ho~(3+) ions and to assess the up-conversion fluorescence yields of the centres. For dopant levels of up to 0.05%, the two centres predominant in CaF_2:Ho~(3+) are monomers in which the Ho~(3+) ions are in sites of either tetragonal or trigonal symmetry. The centres do not exhibit up-conversion. At higher Ho~(3+) concentrations, dimer centres are more prevalent and, largely, these are aggregates of the centres present in the lowly doped samples. In CsCdBr_3:Ho~(3+) the predominant centre is a dimer of the form Ho~(3+)-vacancy-Ho~(3+), even at low dopant concentrations. The dimer centre in CsCdBr_3 yields much stronger up-conversion fluorescence compared to those in CaF_2.