SITE-SELECTIVE UP-CONVERSION LUMINESCENCE OF HO3-DOPED CAF2 CRYSTALS()

With red dye laser excitation, site-selective upconversion properties of three Ho3+ centres (C4v, C3v, and dimer) in CaF2:0.1 at% Ho3+ cryst als are studied. The energy level diagrams for the three centres are d educed from the spectral data. The excited state absorption from the I -5(7), level plays a very important role in the upconversion coming fr om C4v and C3v centres, whereas the ground state absorption of I-5(8) dominates for that from the dimer centre. The upconversion of C4v and C3v centres is explained by a sequential two-photon excitation mechani sm. For the dimer centre, the predominant upconversion mechanism is en ergy transfer.