Low-temperature up-conversion spectroscopy was used to deduce the symmetry of the sites that were occupied by Tm3+ ions. Single Tm3+ centers with cubic, trigonal, tetragonal and orthorhombic symmetries, and established energy level diagrams, were found. Single Tm3+ centers of cubic symmetry tended to predominate in CaF2:TmF3 when the Tm3+ concentration exceeded 1at%.

Red-to-Blue Up-Conversion Spectroscopy of Tm3+ in SrF2, CaF2, BaF2 and CdF2. M.Bouffard, J.P.Jouart, M.F.Joubert: Optical Materials, 2000, 14[1], 73-9