Laser spectroscopy, energy-transfer and bleaching processes in crystalline rare-earth centres
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Laser selective excitation spectroscopy has been applied to investigate electronic structure, excited-state relaxation, inter-ion energy transfer and fluorescence bleaching in several species of crystalline rare-earth centres. Group theoretical techniques have been applied to understand the polarisation behaviour of ensembles of optically-active rare-earth centres in the fluorite lattice. Polarisation experiments were conducted on the Pr³⁺and Tb³⁺ centres in both pure and hydrogenated CaF₂ and SrF₂ crystals, to determine their site-symmetries and to identify their transitions and electronic energy levels. Model configurations and bleaching mechanisms were either proposed or confirmed for all of the known praseodymium Cs centres and Cs* photoproduct centres. These models account for the polarised-bleaching behaviour of each Cs centre. Six new Tb³⁺centres were found in the hydrogenated crystals and at least three of these exhibit fluorescence bleaching. One centre exhibited fluorescence enhancement, a related phenomenon reported for the first time in this study. Crystalfield calculations yielded fitted values for the crystal-field parameters of the terbium C₄ʋF- centres. Bleaching is presented as a phonon-scattering process, involving light-induced barrier crossing in the double-well potential of two distinct ionic configurations of a RE³⁺centre. Thermal-reverting temperatures were measured for most of the Cs centres and used to estimate their barrier potentials. The results confirm that bleaching involves barrier crossing and not quantum tunnelling. Inter-ion energy transfer was studied in both pure and mixed praseodymium and thulium doped CsCdBr₃ crystals. New cross-relaxation and upconversion mechanisms were identified, including several which enable infrared to blue or ultraviolet upconversion. Some novel two-laser techniques for probing both sequential and cooperative-upconversion processes are demonstrated.