The optical absorption and fluorescence of Pr 3ϩ ions in yttria-stabilized zirconia single crystals are investigated. Fluorescence emissions from the 1 D 2 level are clearly dominant and low intensity emission lines from the 3 P 0 and 1 G 4 states are also observed. Analysis with the Judd-Ofelt theory of the absorption intensities has been made assuming that only ϳ40% of the praseodymium ions contribute to the optical absorption bands.Quantum efficiency values of ( 3 P 0 )ϳ0.2 and ( 1 D 2 )ϳ 1 are obtained at room temperature. 1 D 2 fluorescence quenching has been observed in heavily-doped samples due to cross relaxation processes among neighboring Pr 3ϩ ions. Analysis using the Inokuti-Hirayama model shows that electric dipole-dipole interactions are mainly responsible for the quenching effect. Pr 3ϩ ions are present in seven and sixfold configurations with a statistical distribution. The energy position of the 4 f 5d configuration is very different for each center. The fluorescence dynamics is explained by a mechanism involving thermally assisted population of the 3 P 1,2 ϩ 1 I 6 upper levels and fast relaxation to the 1 D 2 level via states of the excited 4 f 5d configuration.

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Optical absorption and luminescence experiments were used to study the photoconversion of neutral oxygen vacancies ͑F centers͒ in MgO single crystals thermochemically reduced at elevated temperatures. In crystals with an undetectable concentration of hydride ions and a moderate concentration of F centers (Ϸ10 17 cm Ϫ3), excitation with UV-light produces positively charged anion vacancies (F ϩ centers͒ and electrons which are subsequently trapped at impurities. Under continuous excitation, the F ϩ centers release holes which are trapped at cation vacancies charge compensated by impurities. In crystals with high concentrations of both hydride ions and F centers (Ϸ10 18 cm Ϫ3), the electrons from the F to F ϩ photoconversion are trapped mainly at the hydride ions to form H 2Ϫ ions, which are metastable at room temperature. ͓S0163-1829͑99͒12101-5͔

Optical absorption and emission experiments were used to characterize defects and defect aggregates in Mg-doped Al2O3 crystals due to thermochemical reduction at high temperatures. Oxygen vacancies and higher-order defects are produced much more readily in Mg-doped than in undoped Al2O3 crystals. F+ and F centers (oxygen vacancies with one or two electrons, respectively) were monitored by their optical absorption bands at about 4.8 and 6.0eV, respectively. In contrast with undoped crystals, where the reduction produces primarily F centers and a small amount of F+ centers, in Mg-doped crystals both F and F+ centers are created in comparable concentrations. These thermally generated F and F+ centers are much more stable than those produced in undoped crystals irradiated with neutrons. Clustering of individual oxygen vacancies forming higher-order defects, such as anion divacancy F22+ and F2+ centers, was investigated by low temperature absorption and luminescence experiments, in conjunction with UV irradiation and thermal treatments. The strong absorption bands at 2.87 and 3.69eV were shown to be due to Mg-perturbed F22+ and F2+ centers, respectively. In addition, photoconversion of F22+ and F2+ centers was observed. In crystals containing large concentrations of F22+ centers, electrons excited by 5.0eV light are trapped by F22+ and F2+ centers resulting in the conversion of F22+ centers into F2+. A model of F-type centers was extended to F22+ and F2+ centers. The calculated optical parameters are in very good agreement with those determined experimentally. A simple analysis of the lattice energy suggests that the environments of the F2-type centers are different in TCR Al2O3 crystals and in n-irradiated undoped crystals.

A systematic investigation is presented of the polarized optical absorption of the Nd3+ ion in LiNbO3 and LiNbO3(MgO) crystals. Energy levels of Nd3+ are identified and labeled by appropriate crystal quantum numbers μ=1/2 or μ=3/2, in agreement with C3 local symmetry. The anisotropic Judd–Ofelt theory is applied to estimate spectroscopic parameters relevant for laser applications (radiative lifetimes and branching ratios from the 4F3/2 laser state). Fluorescence lifetimes are also determined to estimate the quantum efficiency from the 4F3/2 state: η=0.6 for LiNbO3:Nd and η=0.5 for LiNbO3(MgO):Nd. The effect of MgO codoping in the energy levels as well as in the spectroscopic parameters of the Nd3+ ion is evaluated and discussed.