Radiative Properties of Rare Earth Elements - Free and Bound in Crystals

Abstract: This thesis is devoted to fundamental aspects, and to spectroscopic applications of radiative properties in free atoms and ions, as well as in ions doped into crystals. Using the time-resolved laser-induced fluorescence technique, measurements of radiative lifetimes for rare earth elements have been performed in neutral Pr, Nd, Tm, and in singly ionized Sm, Er, Nd, Tm, Yb, as well as in doubly ionized Tm and Tb ions. Free atom and ions were produced by laser ablation. This technique was also applied to Mn I and Cd I, II. The excitation schemes include single-step and two-step processes. The levels range in energy from 18000 up to 98000 cm-1 and the lifetimes fall in the range between 1.5 to 200 ns. Data were compared to theoretical results obtained in relativistic Hartree-Fock calculations taking core-polarization effects into account. Radiative lifetimes of trivalent RE ions doped into several host crystals, including YAG and YAlO3, have been measured at room temperature. Upconversion emissions in the green, blue, violet, and ultraviolet ranges were observed. The upconversion mechanisms were studied by making use of time-resolved luminescence spectroscopy. It appears that, depending on the excitation energy, the upconversion luminescence is dominant due to either energy-transfer upconversion or excited state absorption when the samples were excited into the 4S3/2 multiplet. The studies of the upconversion phenomena in YAG: Er3+ crystal have also been focused on the 4F9/2 multiplet, in which one-colour and two-colour laser excitation processes were employed. Radiative properties of RE-ion-doped crystal have also been investigated at liquid helium temperature using spectral hole burning and coherent transient spectroscopy. The hyperfine level splittings of the 1D2 and 3H4 states and the homogeneous linewidth of the lowest crystal-field states of the zero-phonon 3H4 - 1D2 transition in Pr3+:KY(WO4)2 were obtained. The radiative lifetime of the 1D2 state was measured with the time-resolved fluorescence and three-pulse photon echo techniques. In addition, the spectroscopy of Ho3+:LuYO4 and Ho3+:YVO4 was studied in this thesis as well. Experiments related to laser stabilization using hole burning in combination with a radio frequency (rf) field were partially performed in Pr3+:Y2SiO5.