Ion-Ion Interactions and Their Impact on Fluorescence Decay and Stimulated Emission Properties of ERBIUM:YTTRIUM(3)ALUMINUM(5)OXYGEN(12) and (erbium, NEODYMIUM):YTTRIUM(3)ALUMINUM(5)OXYGEN(12) Laser Crystals.

Unusual fluorescence decay properties of Er ^{3+} ions in high dopant density Er:YAG laser crystal are observed. Studies show that the cross relaxation of Er^{3+} ions in both the upper and lower levels of 2.94 μm laser transition is responsible. These cross relaxation processes affect the lifetimes of the energy levels relevant to the laser action at 2.94 μm and lasing characteristics as well of Er:YAG lasers. They therefore make Er^ {3+} lasers different from other solid state lasers such as Nd:YAG. In the first part of this dissertation, these cross relaxation processes are investigated and their effects on both fluorescence decay and stimulated emission properties of high concentration Er:YAG are discussed. Because the lifetime of the lower laser level of Er^{3+} is longer than that of the upper level, laser emission at 2.94 μm from Er:YAG is self-terminated and only pulsed operation is possible. In order to remove this bottleneck so that room temperature continuous wave operation of Er:YAG lasers at 2.94 μm may be possible, it is necessary to codope another ion in the Er:YAG lattices, which can help deactivate the Er ^{3+} lower level. With this in mind, in the second part of this thesis, the (Er, Nd):YAG laser crystal is investigated. In this material Nd ^{3+} ion is introduced to accelerate the relaxation of the Er^{3+} lower level. It is found in this multiply doped crystal that strong interaction occurs between the two species of dopant ions, namely Er^{3+ } and Nd^{3+}. This interaction has been investigated in detail both experimentally and theoretically. An ion-ion interaction model has been proposed, based on which the interaction parameters are numerically evaluated. As far as the stimulated emission properties of this new laser crystal is concerned, ion -ion interactions lead to novel effects and characteristics. Lasing tests reveal that both ions in (Er, Nd):YAG can simultaneously lase so that multiple wavelength laser action are obtained. In addition, the (Er, Nd):YAG laser has shown stimulated emission over a broad band from 1.01 to 1.15 μm with a peak at 1.06 mum. (Copies available exclusively from Micrographics Department, Doheny Library, USC, Los Angeles, CA 90089-0182.).