EPR and Optical Characterization of Photorefractive Materials Used in Agile Laser Protection

摘要

This is the final technical report for a project to study point defects in photorefractive crystals used in optical limiting applications. The specific materials investigated were LiNbO3 and LiTaO3. The experimental techniques used to characterize these crystals were optical absorption, thermoluminescence, and electron paramagnetic resonance (EPR). We obtained congruent and Mg-doped stoichiometric LiNbO3 crystals from Deltronic and vapor- transport-equilibrated LiTaO3 crystals from Stanford University. The stoichiometric LiNbO3 crystal exhibited a large thermoluminescence peak near 94 K. There was no emission from similar congruent crystals. The thermoluminescence from the stoichiometric crystal could be excited with x-rays or 355-nm pulses from a tripled Nd:YAO laser. It is also shown that congruent LiTaO3 crystals do not give thermoluminescence. However, LiTaO3 crystals made stoichiometric by vapor-phase-equilibration (VTE) treatments have a large thermoluminescence peaks near 94 and 98 K. Self-trapped electrons are participating in the recombination process in both materials. The Fe3+ ions in the VTE-treated LiTaO3 have sharp EPR lines, and a complete angular dependence set of data were collected and analyzed to obtain the g matrix and the second and fourth order crystal-field parameters.