Wei Hong

Date of Graduation


Document Type



Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (EN-DOR) have been used to identify and characterize point defects in nonlinear optical crystals of lithium triborate (LiB3O5) and beta barium borate (β-BaB2O4). Two similar hole centers, one (Hole Center A) stable below approximately 130 K and the other one (Hole Center B) stable below approximately 170 K, were observed in LiB3O 5. For both centers, there is a major hyperfine interaction with only one 11B nucleus. The g matrix and the 11B hyperfine and nuclear quadrupole matrices have been determined for each hole center. I suggest Hole Center A is a self-trapped hole with the hole localized on an oxygen ion between a threefold-bonded boron and a fourfold-bonded boron. The self-trapping occurs because of a significant relaxation of the neighboring fourfold boron away from the hole. A similar model is suggested for Hole Center B, except a neighboring lithium vacancy is included to provide the increased thermal stability. For undoped β-BaB2O4, the dominant electron center is an oxygen vacancy on a bridging oxygen site. The EPR spectrum of this center exhibits a 16-line hyperfine pattern from two boron nuclei. The dominant hole center in β-BaB2O4 exhibits a four-line hyperfine pattern due to one boron, and is assigned to a hole localized on a nonbridging oxygen ion with no other defects nearby. Warming the crystal to approximately 80 K destroys the dominant hole spectrum and introduces other perturbed hole centers. For silver-doped β-BaB 2O4, after room temperature irradiation of the sample, EPR reveals a Ag2+ center (formed when a hole is trapped by a Ag + substituting on a Ba2+ site) and two distinct interstitial Ag0 centers (formed when an electron is trapped by a Ag + on interstitial sites). The g and A matrices of the Ag2+ center have been determined. These centers in LiB3O5 and β-BaB2O 4 are of interest because of their possible role in the unwanted transient optical absorption produced in these nonlinear optical crystals by high-power pulsed ultraviolet lasers.