Raman and Luminescence Investigation of Rare Earth Doped Ferroelectric Crystals and Laser-Induced Crystals-in-Glass

Rare earth-doped ferroelectric crystals are an interesting and important class of materials due to their wide array of favorable properties. These properties make them attractive for many different applications. However, as smaller, better-performing devices are sought after, novel processes for engineering optical materials must be developed and understood. Additionally, the response of the desirable properties to a reduction in size, and to the fabrication techniques, must be quantified and controlled. This work is aimed at advancing the understanding of two of these materials, namely lithium niobate (LiNbO3) and lanthanum borogermanate (LaBGeO5), and their properties via the development and utilization of multifaceted measurement techniques. In LiNbO3, Raman spectra collected continuously during application of an external electric field reveal two different effects: (1) the energies of the Raman modes shift linearly in response to the electric field because of the distortion of the crystal via the piezoelectric effect and (2) the zero-field frequencies of the Raman modes are shifted following ferroelectric domain inversion. The former effect may be used as a calibration in order to quantify different phenomenon which produce internal electric fields. The latter effect is due to the presence of polar defects whose dipole moment does not flip during domain inversion. Using effect (1) to quantify effect (2) forces the conclusion that additional polar defects with dipole moment components orthogonal to the spontaneous polarization must exist. This finding has important consequences for the understanding of domain inversion. In a separate set of experiments, space charge fields were produced in \lin{} by laser-induced photoionization of defects at low temperature, and observed via energy shifts of both Raman modes and the erbium fluorescence emission. Occasional electrical breakdown resulted in discharging of these space charge fields. These breakdowns are seemingly random and occur inconsistently, and therefore multiple parameters were investigated in order to determine those responsible.In LaBGeO5, low temperature Combined Excitation Emission Spectroscopy (CEES) revealed that erbium incorporates into both glass-ceramics and laser-induced crystals-in-glass in predominantly one type of environment (site). However, other minority sites were also observed. The energy levels of the primary site were quantified. The fluorescence characteristics of the erbium ions in any site in the laser-induced crystals were found to be only weakly influenced by the irradiation conditions during growth. On the other hand, a hidden parameter, potentially boron deficiency-related defects, resulted in a significant change of the relative numbers of erbium ions incorporating at the minority sites compared to at the primary site. Scanning confocal Raman spectroscopy showed that the energies of the Raman modes are shifted across the cross-sections of laser-induced crystals in glass. The source of these shifts is potentially strain due the sharp temperature gradient during the laser-induced crystallization process. Fluorescence spectra collected simultaneously with Raman spectra, which for erbium is possible using a single fixed-wavelength excitation source with a wavelength of 488nm, showed that the erbium fluorescence intensity is inhomogeneous over the crystal cross-section, despite the host glass being homogeneously doped. These fluctuations were spatially correlated with small shifts in the Raman spectra, which implies that changes to the structure shift the absorption peaks of the erbium sites either toward or away from the energy of the probe laser. Finally, Raman and fluorescence spectra from laser-induced crystals in a LaBGeO5 glass prepared prior to this work exhibited anomalous behavior, including evidence of strong elemental diffusion at the center of the crystal cross-section which resulted in the crystallization of an unknown LaBGeO5 subphase