Transient and steady-state measurements of photoinduced birefringence in single-domain and periodically poled lithium niobate (LN) crystals containing different nonphotorefractive impurities are presented. The birefringence change is induced by a 532 nm beam and probed by a 633 nm beam. Data were taken at 25 and 50 °C. We find that doping by HfO2 is very effective in reducing the photorefraction. This is interesting also because it is known that hafnium-doped LN crystals can be periodically poled during growth. The analysis of the rise and decay of the induced birefringence shows that doping considerably increases both the photoconductivity and the dark conductivity of the LN crystals.

Transient and steady-state measurements of photoinduced birefringence in Hf-doped congruent lithium–niobate crystals are presented. Through a systematic investigation of crystals with different dopant concentrations, we find that Hf doping induces a progressive decrease of photorefraction up to a threshold concentration of about 4 mol %. Such a decrease is correlated to a corresponding increase in photoconductivity. The interpretation of measurements has to take into account heating effects that may be attributed to nonlinear absorption processes.

Trivalent terbium absorption intensities in single-crystal TbAlO3 are analyzed using the Judd-Ofelt model to assess the crystal’s potential as a solid state laser system. The standard Judd-Ofelt model was applied to the room temperature absorption intensities of Tb3+ (4????8) to determine the phenomenological intensity parameters ????2, ????4, and ????6. Seven multiplet manifolds are identified and the absorption intensities of these manifolds are least-squares fitted to the calculated intensities to obtain the intensity parameters: ????2=40.52×10−20cm2, ????4=8.74×10−20cm2, and ????6=2.26×10−20cm2 in TbAlO3. These intensity parameters are then applied to determine the radiative decay rates and branching ratios of Tb3+ transitions from the ????45 to the ????????′7 multiplet manifolds. Based on the results, the radiative lifetime of the excited state manifold ????45 is determined from the radiative decay rates and found to be 3.5ms. The calculated lifetime is longer than the measured lifetime, reflecting the nonradiative interactions between the Tb3+ ions and the lattice in the pure compound. The intensity parameters, radiative lifetime, and emission cross sections are then compared to those reported in other laser hosts. The quantum efficiency of the laser transition ????45→????57 of Tb3+ is approximately 57.0% in TbAlO3.

A systematic analysis and classification of clusters, consisting of impurity (extrinsic) and intrinsic defects in lithium niobate crystals, were made in order to understand the main features of the observed EPR spectra. It is shown that possible configurations for lithium substitution with charge compensators such as a lithium vacancy ${(X}_{\mathrm{Li}}{-Y}_{\mathrm{Li}}),$ niobium vacancy ${(X}_{\mathrm{Li}}{-Y}_{\mathrm{Nb}}),$ and oxygen interstitial ions ${(X}_{\mathrm{Li}}{-Y}_i)$ belong to $C_3$ or $C_1$ point group symmetries; this is also valid for the complexes with niobium substitution and the ${\mathrm{Nb}}_{\mathrm{Li}}$ antisite as the compensator ${(X}_{\mathrm{Nb}}{-Y}_{\mathrm{Li}}).\mathrm{}$ Clusters of an oxygen vacancy and an impurity on a niobium site ($X_{\mathrm{Nb}}{-Y}_{\mathrm{O}}$ configurations) have $C_1$ symmetry only, but never $C_3$ symmetry. A detailed study of EPR spectra for a wide set of crystals with different chromium concentrations and [Li]/[Nb] ratios was carried out. Besides the main axial ${\mathrm{Cr}}^{3+}$ center, eight satellite chromium centers were experimentally resolved and parameters of their spin Hamiltonians were determined by fitting angular dependences of EPR lines. It was found that in stoichiometric material less chromium is incorporated into the crystal and that the satellite centers disappeared. A correlation of EPR, optical absorption, and luminescence spectra was observed and analyzed. The existence of the family of chromium centers was explained on the basis of one common hypothesis about charge compensation by intrinsic defects. In a minimal model, sufficient to explain all experimental data, it is assumed that the satellite centers include two defects — ${\mathrm{Cr}}_{\mathrm{Li}}$ and niobium vacancy $v_{\mathrm{Nb}}$ in the first or further neighboring shells. Two $v_{\mathrm{Nb}}$’s compensate five ${\mathrm{Cr}}_{\mathrm{Li}}.$ Since in conventional congruent crystals the relative concentration of additional satellite centers is comparable with the concentration of the main center, the conclusion was made that both kinds of centers are equally responsible for many of the lithium niobate properties.