Owing to the extraordinary richness of its physical properties, congruent lithium niobate has attracted multidecade-long interest both for fundamental science and applications. The combination of ferro-, pyro-, and piezoelectric properties with large electro-optic, acousto-optic, and photoelastic coefficients as well as the strong photorefractive and photovoltaic effects offers a great potential for applications in modern optics. To provide powerful optical components in high energy laser applications, tailoring of key material parameters, especially stoichiometry, is required. This paper reviews the state of the art of growing large stoichiometric LiNbO₃ (sLN) crystals, in particular, the defect engineering of pure and doped sLN with emphasis on optical damage resistant (ODR) dopants (e.g., Mg, Zn, In, Sc, Hf, Zr, Sn). The discussion is focused on crystals grown by the high temperature top seeded solution growth (HTTSSG) technique using alkali oxide fluxing agents. Based on high-temperature phase equilibria studies of the Li₂O–Nb₂O₅–X₂O ternary systems (X = Na, K, Rb, Cs), the impact of alkali homologue additives on the stoichiometry of the lithium niobate phase will be analyzed, together with a summary of the ultraviolet, infrared, and far-infrared absorption spectroscopic methods developed to characterize the composition of the crystals. It will be shown that using HTTSSG from K₂O containing flux, crystals closest to the stoichiometric composition can be grown characterized by a UV-edge position of at about 302 nm and a single narrow hydroxyl band in the IR with a linewidth of less than 3 cm⁻¹ at 300 K. The threshold concentrations for ODR dopants depend on crystal stoichiometry and the valence of the dopants; Raman spectra, hydroxyl vibration spectra, and Z-scan measurements prove to be useful to distinguish crystals below and above the photorefractive threshold. Crystals just above the threshold are preferred for most nonlinear optical applications apart holography and have the additional advantage to minimize the absorption even in the far-infrared (THz) range. The review also provides a discussion on the progress made in the characterization of non-stoichiometry related intrinsic and extrinsic defect structures in doped LN crystals, with emphasis on ODR-ion-doped and/or closely stoichiometric systems, based on both spectroscopic measurements and theoretical modelling, including the results of first principles quantum mechanical calculations on hydroxyl defects. It will also be shown that new perspective applications, e.g., the generation of high energy THz pulses with energies on the tens-of-mJ scale, are feasible with ODR-doped sLN crystals if optimal conditions, including the contact grating technique, are applied.

In this review article, we discuss photoluminescence phenomena mediated by polarons in lithium niobate (LNO). At first we present the fundamentals on polaron states in LNO and their energy levels, i.e., on free and bound electron polarons, on hole polarons as well as on bipolarons. We discuss the absorption measurements on reduced as well as on doped LNO that made the characterization of the formed polaron states possible by their absorption bands. Next, we proceed by reporting on the two polaron-mediated photoluminescence bands that have been observed in LNO: (1) A near-infrared luminescence band in the range of 1.5 eV shows a mono-exponential decay and a strong dependence on iron doping. This luminescence is emitted by bound polarons returning from an excited state to the ground state. (2) A luminescence band at visible wavelengths with a maximum at 2.6 eV shows a stretched-exponential decay and is strongly enhanced by optical damage resistant doping around the doping threshold. This luminescence stems from the recombination of free electron and hole polarons. The next major topic of this review are domain contrasts of the visible photoluminescence that have been observed after electrical poling of the substrate, as singly inverted domains show a slightly reduced and faster decaying luminescence. Subsequent annealing results in an exponential decrease of that domain contrast. We show that this contrast decay is strongly related to the mobility of lithium ions, thus confirming the role of polar defect complexes, including lithium vacancies, for these domain contrasts. Finally we discuss the extension of our investigations to lithium tantalate (LTO) samples. While the results on the domain contrast and its decay are similar to LNO, there are remarkable differences in their luminescence spectra.