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The absorption coefficient and the index of refraction of undoped and Mg-doped stoichiometric and congruent LiNbO3 crystals were determined for polarization parallel to the z axis in the far-infrared (FIR) 30−180-cm−1 frequency range for different Mg-doping levels and temperatures down to 10 K. For stoichiometric LiNbO3, smaller absorption and index of refraction were found, than for congruent samples. At a Mg content near the photorefractive threshold, the FIR absorption coefficient has the lowest value in the stoichiometric crystals, which are most suitable for efficient pulse generation in the terahertz range.

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 LiNbO 3 (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 Li 2 O-Nb 2 O 5 -X 2 O ternary systems (X ¼ 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 K 2 O 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 À1 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. V C 2015 AIP Publishing LLC.

High-resolution (0.05 cm-') m R spectroscopy in the temperature range 9-320 K has been used to monitor the narrow line absorption spectra, induced by the stretching modes of OH and OD impurities in as-grown and in-deuterated samples of BSO, BGO, mixed (1x ) ~s o ~x s c o .and BTO silleaite single crystals (i.e. BillMeOar with Me=Si, Ge and Ti). The model of the anharmonic oscillator in the Morse potential was applied to interpret the observed spectra and the anharmonicity parameter xc determined. For the line peaking at 3442.84 cm'l in BSO both the fundamental and the first overtone vansitions were detected: in this way the complete set of the Morse parameters could be determined. A model for the OH centre is proposed on the basis of the dependence of the line position against lattice constant in all the systems examined. The phonon coupling of the stretching modes observed was studied by analysing the detailed temperature dependence of the line position and width. In the framework of weal coupling theory the frequencies of coupled phonons were determined md found to be in agreement with those obmned by reflectance, Raman scattering measurements m d theoreucal calculations.