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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.

A convenient and fast method to determine the composition of LiNbO3 single crystals consisting of a measurement of the position of the fundamental absorption edge in the near UV region is refined and extended to the stoichiometric range. Using unsophisticated apparatus and simple reflection correction, the proposed two-parameter calibration equation provides an absolute accuracy of 0.1 mol % with the relative accuracy reaching an unusual value of better than 0.01 mol % for near stoichiometric compositions.

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.

We report large and nonmonotonic dependences of the electro-optic (EO) coefficient r22 and the corresponding dielectric permittivity in pure LiNbO3 on the concentration of intrinsic defects related to the deviation from the stoichiometry of the crystal. This variation is caused mainly by the electro-mechanical contribution. A stoichiometric crystal possesses both large EO properties and resistance to optical damage which renders it much more suitable for use in laser Q-switching than the congruent crystal.

The shift of t h o UV-absorption edge was applied to determine the real LiNbO, crystal cornposition. The accuracy of the method is about lo-* iiiole% iisiiig a siriiple comrnercia1 UV-sprctrophotometer. IiNhO, is a typical nonstoichionietric compound, the maximum of the phase diagram curve is shifted from the stoichiornetric 1,i:Nb = 1 ratio to the Kb rich direction (LRRNER et al.). A relatively wide homogeneity range, however, allows to produce crystal sections out of the congruent composition. The crystal quality as well as various physical parameters highly depend on the crystal composition. Both scientific investigations as well as the extended practical application demand controlled high crystal quality. Claim arose therefore to determine the real cr>stal composition in some cases. Chemical analysis and some physical methods were found to/be more or less suitable for this purpose.The chemical analysis presents an absolute method but generaly is not sensitive enough for any of the main components of the host material ( -10-l mol",,). Besides, the chemical processing of LiNbO, is rather complicated (POLGAR et al.).The practical physical methods are more sensitive ( -lop2 rnol(;b) though generally relative and are strongly affected by various impurities. Only some of then1 can be calibrated by sintered powder mixtures. (e. g. : Curie temperature measurement, phase matching temperature measurement.) I n addition these physical methods are rather inconvenient. Curie temperature measurement can be performed over thousand degrees centrigrade (BERGMANN et al.). The phase matching temperature of the second harmonic generation can be measured only by a precision laser optical system. The installation of this method therefore is too expensive. The measurement of the extraordinary refraction index with acceptable accuracy demands very high level of crystal processing (MIDWINTER). The influence of impurities on refractivity has been pointed out recently (BODNAR, YARUNICHEV).A remarkable shift of the intrinsic UV absorption edge was observed between the crystals grown from stoichiometric and congruent melt compositions ( FOLDVLRI et al.; RBDFIELD, BRUKE). This effect can easily be used to determine the crystal conlposition in the range from congruent to stoichiometric cases. Figure 1 shows the shape of the absorption curves of various crystal compositions after correcting for the reflected light from the surfaces. Relatively high absorbance is suggested t o take as reference point for the absorption edge analysis (abs. coeff. = 20 em-l). In this

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The number densities and absorption cross sections of both optically generated and reduction-induced small electron and hole polarons in LiNbO(3) are determined by means of time-resolved pump-multiprobe spectroscopy. The data are obtained for free (Nb(Nb)(4+)) and bound (Nb(Li)(4+)) electron polarons, bound Nb(Li)(4+):Nb(Nb)(4+) electron bipolarons, and bound O(-) hole polarons. The peak absorption cross sections are in the range of σ(pol)≈(4-14) × 10(-22) m(2), comparable to that for Fe(2+). In all cases the ratio of occupied to unoccupied polaronic sites is less than 10(-2).