Zs. Szaller

Growth of stoichiometric LiNbO3 single crystals by top seeded solution growth method

Abstract Stoichiometric LiNbO 3 single crystals have been grown using the top-seeded solution growth method from potassium containing flux. Spectroscopic properties (e.g. the UV absorption edge, OH-vibrational band) have been used for the characterization of the crystal composition. Optical homogeneity and the formation of lattice defects were compared with those of undoped off-congruent composition crystals prepared by the Czochralski method.

On the lattice site of trivalent dopants and the structure of Mg2+-OH--M3+ defects in LiNbO3:Mg crystals

The lattice sites of a number of trivalent co-dopants (In, Nd, Gd, Er, Tm, and Lu) have been determined in LiNbO3 crystals also with Mg, using the Rutherford back-scattering and proton-induced X-ray emission channelling techniques. In, Er and Lu have been found mainly on Li sites, but a small fraction of In and Lu may substitute for Nb ions. Gd and Tm ions also replace mainly Li, but some of these ions seem to be present in other positions, possibly related to some kind of aggregate, e.g. self-compensating pairs. A fraction of Nd ions probably occupies interstitial sites in structurally empty oxygen octahedra, while other Nd ions may substitute for host cations. In addition to the already known effect of Cr, Fe and Nd ions, In, Lu and also Sc give rise to an OH- absorption band between 3499 and 3523 cm-1, which is attributed to Mg2+ (Li site)-OH-(O site)-M3+ (Nb site) complex defects. No such band is observed for Y, Gd, Er and Tm co-doped crystals.

Growth, defect structure, and THz application of stoichiometric lithium niobate

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.

Lattice site of trivalent impurities in Mg-doped lithium niobate crystals

Abstract The lattice site of trivalent impurities (lutetium, neodymium and indium) was determined in magnesium-doped LiNbO3 crystals by Rutherford backscattering and proton-induced X-ray emission channelling experiments. It was shown that lutetium and indium substitute mainly for lithium ions, while most of the neodymium ions occupy structural vacancy positions. It cannot be excluded, however, that a fraction of the impurities may replace niobium sites. In all of the three crystals an IR absorption band was detected which is attributed to OH− stretching vibrations in M3+ (niobium site)-OH−Mg2+ (lithium site) complex defects.

X-ray photoelectron spectroscopy and optical studies of Bi12(GaxBi1-x)O20-δ and Bi12(ZnxBi1-x)O20-δ single crystals

Abstract Bi12(MxBi1−x)O20−δ transparent crystals, where M = Ga (x = 0.63 − 0.72)orM = Zn (x = 0.57), have been grown by Czochralski technique. A change in the bonding conditions of a minor fraction of bismuth in the sample surface has been confirmed by X-ray photoelectron spectroscopy. This has been attributed to the surface enhanced presence of bismuth in tetrahedral lattice positions (BiM). It is suggested that defect centers related with BiM cations are responsible for a pre-edge absorption observed for M = Ga. Excitation in this band produce a photoluminescence emission peaking at 608 nm, observed at 15K. Infrared absorption bands at 636 cm −1 for M = Ga and 635 cm −1 for M = Zn were detected and attributed to BiM O vibrations.

The distribution of metaphosphate ion. A new critical impurity in LiNbO3 single crystals grown by the Czochralski technique

LiNbO3 single crystals doped with metaphosphates were subjected to chemical analysis and studied by EPM. The distribution of P found in the crystal can be described by the NPM model. The results indicate that the metaphosphates belong to the “critical” impurities for this substance.

Effect of alkali doping on the Raman spectra of potassium lithium niobate crystals

Raman spectra have been measured in both pure and alkali doped potassium lithium niobate (KLN) crystals. One of the A1(TO) Raman modes at about ≈640 cm−1 measured in x(zz)x backscattering geometry is sensitive to the changes of Nb concentration. Other phonon modes at ≈306 cm−1 and ≈350 cm−1 corresponding to K+ and Li+ ion vibrations, respectively, have been identified by Na doping and 6Li isotope substitution. Na+ ions substitute both for Li+ and K+ ions, while Rb+ ions incorporate only at K+ sites in KLN.

Microscopic and Raman Spectroscopic Investigation of the Domain Structures in LiNbO3:Y:Mg Crystals

Formation of periodic domain structure in Mg, Y and Mg+Y co-doped congruent LiNbO 3 single crystals grown by off-centered Czochralski method in the X and 148°Y pulling directions has been investigated. The developed domain structures were explained as functions of the thermal gradient during growth and the amount of Y and Mg dopants, the latter dopant determining the coercive field in the crystal. Raman spectra were measured in Z(YY)Z and Z(YX)Z geometry on periodically poled LiNbO 3 crystal doped with 2 mol% Mg and 0.65 mol% Y. A Raman band at about 610 cm−1 (presumably related to the E(TO 9 ) mode) occured at the ferroelectric domain boundaries related to periodic step-like forms on the surface as proved by time-dependent etching experiments.