Stoichiometric LiNbO3 Single Crystals Research Articles

Study of laser-induced defects in LiNbO3:Zn crystals via fractal analysis

Fractal analysis is used to study patterns of photo-induced light scattering in stoichiometric LiNbO3 single crystals and in a series of LiNbO3:Zn (0.018–0.88 wt %) single crystals. It is found that the concentration of laser-induced defects depends on that of Zn, and that energy is redistributed between the layers of laser-induced scattering.

Growth of MgO doped near stoichiometric LiNbO3 single crystals by a hanging crucible Czochralski method using a ship lockage type powder feeding system assisted by numerical simulation

MgO doped near stoichiometric LiNbO3 (MgOSLN) crystals have great potential for use in periodically poled structured LiNbO3 (PPLN) for doubling frequency lasers and optical parametric oscillation infrared lasers, because of their high optical damage threshold, single domain characteristics, and very low coercive field. However, few MgOSLN crystals are commercially available in the present market because of the great difficulty in growing them. This paper describes how a hanging crucible Czochralski technique with a newly-designed ship lockage type powder feeding system assisted by numerical simulation was developed for the successful growth of high quality near stoichiometric LiNbO3 (SLN) and MgOSLN (~1 mol%) crystals. The physical properties of the crystals were assessed by measuring their crystallinity, Curie temperature, optical properties, coercive field and thermal properties. These properties, including a coercive field of 1.4 kV mm−1 and a thermal conductivity of 6.61 W (m K)−1 demonstrated that the crystals meet the demands for manufacturing periodically poled crystal devices. This growth method has great potential in the mass production of SLN and other incongruently melting crystals.

Photorefractive damage resistance threshold in stoichiometric LiNbO_3:Zr crystals

Several optical methods including ultraviolet absorption, infrared absorption of the hydroxyl ions, Raman spectroscopy, and the Z-scan method have been used to determine the damage resistance threshold in 0-0.72 mol. % Zr-containing, flux-grown, nearly stoichiometric LiNbO₃ single crystals. All spectroscopical methods used indicate that samples containing at least ≈0.085 mol. % Zr in the crystal are above the threshold while Z-scan data locate the photorefractive damage threshold between 0.085 and 0.314 mol. % Zr.

Formation of dendrite domain structures in stoichiometric lithium niobate at elevated temperatures

Formation of the dendrite-type self-organized domain structures during polarization reversal at elevated temperatures (above 230 °C) has been revealed and studied in stoichiometric lithium niobate LiNbO3 single crystals. Optical, confocal Raman, scanning electron, and piezoelectric force microscopy have been used for domain visualization. It has been shown experimentally that formation of the dendrite-like structures has been attributed to correlated nucleation caused by a field distribution in the vicinity of the charged domain walls.

Vibrations of H+(D+) in stoichiometric LiNbO3 single crystal

A first principles quantum mechanical calculation of the vibrational energy levels and transition frequencies associated with protons in stoichiometric LiNbO(3) single crystal has been carried out. The hydrogen contaminated crystal has been approximated by a model one obtains by translating a supercell, i.e., a cluster of LiNbO(3) unit cells containing a single H(+) and a Li(+) vacancy. Based on the supercell model an approximate Hamiltonian operator describing vibrations of the proton sublattice embedded in the host crystal has been derived. It is further simplified to a sum of uncoupled Hamiltonian operators corresponding to different wave vectors (ks) and each describing vibrations of a quasi-particle (quasi-proton). The three dimensional (3D) Hamiltonian operator of k=0 has been employed to calculate vibrational levels and transition frequencies. The potential energy surface (PES) entering this Hamiltonian operator has been calculated point wise on a large set of grid points by using density functional theory, and an analytical approximation to the PES has been constructed by non-parametric approximation. Then, the nuclear motion Schrödinger equation has been solved by employing the method of discrete variable representation. It has been found that the (quasi-)H(+) vibrates in a strongly anharmonic PES. Its vibrations can be described approximately as a stretching, and two orthogonal bending vibrations. The theoretically calculated transition frequencies agree within 1% with those experimentally determined, and they have allowed the assignment of one of the hitherto unassigned bands as a combination of the stretching and the bending of lower fundamental frequency.

Combination transitions due to stretching and librations of OH^− ions in LiNbO_3

A new absorption band has been detected at 4009 cm⁻¹ in stoichiometric LiNbO₃ single crystals. Based on present and earlier experiments and recent calculations, this band has been assigned as a combination transition involving the OH stretching and an OH librational mode. The librational mode participating in this combination is of lower fundamental frequency than that contributing to the combination band at 4415 cm¹ observed by Gröne and Kapphan [J. Phys. Condens. Matter 7, 3051 (1995)].

Kinetics of Photorefractive Light Scattering in Stoichiometric LiNbO3 Single Crystals Grown from Melt Containing 58.6 Mole% of Li2O

A study of kinetics of photo-refractive light scattering in ostensibly pure stoichiometric LiNbO3 (Li/Nb = 1) single crystal grown from melt containing 58.6 mole% Li2O excited by laser radiation of 0.53 μm is reported. Asymmetry of the distribution of scattered light intensity is revealed and found to be a linear function of the intensity of the exciting radiation in the 35–160 mW range. Due to heating of the crystal, at farther increase of the radiation intensity the distribution of scattered light contracts.

Growth and characterization of near stoichiometric LiNbO3 single crystal

AbstractA near stoichiometric LiNbO3 single crystal has been grown by the Czochralski method from a 58.5% Li melt hold in a large platinum crucible. High resolution X‐ray rocking curves of 30$ \bar 3 $0 and 0006 reflections indicated that the near stoichiometric LiNbO3 crystal possesses the high structural quality. Compared with the congruent LiNbO3, the near stoichiometric LiNbO3 possesses shorter ultraviolet absorption edge, thus higher Li concentration. The OH– infrared absorption band analyses showed that the Li concentration in the near stoichiometric LiNbO3 crystal is higher than that in the congruent LiNbO3 crystal. This result is in good agreement with that of the ultraviolet absorption edge. The electro‐optic (EO) coefficient γ22 of the near stoichiometric LiNbO3 crystal was measured to be 6.75 pm/V higher than that of congruent LiNbO3 crystal. It also proves the near stoichiometric LiNbO3 electro‐optic Q‐switched requires a low driving voltage and it is advantageous for the device performance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hologram characteristics of Mn, Tb co‐doped stoichiometric LiNbO3 single crystals

AbstractMn, Tb co‐doped stoichiometric LiNbO3 (Mn:Tb:SLN) single crystals of 0.5∼1.0 mm in diameter and 30∼35 mm in length were grown using the micro‐pulling down (µ‐PD) method. Mn:Tb:SLN single crystals were also grown to 20‐25 mm in diameter and 30‐35 mm in length using the Czochralski (CZ) method. The homogeneous distributions of Mn and Tb concentration were confirmed by the EPMA and defects were observed by optical microscopy. We investigated the photoluminescence (PL) and hologram grating properties of Mn:Tb:SLN single crystals. The effect of concentration of dopant MnO2 and Tb4O7 on the luminescnce properties of the Mn:Tb:LiNbO3 single crystals excited by laser line 325 nm, 488 nm and 514 nm is reported. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Domain growth kinetics in lithium niobate single crystals studied by piezoresponse force microscopy

The kinetics of sidewise domain growth in an inhomogeneous electric field has been investigated in stoichiometric LiNbO3 single crystals by measuring the lateral domain size as a function of the voltage pulse magnitude and duration using piezoresponse force microscopy. The domain size increases linearly with the voltage magnitude suggesting that the domain size is kinetically limited in a wide range of pulse magnitudes and durations. In spite of that, the written domains exhibit strong retention behavior. It is suggested that the switching behavior can be described by the universal scaling curve. Domain kinetics can be described as an activation process by calculating the field distribution using the charged sphere model under the assumption of an exponential field dependence of the wall velocity. The activation energy is found to be a function of the external field.

Characteristic Investigations of Stoichiometric LiNbO<sub>3</sub> Single Crystal Fiber Co-Doped with Tb and Mn

[MnO2(1.0mol%) : Tb4O7(0.5mo%)] doped stoichiometric LiNbO3 (Mn:Tb:SLN) single crystals of 0.5~1.0 mm in diameter and 30~35 mm in length were grown by micro pulling down(µ-PD) method. We investigated the photoluminescence (PL) properties of Mn:Tb:SLN single crystal. The OH- absorption band of the single crystals observed infrared the absorption spectra by using an FT-IR spectrophotometer at room temperature. Homogeneous distributions of Mn and Tb concentration were confirmed by the EPMA and observed defects by optical microscopy.

Complex Elastic Properties and Piezoelectric Activity in Stoichiometric and Congruent LiNbO3 Single Crystals

Piezoelectric effect has been measured for stoichiometric (Li/Li + Nb = 50 mole%) and congruent (Li/Li + Nb = 48.6 mole%) lithium niobate crystals. Observations of the piezoelectric resonance allowed to investigate the temperature dependence of the complex elastic coefficient s*11. Distinct local maxima on the s″11(T) dependencies have been found.

Non-stoichiometric control of LiNbO3 and LiTaO3 in ferroelectric domain engineering for optical devices

Photorefractive damage, photoconductivity, and photogalvanic currents of stoichiometry controlled LiNbO3 and LiTaO3 have been investigated. A nearly stoichiometric LiNbO3 single crystal shows a lower photorefractive damage resistance than congruent LiNbO3, however, crystals doped with a small amount of MgO (> 0.78 mol.%) exhibit no measurable photorefractive damage at 532 nm up to intensities of 2 MW/cm2. In the case of LiTaO3, it turned out that nearly stoichiometric crystals exhibit considerably high damage resistivity even without MgO doping. Green-induced infrared absorption (GRIIRA) in the crystals was also investigated by a photothermal technique. The GRIIRA in nearly stoichiometric LiNbO3 is found to be remarkably decreased by a small amount of MgO. GRIIRA in nearly stoichiometric LiTaO3 was strongly suppressed even without MgO doping.

Photorefraction in LiNbO3 as a function of [Li]/[Nb] and MgO concentrations

Photorefractive damage, photoconductivities, and photogalvanic currents of LiNbO3 single crystals have been investigated as a function of [Li]/[Nb] and MgO concentrations. Near stoichiometric LiNbO3 single crystal, having large [Li]/[Nb] ratios of 0.993, shows lower photorefractive damage resistance than congruent LiNbO3. However, crystals doped with a small amount of MgO (>0.78 mol %) exhibit no measurable photorefractive damage at 532 nm to intensities of as much as 2 MW/cm2. This remarkable decrease of photorefraction that was found can be attributed to the decrease of saturated space charge field results from a combination of the increased photoconductivity and the decreased photogalvanic current of the crystal.

Improved Properties of Stoichiometric LiNbO3 for Electro-Optic Applications

High quality stoichiometric LiNbO3 single crystals, 40 mm in diameter and 80-100 mm in length, have been successfully grown by the double crucible Czochralski method equipped with automatic powder supply system using radio-frequency heating and automatic diameter control system. Although some difficulties still exist in growing 3 inch size single crystals from offcongruent composition melts, our results demonstrate that stoichiometric LiNbO3 crystals containing much less nonstoichiometric defects than congruent LiNbO3 exhibit some advantageous properties for holographic data storage applications. This was confirmed by the results of two-beam coupling experiment and digital hologram test. In digital holographic data storage, we have demonstrated that stoichiometric LiNbO3 shows more than 10 times larger photorefractive sensitivity than congruent LiNbO3 In order to interpret the high performance of stoichiometric LiNbO3, we compared several relevant parameters such as electro-optic constants, photoconductivity, and photogalvanic constants between stoichiometric and congruent LiNbO3 crystals. We report that material parameters are strongly dependent on the density of nonstoichiometric defects and control of these defects is of key importance in improving and tailoring those properties.

Electron paramagnetic resonance studies of Co2+ ions in congruent and nearly stoichiometric LiNbO3 single crystals

Congruent and nearly stoichiometric LiNbO3 single crystals doped with Co2+ ions have been investigated by the electron paramagnetic resonance technique. The spin-Hamiltonian parameters, g|| = 2.671(2), g = 5.052(2), A|| = 40(2) × 10-4 cm-1, and A = 154(1) × 10-4 cm-1, are obtained without including the nuclear Zeeman and quadrupole interactions. The factor, k, of reduction due to the covalency is estimated as 0.92 k1 using the above g-values. On the basis of the temperature dependence of the linewidth, the spin-lattice relaxation and the spin-spin relaxation mechanisms are investigated. The former is a sum of a weak direct process with a phonon bottleneck (~T2.4) and an Orbach process with ~156 cm-1 (or 225 K), and the latter is a sum of a typical temperature-independent process and an anomalous process with a temperature dependence T-3.6. This anomaly is considered as a shortening of T2 due to the increase of the correlation length of the spin-spin interaction as the temperature decreases. The integrated signal intensity also showing an anomaly is a consequence of the linewidth anomaly.

Stoichiometric Mg:LiNbO_3 as an effective material for nonlinear optics

Photorefractive damage, optical absorption, photoconductivities, and photogalvanic currents of stoichiometric LiNbO(3) single crystals with different Mg doping levels have been investigated. Nominally pure stoichiometric LiNbO(3) shows lower photorefractive damage resistance than congruent crystal; however, stoichiometric crystals doped with MgO of more than 1.8 mol. % exhibit no measurable photorefractive damage at 532 nm to intensities of as much as 8 MW/cm(2) . This remarkable damage resistance can be attributed not only to increased photoconductivity but also to decreased photogalvanic current. Stoichiometric Mg:LiNbO(3) also demonstrates the shortest absorption edge, 302 nm, and a single-domain nature with low scattering losses.

Stoichiometric LiNbO 3 single crystal growth by double crucible Czochralski method using automatic powder supply system

Stoichiometric LiNbO3 single crystals 40 mm in diameter and 50–100 mm in length were grown by the Czochralski method using a double crucible and automatic powder supply system. The crystals were grown from a Li-rich melt (Li2O 58–60 mol%) in the inner crucible, and the stoichiometric LiNbO3 powder was supplied smoothly and continuously to the outer melt. From the lattice parameter and Curie temperature measurements, the chemical composition through the grown crystal was found to be close to the stoichiometric composition. The as-grown crystals were of single domain except at the periphery. From a comparison of the transmittance between stoichiometric and congruent LiNbO3 single crystals, the absorption edge wavelength of the stoichiometric crystal was found to be 10 nm shorter than the congruent crystal. From these results, it is expected that this double crucible Czochralski method can be effectively applied to control crystal stoichiometry during growth and to produce uniformly doped single crystals of various oxide compositions.