Congruent LiNbO3 Crystals Research Articles

Comparative study of the singularity in the optical properties of congruent doped LiNbO3 crystals

A comparison of the optical properties such as optical absorption band edge, ordinary and extraordinary refractive indices, powder second harmonic generation and the non-linear coefficient, d33, of congruent LiNbO3 crystals doped with di-, tri- and hexa-valent cations is reported. It has been found that the referred parameters show a clear concentration singularity depending on the valency of the cation. A charge compensation mechanism is proposed to explain the role of the cation. The experimental data of no and ne are described by a generalised Sellmeier equation considering the different ions sites in the crystal.

The fine structure of spectral response of Cr3+ sites in LiNbO3

We studied Cr3+ centers in congruent LiNbO3 crystals using the method of combined excitation emission spectroscopy. Performing a coordinate transformation of the measured experimental data the inhomogeneous broadening of the 2E → 4A2 transitions of Cr3+ was directly separated into contributions from the variations of the low symmetry crystal field and of free ion parameters. At liquid helium temperatures fine structure in the spectral response of Cr3+ “defect” centers in high crystal field sites was observed. Our results are discussed on the basis of a microscopic hierarchic model for perturbed Cr3+ ions in the lithium niobate crystal lattice.

Photorefractive properties of near-stoichiometric LiNbO3 grown from congruent melts containing K2O

The photorefractive properties of nominally pure near-stoichiometric LiNbO3 crystals grown from congruent melts containing 11 mol % K2O are studied, and some photorefractive properties which are very different from those in congruent LiNbO3 crystals are observed. Large stationary energy transfer (Γmax=25 cm−1) exists in near-stoichiometric crystals though the photovoltaic field is dominant. The light-induced scattering fanning is markedly asymmetric in the crystals, which is very different from the symmetric fanning observed in congruent ones. Our analysis shows that these differences are caused by the large photovoltaic field and the relatively small density of filled traps in as-grown near-stoichiometric LiNbO3.

Electron-lattice coupling incongruent Co-doped LiNbO3:Cr3+:ZnO crystal

The importance of the role of the ion-lattice coupling indetermining the energy levels of Cr3+ ions in congruentLiNbO3 crystals doped with 2.9% of ZnO is demonstrated in thispaper. The Racah parameters: B = 533 cm-1, C = 3244 cm-1 and the crystal-field intensity Dq = 1527 cm-1 were determined and the Tanabe-Sugano diagram was constructed.The characteristics of the absorption and the emission spectra ofCr3+ ions in these crystals can be explained in terms of theconfigurational-coordinate diagram in the harmonic approximation,with good agreement obtained. The Huang-Rhys parameter S = 6.86 andthe lattice phonon energy ℏω = 343 cm-1 are alsoreported in this work.

Nonvolatile two-color holography in indium-doped lithium niobate

We demonstrate nonvolatile two-color holography in an as-grown congruent LiNbO3:In crystal of 2.2 mm thickness. We show that the nonvolatile two-color holographic performance of the crystal is significantly improved as compared with that of other congruent LiNbO3 crystals reported so far. The figure of merit M/# and the two-color recording sensitivity S are measured to be ∼0.04 and ∼3.5×10−4 cm/J, respectively, with a writing intensity of 4 W/cm2 at 780 nm and a gating intensity of 1.15 W/cm2 at 488 nm. These values are comparable to those of unreduced near-stoichiometric LiNbO3 crystals under similar recording conditions.

Subsecond relaxation of internal field after polarization reversal in congruent LiNbO3 and LiTaO3 crystals

We developed an experimental method for measuring subsecond temporal evolution of the internal field after polarization reversal in ferroelectric crystals, congruent LiNbO3 and LiTaO3 crystals. In each crystal the internal field exhibited two distinct relaxations, suggesting that they originate from two different types of defects. The fast one decays in subsecond range while the slow one lasts over days at room temperature. The subsecond relaxation could be fit to a stretched exponential. This method can be applied to other ferroelectric crystals to investigate the properties of defects.

Site-selective spectroscopy of Cr3+ in congruentco-doped LiNbO3:Cr3+:ZnO crystals

This work reports the energy levels of chromium ions for differentsites in congruent LiNbO3 crystals doped with ZnO. Opticalabsorption and luminescence (fluorescence and excitation)experimental techniques were used. For crystals with Zn2+concentration less than 4.7%, optical signals correspond toCr3+ ions substituting for Li+ ions; for higher Zn2+concentration, signals show two components coming from differentsites for active Cr3+ ions. It is accepted that these twocomponents can be assigned to Cr3+ ions substituting for Li+ions and Cr3+ ions substituting for Nb+ ions. The energylevels of the former site are reported for the first time in this work;the 4T2 and 4T1 levels lie 13 421 cm-1 and19 550 cm-1 above the 4A2 ground level,respectively. Analysis of the excitation spectra was the keyresult for achieving the resolution of the new site.

Direct x-ray synchrotron imaging of strains at 180° domain walls in congruent LiNbO3 and LiTaO3 crystals

We present direct evidence for wide regions of strain on length scales of many micrometers associated with 180° domain walls in congruent LiNbO3 and LiTaO3 crystals. Strain contrast in symmetric high-resolution diffraction images of these crystals in Bragg geometry arises in part from curvature in the basal planes (0001) across a domain wall as well as from lateral variation in the lattice spacing of the basal planes extending over many micrometers in going across a wall.

Domain Reversal Characteristics of Near-Stoichiometric LiNbO3 Crystals

The domain reversal characteristics of near-stoichiometric LiNbO3 ([Li]/[Li+Nb] = 0.496) single crystals, which are grown by the Czochralski method with K2O flux, are investigated. The switch field for 180° ferroelectric domain reversal in the near-stoichiometric LiNbO3 crystal is 8.0 ± 0.5 kV/mm, which is only one third of the switching field required for the congruent LiNbO3 crystals. We have successfully achieved 180° domain reversal in near-stoichiometric LiNbO3 samples of 1.0 mm thickness.

An Increase of More Than 30% in the Electrooptic Coefficients of Fe-Doped and Ce-Doped Stoichiometric LiNbO3 Crystals

The composition dependences of the electrooptic coefficients in LiNbO3 crystals and the influence of doping with Ce-ions and Fe-ions were measured precisely and investigated in detail for the first time. The electrooptic coefficient r33 increases with an increase in the Li/Nb ratio. The r33 in stoichiometric LiNbO3 crystal was 1.14 times as large as that in conventional congruent LiNbO3 crystal whose electrooptic coefficient is 32.6 pm/V. The electrooptic coefficient r33 is also increased by doping with some ions. An increase of more than 30% in electrooptic coefficients (from those in nondoped congruent LiNbO3) can be obtained by doping with Ce- or Fe-ions. The lattice constants are also measured. The lattice constant along the c-axis decreases with an increase in the Li/Nb ratio, or by doping with Ce- or Fe-ions, and in this way the crystal structure changes into a less symmetric structure. This may be the reason why the electrooptic coefficient increases with an increase in the Li/Nb ratio or by doping with Ce- or Fe-ions.

Influence of the MgO doping concentration on the width of the E(TO1) raman mode in Congruent LiNbO3 crystals

We study the concentration dependence of Raman spectra of MgO doped LiNbO3 crystals. We focus our attention on the width of the E(TO1) phonon located at 153 cm−1, which is used in pure lithium niobate to determine precisely the composition of the crystal. The concentration dependence of this peak differes with the nature of doping. The results support the Li site vacancy model and suggest that the ions incorporate on Li sites, thus removing the intrinsic defect NbLi (Nb*, Nb-antisite defects) from the lattice.

In situ video observation of 180° domain kinetics in congruent LiNbO3 crystals

We report in situ observation of the nucleation and growth of ferroelectric 180° domains in congruent LiNbO3 crystals. The domains nucleate and grow as six-sided polygons. The wall velocity measured at a constant field shows a spike-like behavior with time, suggesting a strong pinning–depinning type of growth mechanism. This behavior is similar to the corresponding transient current measurement which also shows current spikes during domain reversal.

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.

Comparison of electro-optic effect between stoichiometricand congruent LiNbO 3

Comparison of the electro-optic (EO) effect between stoichiometric and congruent lithium niobate (LiNbO3) crystals has been investigated using the Mach-Zehnder interferometer at 0.633 µm wavelength. Anisotropic changes of EO coefficients, r33 and r13, have been found between the stoichiometric and congruent LiNbO3 crystals. The measured r33 value in the stoichiometric LiNbO3 is at least 20% larger than that of LiNbO3 with congruent composition, whereas the r13 coefficient of stoichiometric LiNbO3 is the same as that of congruent LiNbO3.

Progress in single crystal growth of LiNbO3 using double crucible Czochralski method

In order to grow single crystals with high quality and high homogeneity from off-congruent melts, a double crucible Czochralski method has been developed. In this method, the melt was divided into two parts by a double structure (two chamber system) crucible. The crystal was grown from the inner melt, and powder with the same composition as the growing crystal was supplied to the outer melt. The powder was supplied smoothly at the rate of weight increase of the growing crystal. This rate was automatically controlled to match the weight increase of the growing crystal as monitored by a load cell. In combination with an automatic diameter control system, a fully automatic double crucible Czochralski method was established. Using this method, LiNbO3 single crystals with a composition close to stoichiometric were grown, and their photorefrative properties were compared with those of Nb-rich congruent melt LiNbO3 crystals. Two beam coupling experiments show that both non-doped and Fe-doped stoichiometric crystals exhibited larger exponential gain coefficients than the exponential gains obtained in Fe-doped congruent crystals. This fact suggested that nonstoichiometry control of LiNbO3 is a key for improvement of these properties.

Frequency dependence of the dielectric properties in MgO:LiNbO3crystals

Abstract MgO-doped congruent composition LiNbO3 crystals were grown by the Czochralski method. All MgO:LiNbO3 melts have a higher melting temperature than that of a pure one. Frequency dependence dielectric spectra were measured to study the relaxation and conduction mechanism of these crystals. Strong, low frequency dispersion indicated that the main conduction of MgO-doped LiNbO3 is the hopping charge carrier which may be caused by the defects or Mg-ions. Impedance spectra measurements show that this system does not satisfy a Debye but Jonscher's “Universal Law”

Effect of doping on the Raman modes in MgO‐doped lithium niobate crystals

AbstractRaman spectra of a series of MgO‐doped congruent LiNbO3 crystals were obtained with MgO concentrations up to 30 mol.%. Single crystals were successfully grown and no precipitates were discovered. The intensity, frequency and line width of the Raman phonons were studied as a function of doping percentage. An abrupt change was observed near the composition of 7.5 mol.% of MgO‐doped LiNbO3 crystal. From the spectral changes it is proposed that Mg2+ replaces the mis‐sited Nb5+ cation at Li sites first. When these mis‐sited Nb5+ cations have been completely replaced, the Mg2+ cation then replaces the Li+ cation and creates vacancies at Li+ sites.

Nonstoichiometry as a source of “intrinsic impurities” in LiNbO3 crystals

The UV absorption edge of stoichiometric, congruent and Mg doped congruent LiNbO3 crystals was measured in the temperature range 10–300 K. The shape of the absorption edge of stoichiometric LiNbO3 was found to obey the Urbach rule reflecting the intrinsic character of the absorption. The shift of the absorption edge to longer wavelengths observed in the case of the congruent material can be, explained by the presence of localized excitons due to excess Nb situated in Li positions. The possibility of removing the excess Nb by doping the congruent material by MgO is pointed out.