High-temperature Top-seeded Solution Growth Research Articles

Structure, Optical Properties and Physicochemical Features of LiNbO3:Mg,B Crystals Grown in a Single Technological Cycle: An Optical Material for Converting Laser Radiation.

Two series of LiNbO3:Mg:B crystals have been grown and studied. Two doping methods-have been used. The crystals-have been co-doped with Mg and a non-metallic dopant, B. The physicochemical features of the growth-have been considered for LiNbO3:Mg:B crystals obtained from a boron-doped melt. The charge-has been prepared using different technologies: homogeneous (HG) and solid-phase (SP) doping. The same two methods have been used to grow single-doped LiNbO3:Mg crystals. A control near-stoichiometric (NSLN) crystal-has been grown via the HTTSSG (high-temperature top-seeded solution growth) method from a congruent melt (Li/Nb ≈ 0.946) with 5.5 wt% K2O. The characteristics of the LiNbO3:Mg:B crystals-have been compared with those of the LiNbO3:Mg and NSLN crystals. Physicochemical and structural reasons have been established for the differences in the distribution coefficients of magnesium (KD) during the growth of the HG- and SP-doped LiNbO3:B:Mg and LiNbO3:Mg crystals. The optical characteristics of the LiNbO3:B:Mg crystals-have been studied via optical spectroscopy, laser conoscopy and photoinduced light scattering (PILS). The influence of boron on the microstructure, compositional and optical uniformities and optical damage resistance of the LiNbO3:Mg:B crystals-has been estimated. Optimal technological approaches to growing optically uniform LiNbO3:B:Mg crystals have been determined. LiNbO3:Mg:B crystals have been shown to have a significant advantage over the commercially used LiNbO3:Mg crystals since large LiNbO3:Mg:B crystals can be grown without stripes. Such stripes usually appear perpendicular to the growth axis. In addition, the photorefractive effect is suppressed in LiNbO3:Mg:B crystals at lower magnesium concentrations ([Mg] ≈ 2.5 mol%) than in LiNbO3:Mg ([Mg] ≈ 5.5 mol%).

Flux Growth and Properties of Volatile Bromine-Containing UV Nonlinear Optical Crystal K3B6O10Br

A UV Nonlinear optical (NLO) crystal is one of the key devices in all-solid-state laser technology, and borate halides show outstanding potential due to their abundant structural diversity and short UV cut-off edges. In this article, the sizable UV NLO crystal of K3B6O10Br (KBOB) has been grown with lead-containing and lead-free fluxes systems using the high-temperature top-seeded solution growth (TSSG) method. Energy Dispersive X-ray Spectroscopy (EDS) and transmittance spectra illustrate the influence of Pb2+ ions on the transmittance properties and laser-induced damage threshold (LDT). The thermal property, namely, thermal expansion, thermal conductivity, and thermal diffusivity curves, were characterized. Moreover, a small variation of thermal refractive indexes was analyzed to illustrate the advantage of KBOB in the application for temperature-fluctuated specific regions.

Hydroxyl ion probing transition metal dopants occupying Nb sites in stoichiometric LiNbO3

Transition metal ion (TM = Fe3 +, Cr3+, Ti4+) doped stoichiometric LiNbO3 crystals have been grown by the high temperature top-seeded solution growth and Czochralski methods. Vibrational bands of hydroxyl ions (O H −) have been observed for dopants above a threshold concentration at wavenumbers of 3502 cm−1 for Fe3+ and Cr3+, and 3486 cm−1 for Ti4+. The absorption bands have been attributed to the stretching vibration of O H − ions in M N b n+ – O H − type complexes, where the dopant M occupies a Nb site. The observed vibrational frequencies of the O H − ions and their polarization dependences agree well with the model established for LiNbO3 doped with optical damage resistant (Mg2+, Zn2+, In3+, Sc3+, Hf4+, Zr4+, Sn4+) and rare-earth ions (Nd3+, Er3+, Yb3+), confirming its general validity.

Compositional Homogeneity and Optical Properties of Stoichiometric Lithium Niobate Crystals of Various Geneses

It is shown that the method of high temperature top-seeded solution growth (HTTSSG) can be used to obtain near stoichiometric LiNbO3 crystals from a congruent melt with the addition of 6.0 wt.% of the K2O flux. The structure of complex defects containing OH− groups in these crystals is close to those in stoichiometric crystals grown from a melt containing ∼ 58.6 mol.% of Li2O. However, the former are significantly superior to the latter in terms of total optical and structural homogeneity. The obtained crystals are studied using laser conoscopy, photoinduced light scattering, measuring the fundamental absorption edge, IR absorption spectroscopy in the region of OH− stretchings, Raman spectroscopy in the region of two-particle states of acoustic phonons with zero total wave vector.

Defect Complexes in Stoichiometric Lithium Niobate Crystals Prepared by Different Processes

Using IR absorption spectroscopy in the stretching region of OH groups and Raman spectroscopy in the region of two-particle states of acoustic phonons, we have demonstrated that high-temperature top-seeded solution growth from a melt of congruent composition containing 6.0 wt % K2O flux in Kristall-2 commercially available growth units makes it possible to obtain nearly stoichiometric LiNbO3 crystals having approximately the same perfection of the proton sublattice as in stoichiometric crystals grown from a melt containing 58.6 mol % Li2O, but considerably surpassing them in general optical and structural homogeneity.

Lattice Site of Rare-Earth Ions in Stoichiometric Lithium Niobate Probed by OH− Vibrational Spectroscopy

Rare-earth (RE = Er3+, Nd3+, or Yb3+) ion-doped stoichiometric LiNbO3 crystals were grown by the Czochralski and the high-temperature top-seeded solution growth methods. For the 0.22–0.87 mol% concentration range of the RE oxides in the melt/solution, in addition to the well-known hydroxyl (OH−) vibrational band in undoped stoichiometric LiNbO3, a new infrared absorption band was observed at about 3500 cm−1, similar to the case of the trivalent optical damage resistant (ODR) dopants In3+ and Sc3+. By comparing the frequencies and polarization dependences of the bands to those detected for ODR ion containing crystals, they are attributed to the stretching vibration of OH− ions in RE3+Nb-OH− complexes. Consequently, above a given concentration threshold, some of the rare-earth ions are assumed to occupy niobium sites in the LiNbO3 lattice. The same model is also suggested for RE-doped congruent LiNbO3 crystals containing over-threshold (>5 mol %) amounts of the Mg-co-dopant.

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.

Controlled growth of large β-BaB2O4crystals based on theoretical guidelines

The diagram of phase equilibria in the BaB2O4–NaF system has been used to deduce the relationship between the cooling speed (ΔT), the pulling speed (v), the crystal radius (Rs), the slope of liquidus (m), the solution component (x) and the total quantity of melt, namely ΔT= 0.00159Rs2vmx2/G. The theoretical curves of the crystal thickness dependence on cooling rate and pulling rate have also been drawn. Under the guidance of the deduced formulas, the controlled growth of β-BaB2O4(BBO) crystals to a desired size has been achieved. A typical as-grown BBO crystal with dimensions of Ø76 × 33 mm (525.25 g) has been grown successfully by using the high-temperature top-seeded solution growth method. The measured optical homogeneity indicates that the as-grown BBO crystal has high optical quality (Δn≃ 6.9 × 10−6). The experimental curves of the crystal thicknessversusthe cooling rate and pulling rate were in line with the theoretical curves. The phenomenon of diameter shrinkage in the crystal growth has also been explained according to theory and practice. The theoretical derivation and experimental results provide the rationale for further growth of large BBO crystals with high optical quality.

Judd–Ofelt analysis and transition probabilities of Er3+ doped KY1−x−yGdxLuy(WO4)2 crystals

In this work the transition probabilities, radiative lifetimes and branching ratios of Er3+ ions in KY1−x−yGdxLuy(WO4)2 crystals have been determined. With this aim, Er3+ doped KY1−x−yGdxLuy(WO4)2 crystals have been grown by means of the high temperature Top-Seeded Solution Growth. A spectroscopic analysis of Er3+ ions in this crystal has been performed following anisotropic Judd–Ofelt analysis to obtain basic spectroscopic properties. A quantum efficiency over 97% has been determined for the 4I13/2→4I15/2 transition, indicating that this material is adequate for the development of lasers and amplifiers working in the third telecommunication window.

Influence of oxygen partial pressure on SrTiO3bulk crystal growth from non-stoichiometric melts

Cylindrical SrTiO3 single crystals with diameters and lengths exceeding one inch were grown from non-stoichiometric melts below 1800 °C using the high temperature top-seeded solution growth (TSSG) method. Despite the large crystal diameter the quality has been improved compared to Verneuil grown crystals. Typical etch pit density (EPD) values are in the range between 2 × 104 and 2 × 105 cm−2 for bulk crystals pulled in the <100> direction and using seeds with EPD values of 5 × 105 cm−2. So far pulling SrTiO3 crystals from the melt suffers from growth instabilities like foot formation and subsequent spiraling. To avoid them an effective heat transport via the crystal is indispensable. Especially at elevated temperatures it requires a high optical transparency in the infrared region. In this paper we report the correlation between oxygen partial pressure, composition, IR-transmittance and coloration of the crystals. The growth at a sufficiently high oxygen partial pressure leads to an increase in the optical transparency of the crystals and can thereby suppress foot or spiral formation.

Polytypic modifications in heavily Tb and Eu doped gadolinium aluminum borate crystals

The evidence of a new polytypic phase was demonstrated in double doped gadolinium aluminum borate (GAB) crystals grown by the high temperature top seeded solution growth (HTTSSG) method. The structures of the rhombohedral and the new monoclinic phases in the Eu0.02Tb0.12Gd0.86Al3(BO3)4 and Eu0.18Tb0.19Gd0.63Al3(BO3)4 compositions were determined by single crystal and powder X-ray diffraction methods. The monoclinic crystals with different compositions have identical structures with space group C2/c. It was concluded that the starting crystallization temperature was the dominant factor in formation of the rhombohedral and monoclinic phases. The crystals with non-centrosymmetric rhombohedral structure (R32) may possess similar compositions as the previous ones but their starting crystallization temperature was lower.

Crystal defects revealed by Schlieren photography and chemical etching in nonlinear single crystal LYSB

Large single crystals of a nonlinear optical material LaxYyScz(BO3)4 have been obtained by High Temperature Top-Seeded Solution Growth (HTTSSG). This material is very interesting due to its easy growth procedure, its non hygroscopic properties, a suitable hardness to be reliably cut and polished, a large transparency wavelength range and also good nonlinear properties with potential for UV generation. However, the crystals show inhomogeneities and growth imperfections which can be observed by light scattering and by Schlieren photography method. Chemical etching is used to reveal defects. Observed striations can be related to the growth of rhombohedral facets.

Gd xLa 1− xSc 3(BO 3) 4: A new nonlinear optical crystal

A new nonlinear optical material Gd x La 1− x Sc 3(BO 3) 4 (GLSB) has been synthesized in the trigonal huntite-type structure. The composition parameter x, which defines the existence region of the trigonal huntite-type structure, was determined. Large single crystals were successfully grown by a high-temperature top-seeded solution growth method using Li 6B 4O 9–LiF as a flux. Evidenced by single-crystal X-ray diffraction measurements, the member Gd 0.22La 0.78Sc 3(BO 3) 4 crystallized in space group R32 with cell dimensions a=9.7933 and c=7.9540 Å. The UV absorption edge for polished pieces exhibited at a wavelength shorter than 200 nm. Sellmeier equations were established on the basis of the refractive indices measured by the auto-collimation method. The nonlinear optical coefficient d 11 was determined to be 2.08 pm/V by the modeling and optical measurements of powders.

Development of a versatile high temperature top seeded solution growth unit for growing cesium lithium borate crystals

The design and fabrication of a state of the art high temperature top seeded solution growth (HTTSSG) unit with seed and crucible rotation options is discussed. Crystals from solid solutions of high viscosity could be grown using the setup. The fabricated setup is used to grow high optical quality single crystals of cesium lithium borate with dimensions 50x40x40 mm(3) for frequency conversion of ir to UV laser through harmonic generation.

Temperature stability of elastic and piezoelectric properties in LiNbO3 single crystal

Temperature dependences of the piezoelectric modulus d31 and complex elastic compliance have been measured for a lithium niobate single crystal grown by the high temperature top-seeded solution growth technique. The results obtained have shown that the imaginary part of the elastic coefficient is probably sensitive to the lithium ion and proton migration inside the crystal. X-ray photoelectron spectroscopy has revealed the existence of some defects in the crystal structure. While these imperfections only weakly control the magnitude of the piezoelectric coefficients, they influence the elastic properties, even in the relatively low temperature range. It was found that the temperature dependence of the elastic properties is stable as long as a temperature of ∼90 °C is not reached.

Threshold concentration of MgO in near-stoichiometric LiNbO 3 crystals

MgO-doped near-stoichiometric LiNbO 3 crystals have been grown by the high-temperature top-seeded solution growth method from the K 2O–Li 2O–Nb 2O 5 flux. The critical threshold concentration of MgO required to minimize photorefractive damage in the crystals has been determined on the basis of the liquid–solid phase relations. The lowest MgO concentration in the melt where above-threshold crystals can be still grown with near-stoichiometric crystal composition was 0.2 mol%. At near-threshold MgO concentration, a transitional region was detected along the crystal growth axis, where two OH − absorption bands peaking at 3465 and 3534 cm −1 frequency were observed in the IR spectra at the same time.

Crystal growth and optical properties of YbAl 3(BO 3) 4: a promising stoichiometric laser crystal

Single crystals of ytterbium aluminum borate YbAl 3(BO 3) 4 (YbAB) have been grown by the high-temperature top-seeded solution growth (TSSG) method. The structure of the YbAB crystal has been determined by X-ray diffraction analysis. It belongs to the space group R32 with unit cell parameters: a=0.92512 nm, c=0.71893 nm, Z=3, V=0.53284 nm 3, and the density of the crystal is 4.574 g/cm 3. The absorption and fluorescence spectra have been measured at room temperature. The absorption cross-section, emission cross-section and fluorescence lifetime have been estimated as 2.1×10 −20, 1.0×10 −20 cm 2 and 140 μs respectively, and the results indicate that the crystal is a promising stoichiometric laser material.

Chemical and thermal conditions for the formation of stoichiometric LiNbO 3

One of the effective methods of growing stoichiometric LiNbO 3 single crystals is the high-temperature top-seeded solution growth from K 2O–Li 2O–Nb 2O 5 fluxes. The variation of the crystal composition was examined as a function of the K + content of the flux. A series of DSC and X-ray diffraction measurements were performed in order to establish proper chemical and thermal conditions for the formation of the stoichiometric material. By the combination of these results with the crystal growth and composition measurements, it was possible to establish the temperature limit for the growth of stoichiometric crystals, and a compositional range concerning the K 2O–Nb 2O 5–Li 2O system that can be used.

Crystal growth and optical assessment of Nd 3+ : GdAl 3(BO 3) 4 crystal

This paper reports the high-temperature top-seeded solution growth (TSSG) of large single crystal of Nd 3+-doped GdAl 3(BO 3) 4 from a K 2Mo 3O 10-LiF flux cooled at a rate of 2 K/day and using a crystal rotation rate of 4.5 rpm. The polarized optical absorption and fluorescence spectra of Nd 3+ : GdAl 3(BO 3) 4 crystal have been investigated. Polarized absorption cross sections are 4.3×10 −20 and 2.6×10 −20 cm 2 with band widths of 8.7 and 6.1 nm for σ- and π-polarization, respectively. The emission cross sections are 6.3×10 −19 and 6.0×10 −19 cm 2 for σ- and π-polarization, respectively, at 1062 nm corresponding to the 4F 3/2→ 4I 11/2 transition. The fluorescence lifetime is 54.3 and 57 μs at 300 and 20 K, respectively.

Structural defects in flux-grown stoichiometric LiNbO 3 single crystals

The effect of growth conditions on the crystal perfection of stoichiometric lithium niobate LiNbO 3 prepared by the high-temperature top-seeded solution growth technique (HTTSSG) from potassium containing flux was investigated. Flux compositions between 0.09 and 0.21 K 2O/LiNbO 3 ratio were tested, and it was found that LiNbO 3 forms an eutectic system with the K 2O, which is considered as the solvent in the system. The exact composition of the eutectic is close to the 1 : 5 molar ratio of K 2O and LiNbO 3. Stoichiometric crystals with homogeneous composition were grown from fluxes in the range of 0.16–0.195 K 2O/LiNbO 3 ratio. The formation of the structural defects (faceting and ferroelectric domain reversal) have been examined for growth along the Z= 0 0 0 1 , Y= 0 1 1 ̄ 0 , X= 2 1 ̄ 1 ̄ 0 , 0 1 1 ̄ 1 and 0 2 ̄ 2 1 directions. The development of facets occurred primarily along the {0 1 .2} pyramidal planes. Close connection was found between faceted growth and the appearance of domain reversal.