Temperature Top-seeded Solution Growth Research Articles

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.

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.

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.

Dielectric, electrical and nonlinear optical properties of KTiOPO4:La3+

Lanthanum doped KTP crystal was grown using high temperature top seeded solution growth technique. The 0.08wt.% of La3+ doping was confirmed by the EDAX analysis. Structural and variation of lattice parameters by doping were investigated by X-ray diffraction. The ferroelectric hysteresis loop was traced in polar axis of grown crystal. Static high frequency dielectric permittivity, suppressed electrical conductivity, low loss tangent, increase of resistivity and modulus properties were studied for lanthanum doped KTP crystal by broad band dielectric spectroscopy. Desirable low ionic conductivity was found as 4×10−7Ω−1cm−1 in about 40kHz frequencies.

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.

High temperature top seeded solution growth of stoichiometric lithium niobate LiNbO3 (sLN) with planar interface

HTTSSG growth of stoichiometric lithium niobate (sLN) from K2O containing melt was investigated in order to establish optimal condition for facet-free growth of crystals pulled along the Z=〈0001〉 axis. Elimination of pyramidal facets and flow instabilities was achieved by a gradually adjusted rotation rate program at thermal conditions characterized by the relation Gr/Re2>1 assuring a crystallization front conformed to the very low convexity requirement.

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.

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.

Visible and UV effective non-linear optical coefficients of β-BaB 2O 4 as function of the growth technique

We report second and fourth harmonic generation measurements in β-BaB 2O 4 single crystals (β-BBO) cut in the type I phase matching direction for the conversions from 1064 to 532 nm and from 532 to 266 nm. The absolute values of the effective non-linear optical coefficients d eff are determined by measuring the conversion efficiency towards the visible and the UV. Crystals grown by the Czochralski method (directly pulled from a supercooled pure melt) are compared with crystals produced by the usual high temperature top seeded solution growth method. No difference in the effective non-linear optical coefficients was observed for the conversion from 1064 to 532 nm. In contrast, a systematically slightly higher d eff value of crystal grown by the Czochralski method is obtained for the conversion from 532 to 266 nm.

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.

Crystal growth and stoichiometry of LiNbO3 prepared by the flux method

The high temperature top seeded solution growth technique (HTTSSG) is the leading candidate to grow stoichiometric lithium niobate single crystals. The effect of the growth conditions on the crystal perfection prepared by the HTTSSG from potassium containing flux was investigated. Various flux compositions were tested to assign the limits of the preparation of stoichiometric crystals. It was shown that the key parameter is the crystallization temperatures and that it should be below 1160 °C to obtain stoichiometric crystals with homogeneous composition.

Revealing growth defects in non-linear borate single crystals by chemical etching

Inorganic cyclic borates such as β-BaB 2O 4 (BBO), LiB 3O 5 (LBO), CsLiB 6O 10 (CLBO) and Li 2B 4O 7 (LTB) are excellent non-linear optical (NLO) materials, particularly in the ultraviolet region. Another type of borate is YAl 3(BO 3) 4 (YAB) which is utilized as a laser host and for self-frequency doubling. Growth imperfections causing scattering and absorption of laser beams affect the performance and the use of borate single crystals. Defects, like inclusions, twins, grain boundaries and dislocations form readily in crystals grown by both the high temperature top-seeded solution growth (HTTSSG) method (BBO, LBO, YAB) and the Czochralski technique (CLBO, LTB). Chemical etching methods have been developed on various planes of the different borate crystals, in order to reveal the nature and the distribution of these defects. These methods have successfully been used to characterize NLO borate crystals. The origins of lattice defect formation are discussed in terms of the growth parameters.

The growth and optical assessment of Cr 3+-doped RX(BO 3) 4 crystals with R = Y, Gd; X = Al, Sc

This paper reports the high temperature top-seeded solution growth (TSSG) of Cr 3+-doped mixed borate crystals having the general formula RX 3(BO 3) 4, with R 3+ = Y 3+ or Gd 3+ and X 3+ = Al 3+ or Sc 3+. Both YAl 3(BO 3) 4 (YAB) and GdAl 3(BO 3) 4 (GAB) have been grown from a mixed K 2Mo 3O 10B 2O 3 flux, whereas YSc 3(BO 3) 4 (YSB) and GdSc 3(BO 3) 4 (GSB) are grown from K 2Mo 3O 10Li 2B 4O 7 mixtures. Typical crystals ranging in volume from 60 mm 3 to 14 cm 3 have been grown, with Cr 3+ concentrations of order 0.2–5.0 at%. Measurements of the optical absorption and luminescence spectra show that in YAl 3(BO 3) 4 and GdAl 3(BO 3) 4 the Cr 3+ ions occupy strong crystal field sites whereas in YSc 3(BO 3) 4 and GdSc 3(BO 3) 4 these ions occupy weak field sites. The significance of this observation is that the Cr 3+ ions in weak field sites emit via the broadband 4T 2 → 4A 2 transitions, with obvious potential as tunable laser gain media.

Crystal growth and optical characterisation of Cr 3+-doped YAl 3(BO 3) 4

This paper reports the high temperature top-seeded solution growth (TSSG) of large single crystals of Cr 3+-doped YAl 3(BO 3) 4 from a K 2Mo 3O 10B 2O 3 flux cooled at a rate of 2–4 K per day in the temperature range 950–1060°C, using a crystal rotation rate of 4–6 rpm. Optical absorption measurements at T = 300 K reveal broad bands associated with transitions from the 4A 2 ground state of Cr 3+ to the 4T 2 and 4T 1 excited states. The doublet R lines and their one-phonon sideband are clearly resolved at room temperature in both absorption and luminescence spectra: the splitting of the R lines demonstrates the effect of the even-parity trigonal distortions of their environment on the 4A 2 and 2E levels of the Cr 3+ ions.