β-BaB2O4 (BBO) crystals are among the most effective nonlinear optical materials used for frequency conversion. At the same time, some important physical properties of BBO are less studied and its potential applications are not clear yet. In this work we studied the optical and acoustical properties of crystals produced by improved top-seeded solution growth (TSSG) method. Using particular chemicals for molten solution synthesis and periodical adding ofproper compounds into the melt allowed us to control the degree of boron-oxygen polycondensation in the zone of seeding and crystal growth. Hence, growing BBO single crystals contained little or no melt inclusions and low-angle structural boundaries. Optimization of cooling conditions of grown crystals eliminated microscale light-scattering centers. Transmission spectra in mid-IR range show the system of absorption lines related to the presence of OH-groups in crystal lattice. Sound velocity measurements have been conducted using interference acousto-optic technique for longitudinal crystallographic directions. It has been shown that the crystals are characterized by higher stiffness tensor components compared with those reported for BBO grown by traditional TSSG method. The effect appears to be the result of BBO structural quality owing to the progress in crystal growth.
Defects in nonlinear optical barium metaborate crystals (?- and ?-BBO) have been investigated by means of optical spectroscopy and thermal activation methods. Low-temperature absorption peaks have been observed in all samples. The dependence of these peaks upon crystalline phase and type of the flux used while growing is low. This fact indicates that intrinsic defects play the leading role in absorption.. It is shown that while growing BBO crystals by TSSG technique, defects, which form deep electron-type traps in the forbidden gap, are generated.
Defects in nonlinear optical barium metaborate crystals have been investigated by means of optical spectroscopy and thermally activational methods. Low-temperature absorption peaks have been observed at all samples. The dependence of these peaks upon crystalline phase and type of the flux used while growing is low. This fact indicates that intrinsic defects play the leading role in absorption origination. It is shown that while growing BBO crystals by TSSG technique, defects, which form deep electron-type traps in the forbidden gap, are generated.
Some aspects of growth of volume nonlinear-optical KTP and BBO single crystals, specific features of formation of their real structure and its characteristics have been studied. They can be governed by such phenomena as formation and restructuring of adsorption layers at the interface of crystal growth, their possible directing and screening effect on solution-melt clusters, mechanism of formation of the polarity of crystal structure, etc. The most important factor is the tendency to form antipolar structures of these crystals, caused by internal energy reasons. It is shown that for developing efficient technological processes for production of this type of crystals, in addition to the above factors, real processes of temperature-time changes in structure and properties of complex solution-melt crystallization media must be also taken into account.
We have studied features of some phase relations in the system BaO-B2O3-Na2O, using differential thermal, X-ray diffraction, and chemical analyses. Seeding and growth of barium metaborate crystals, (beta) -BaB2O4 (BBO), were investigated by the visual-polythermal method. It was established that most compositions of the system, which might be used as BBO solvents, have a rather small stability zone and during growth of bulk crystals, normally, decompose into boron- and barium-enriched components. Their composition depends on solution temperature and deviations of its components from stoichiometric ratio. The compounds synthesized at phase transformations in the system have a low solubility and the solution-melts, from which the BBO crystals are grown, have emulsion features. The quality of seeding and further formation of defects in the forming crystal structure depends on the degree of chemical homogeneity and phase stability of solution-melts.
Boundaries of the electric macrotwins (antipolar parts of crystal) have always been found in X- or Y-axis crystals. Boundaries are nearly planar and located in subseed zone of crystal along the (00.1) plane. In large crystals two more of such boundaries are usually found at a distance of 15 - 18 mm in both sides from the central boundary. In Z-axis crystals, boundaries of macrotwins have never been found. There are only microtwins up to a few mm in size and insuperable tendency to cellular breakdown across the entire growth interface of crystals after 15 - 30 mm of growth. When Na2O and NaF are used as a solvent, antipolar microtwins can be found near macrotwin boundaries or cellular growth zone and in the whole volume of crystals, respectively. In the latter case, crystals are stressed and show a partial fracture along the (00.1) plane. Misoriented blocks and low-angular boundaries can be found at the growth stages later than seeding and growing up to the required sections. Most of the inclusions in BBO crystals are of flux type with a partially void space and a soli state core. If reactions of synthesis had not been finished before melting, enormous amounts of CO2 and probably some amounts of OH were detected, along with near eutectical alignment of Ba and Na in the core of inclusions. Possible causes of formation of the above-mentioned features in BBO structure are discussed.
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