Bridgman growth and characterization of single crystals of strontium barium niobate and related materials

Strontium barium niobate (Sr_1−xBa_xNb ₂O₆, SBN) crystals are useful for numerous applications including electro-optic modulation, second harmonic generation and holographic data storage. Previously, bulk SBN single crystals were primarily grown by the Czochralski method where several difficulties were encountered: striation formation and diameter control. Striation formation occurred mainly because of crystal rotation in an asymmetric thermal field and unsteady melt convection driven by thermal buoyancy forces. Precise diameter control required modulation of furnace power based on feedback from crystal weighing, but the power regulation can produce temperature changes that could lead to striations. In this thesis research, a system was developed for growing SBN single crystals by the vertical Bridgman method, which offers the following important advantages over the Czochralski method: (1) rotational striations are eliminated since there is no rotation, (2) vertical temperature gradients are intrinsically stabilizing with respect to thermal buoyancy driven melt convection, (3) the crystal is not exposed directly to air convection in the furnace and (4) the crystal diameter and cross-sectional shape are determined by the crucible walls.; Single crystal SBN boules were grown in the c-axis direction up to 2.5 cm in diameter at growth rates up to 12 mm/hr. Fabricated crystals showed an extraordinary refractive index inhomogeneity of better than δn_e ∼ 10−5. The relationship between temperature and refractive index fluctuations was determined by using furnace temperature modulations to produce intentional striations, so that it was possible to specify the temperature stability needed to achieve a required optical homogeneity. The crystal growth results obtained in this thesis research demonstrated that the Bridgman method could be used to obtain SBN crystals of superior quality with greater ease and more versatility than the Czochralski method.; Additionally, light scattering tomography was used to observe the dynamics of ferroelectric poling from an initial polydomain state to a single domain state. Room temperature poling studies showed that poling occurred through the formation of columnar structures along the polarization axis.