This Small Business Innovation Research Phase I project addresses novel vertical Bridgman growth of ferro-electric tungsten bronze materials which are known to have large electro-optic and photorefractive effects. They comprise a class of optical materials that are very promising for applications such as tunable holographic wavelength filtering, holographic mass data storage, electro-optic modulation and beam steering, phase conjugation, and integrated optical switching. Most tungsten bronze crystals are currently grown by the Czochralski method where the main problems are refractive index striations, diameter instability, and susceptibility to volatilization loss of hazardous constituents. Striations can cause significant light scattering and degrade device performance. They arise from spurious growth rate fluctuations, principally due to temperature variations at the melt-solid interface. The dominant source of these variations is crystal rotation in a non-axisymmetric temperature profile. We propose to develop a vertical Bridgman system for growing SBN and PBN because this method inherently avoids crystal rotation and minimizes buoyancy-driven melt convection, thereby minimizing striations. Furthermore, the crystal diameter is determined by the crucible walls so that diameter instability is not an issue, and volatilization can be eliminated by using a sealed-crucible technique. Vertical Bridgman growth of SBN, PBN, and other tungsten bronze crystals will provide higher optical quality samples with larger sizes and significantly lower cost. This technology will help to further advance numerous electro-optic and photorefractive applications.
