Facetting and optical perfection in Czochralski grown garnets and ruby

Studies of ruby (Al₂O₃/Cr³⁺) and rare-earth aluminium garnet single crystals, in particular the mixed garnets formed between Y₃Al₅O₁₂ (YAG) and Dy₃Al₅O₁₂ (DyAG), have shown that the formation of facets on the solid/liquid interface, which give rise to a strained central core within the crystals, is dependent upon the shape of the solid/liquid interface. Development of the strained and facetted core can be prevented by modifying the growth conditions to produce a flat solid/liquid interface and as a result the optical perfection of the crystals is greatly improved. Certain crystals, e.g. DyAG, grow naturally with aflat interface, and in the present work this has been shown to be due to the optical absorption characteristics of this material. In other materials, e.g. YAG and ruby, the interface shape can be controlled by the rate at which the growing crystal is rotated. The changes in temperature gradient produced in a YAG melt by changes of crystal rotation rate have been measured, and their effect upon crystal perfection is described.