OPTICAL MATERIAL, WAVELENGTH CONVERSION ELEMENT, SINGLE CRYSTAL OF FERROELECTRIC FLUORIDE, AND PROCESS FOR PRODUCING SINGLE CRYSTAL OF FERROELECTRIC FLUORIDE

摘要

Disclosed is an optical material formed of a ferroelectric fluoride transparent to ultraviolet, visible and infrared wavelength ranges and can realize polarization reversal. Also disclosed is a single crystal of a ferroelectric fluoride grown in such an environment that the direction of growth of the single crystal is in a range of ±20˚ or less from any one direction of an a axis, a b axis, and a c axis in the crystal axes, and the single crystal growth direction is identical to the direction of temperature gradient within a region in which the single crystal is grown, or the angle between the single crystal growth direction and the temperature gradient direction is ±20˚ or less. Further disclosed is a process for producing a single crystal of a ferroelectric fluoride represented by general formula (1), comprising a melting step of placing a carbon or metallic container containing a fluorine compound-containing starting material and a seed crystal of a ferroelectric fluoride in a high temperature region to melt the starting material, and a crystal growth step of gradually moving the container from the high temperature region to a low temperature region to grow a crystal of the melted starting material from the tip of the seed crystal. Ba1-y(Mg1-xZnx)1+yF4 (1) wherein x and y respectively satisfy 0 ≤ x ≤ 1 and -0.2 ≤ y ≤ 0.2.