Preparation of KNbO₃ Single Crystal for Optical Applications

A modified Kyropoulos technique is applied to obtain large single domain KNbO₃ crystals suitable for optical applications. Colorless and high resistivity crystals up to 30×30×15 mm³ in size are grown with 52∼55 mole% K₂CO₃ content under the condition of shallow temperature gradient. Single domain crystals up to 8 mm cube are obtained through poling procedure with a d-c field of approximately 1 kV/cm at about 190°C under slight stress along an a axis. The poled crystals are found to be of good optical quality by microscopic examination with polarized light and experiments on second harmonic generation using 1.06 µCW Nd:YAG laser. The second harmonic efficiency for up-conversion of KNbO₃ is found to be as high as that of Ba₂NaNb₅O₁₅. KNbO₃ is revealed to be a potentially useful non-linear material for optical device application.