Investigation of KLN single crystals grown by micro-pulling-down method for blue SHG applications

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In order to develop for high quality potassium lithium niobate (KLN) single crystals fibers grown by micro-pulling-down (mu-PD) method, the quality of grown crystals and second-harmonic-generation (SHG) were investigated.Although there are several reports concerning growth of KLN by means of Cz or TSSG methods, bulk crystals with sufficient size and good compositional homogeneity for a practical use have not been obtained yet. High quality KLN single crystals were grown via a mu-PD method. Crystal diameter was well controlled, and the maximum pulling-down rate without cracks was attained to be 120 mm/h. The KLN single crystals were obtained as colorless rectangular-fibers with well developed [100] and [001] facet planes. The transmitted light to an a-plane extinguished uniformly under the crossed polarizers indicting high optical uniformity of the crystal.Distribution of phase-matching wavelength along the pulling-down axis was measured. A KLN crystal had a good compositional homogeneity along the pulling-down axis. On the other hand, relatively large variation of phase-matching wavelength is found along the radial direction. The phase-matching wavelength shows its minimum around the center of the section of the fiber and increase toward the periphery. Polarized transmittance analysis reveals that the SHG wavelength variation is due to the change of refractive index for an extraordinary ray. X-ray topographic images showed that the internal strain was diagonally remained, //[101]. This phenomena was more obviously obsered in a faster-grown crystal. Compositional inhomogeneity and/or internal strain is considered to be the possible cause of refractive-index change. They should be avoided by optimizing growth conditions such as growth velocity, temperature gradient and meniscus shape.