Sources of optical distortion in rapidly grown crystals of KH2PO4

We report the results of X-ray topographic and optical measurements on KH2PO4 crystals grown at rates of 5-30 mm/day. We show that optical distortion in these crystals is caused primarily by three sources: dislocations, differences in composition between adjacent growth sectors of the crystal, and differences in composition between adjacent sectors of vicinal-growth hillocks within a single growth sector of the crystal. We find that the compositional heterogeneities cause spatial variations in the refractive index and induce a distortion of the transmitted-wave front, while large groups of dislocations are responsible for strain-induced birefringence which leads to beam depolarization.