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 d istortion in these crystals is caused primarily by three sources: disl ocations, differences in composition between adjacent growth sectors o f the crystal, and differences in composition between adjacent sectors of vicinal-growth hillocks within a single growth sector of the cryst al. We find that the compositional heterogeneities cause spatial varia tions in the refractive index and induce a distortion of the transmitt ed-wave front, while large groups of dislocations are responsible for strain-induced birefringence which leads to beam depolarization.