Growth-dependent properties of KTP crystals and PPKTP structures

Growth of optically uniform and single ferroelectric domain KTP crystals is of prime importance for frequency-conversion applications. In the course of KTP growth from pure self-fluxes the flux becomes enriched in potassium causing a gradual increase of potassium content in the crystal as well. We have shown that such an effect can be well characterized by a corresponding increase in the Curie temperature of the crystal. Establishment of the potassium concentration gradients is followed by charge separation and production of a built-in electric field, which can be enhanced or diminished also by the incorporation of charge-compensating residual impurities. The magnitude of the built-in electric field is directly proportional to the projection of the potassium concentration gradient on the crystal's Z-axis, and it defines the domain direction in immersion seeded or different configurations of the top-seeded growth of KTP crystals. Detailed investigation of the domain formation mechanisms has allowed us to suggest a number of ways of growing single domain crystals, such as top-seeded growth with pulling in the Z-direction. Pulling in the X-direction is shown to yield predominantly bi-domain crystals. The formation of bi-domains and complex domain structures along the growth sector boundaries is explained in terms of edge-like and apex-like growth perturbations, respectively, which are due to temperature fluctuations at the growth interface. The knowledge of parameters influencing the domain formation mechanisms has allowed us to develop a technique for obtaining as-grown periodic domain structures necessary for large aperture (high-power) frequency conversion applications.