Potassium Vacancy Influences on Domain Structure Reversal of KTiOPO₄ Crystal

With the rapid development of quantum information science, there has been a growing focus on the research and development of high-performance quantum entanglement sources. Based on the principle of quasi-phase match (QPM), periodically poled KTiOPO4 (PPKTP) is widely utilized for generating entangled photon pairs through spontaneous parametric down-conversion (SPDC) processes. However, the mechanism of ferroelectric domain reversal in KTiOPO4 crystals remains rarely studied and presents a significant challenge for producing high-quality PPKTP. In this study, we conducted domain reversal experiments on flux-grown KTiOPO4 crystals with varying potassium contents. The resistivity would be changed during potassium migration under a poling electric field in KTiOPO4 crystals with potassium vacancies. The results indicate that the potassium migration based on the potassium vacancies played a key role in controllable domain reversal of the KTiOPO4 crystal. Without any chemical or physical treatment, we successfully prepared PPKTP with periods from 10 to 46.2 μm at room temperature and verified their nonlinear SPDC properties. Our findings suggest that the presence of potassium ion vacancies significantly influences the reversal of domain structure in the KTiOPO4 crystal, emphasizing the importance of high-quality crystals with reduced K ion vacancies for achieving superior PPKTP quality.