Tuning the Electro‐Optic Properties of BaTiO₃ Epitaxial Thin Films via Buffer Layer‐Controlled Polarization Rotation Paths

Abstract BaTiO₃ thin films emerge as a highly promising material platform for integrated photonics due to their outstanding electro‐optic properties and diverse functionalities. Despite extensive studies, there remains a notable gap in the understanding of the intricate relationship between the phase structure, domain switching kinetics and electro‐optic performance of these materials. By controlling the structural phase evolution, it is possible to gain deep insights into the physical mechanisms underlying the electro‐optic performance. Here, the phase constitution of BaTiO₃ epitaxial thin films is successfully tuned by the insertion of a GdScO₃ buffer layer. Continuous polarization rotation paths are observed from an out‐of‐plane tetragonal‐like phase, to an intermediate rhombohedral‐like phase, and finally an in‐plane tetragonal‐like phase, achieving an enhanced effective electro‐optic coefficient of 175 pm V⁻¹ compared to unbuffered films. Furthermore, by in situ second harmonic generation microscopy and electro‐optic measurements, the domain switching behaviors in both statistical and spatially resolved manners are examined, resulting in a clear delineation of the key domain nucleation processes. The in‐depth explorations of these structural mechanisms and kinetics inform the rational design of strong electro‐optic thin films and help in the realization of high‐performance integrated photonic devices such as electro‐optic modulators and multilevel phase shifters.