Self-buffered epitaxy of barium titanate on oxide insulators enables high-performance electro-optic modulators

Integrated photonics has emerged as a promising alternative for data communication and computing, ferroelectric BaTiO₃ (BTO) stands out for its exceptional electro-optic response among candidate materials. However, direct epitaxial growth of BTO entails a fundamental trade-off: substrates with low refractive index are required for strong optical confinement, yet those with large lattice mismatch degrade film crystalline quality and electro-optic performance. We report a buffer-free, strain-engineered approach to integrate high-performance BTO thin films directly on LaAlO₃-Sr₂TaAlO₆ (LSAT) oxide-insulator substrates. By exploiting a self-buffer layer formed during the initial growth stage, we achieve periodic in-plane strain modulation that stabilizes a polymorphic phase boundary with orthorhombic polar nanoregions, yielding a Pockels coefficient exceeding 358 pm V⁻¹ and a Curie temperature raised to 200 °C. Leveraging this material platform, we demonstrate the first realization of a Mach-Zehnder modulator using epitaxial BTO on LSAT. The device exhibits a half-wave voltage-length product of 0.7 V cm at 1550 nm, which closely matches finite-element simulations, and supports a 6-dB electro-optic bandwidth of 28 GHz. Our results validate BTO on LSAT as a viable photonic platform for scalable, low-voltage and high-speed modulators.