  The integration of complex oxides on silicon presents opportunities to extend
and enhance silicon technology with novel electronic, magnetic, and photonic
properties. Among these materials, barium titanate (BaTiO3) is a particularly
strong ferroelectric perovskite oxide with attractive dielectric and
electro-optic properties. Here we demonstrate nanophotonic circuits
incorporating ferroelectric BaTiO3 thin films on the ubiquitous
silicon-on-insulator (SOI) platform. We grow epitaxial, single-crystalline
BaTiO3 directly on SOI and engineer integrated waveguide structures that
simultaneously confine light and an RF electric field in the BaTiO3 layer.
Using on-chip photonic interferometers, we extract a large effective Pockels
coefficient of 213 plus minus 49 pm/V, a value more than six times larger than
found in commercial optical modulators based on lithium niobate. The
monolithically integrated BaTiO3 optical modulators show modulation bandwidth
in the gigahertz regime, which is promising for broadband applications.
