Low-Loss BaTiO₃–Si Waveguides for Nonlinear Integrated Photonics

Barium titanate (BaTiO 3 ) has become an attractive material to extend the functionalities of the silicon photonics platform because of its large Pockels coefficient of more than 1000 pm/V. BaTiO 3 integrated epitaxially on silicon-on-insulator substrates can be structured in passive and electro-optic silicon photonic devices using slot-waveguide geometries, both of which have been demonstrated. However, all devices demonstrated so far suffer from high optical propagation losses of ∼40–600 dB/cm, which limits their performance compared with state-of-the-art silicon photonics devices (<2 dB/cm). Here, we identify the origin of these high propagation losses and demonstrate a path to fabricate low-loss BaTiO 3 –Si waveguides with propagation losses of only 6 dB/cm. In particular, we identified the thin strontium titanate (SrTiO 3 ) seed layer typically used for the epitaxial deposition of BaTiO 3 on silicon as the main source of absorption: When manufacturing slot-waveguide structures, the BaTiO 3 /SrTiO 3 layer stack is typically exposed to hydrogen, which is incorporated in the SrTiO 3 layer, and causes absorption. We demonstrate that a low-temperature anneal is sufficient to remove hydrogen and to achieve low propagation losses in waveguides. Thus, we found a way to eliminate the previously observed showstopper for incorporating functional and highly nonlinear barium titanate films into silicon photonic structures, ultimately enabling ultra-high-speed switches and novel nonvolatile optical silicon photonic devices.