Low-Loss BaTiO₃-Si Waveguides for Nonlinear Integrated Photonics

Barium titanate (BaTiO3) 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. BaTiO3 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 BaTiO3-Si waveguides with propagation losses of only 6 dB/cm. In particular, we identified the thin strontium titanate (SrTiO3) seed layer typically used for the epitaxial deposition of BaTiO3 on silicon as the main source of absorption: When manufacturing slot-waveguide structures, the BaTiO3/SrTiO3 layer stack is typically exposed to hydrogen, which is incorporated in the SrTiO3 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.