Optical nonlinearity and the anharmonicity of oxides

Integration of crystalline oxides on silicon is one of the most promising ways to continue enhancing the performance and functionality of silicon-based technology. Oxide integration offers access to new physical phenomena not present in current semiconductor platforms to make use of in devices. In this work, we discuss one such phenomenon, the Pockels effect (also known as the linear electro-optical effect), and its connection with crystal anharmonicity. The Pockels effect describes the linear change in a material’s index of refraction in response to an applied electric field, and as such, is only present in non-centrosymmetric crystals. Anharmonic crystals with soft phonon modes can have enormous Pockels responses, and anharmonicity often manifests itself via large thermal expansion. However, one does not imply the other. Highly anharmonic crystals like BaTiO₃ (and, under strain, SrTiO₃) do exhibit large Pockels effect, while the even more anharmonic LiB₃O₅ does not. We will discuss the relationship between the thermal expansion, anharmonicity and the electro-optical effect.