The ability to control the optical properties of a material with an electric field has led to optical memory devices, communication systems, optical signal processing or quantum cryptography. Understanding electro-optic effects, especially in thin films, would improve the efficiency of these applications. In particular, the influence of epitaxial strains is of prime importance. In addition, the active control of these effects would be of great interest to tailor the material to the desired performance. Here, we demonstrate through firstprinciple calculations that the linear electro-optic response (Pockels effect) of two siliconcompatible ferroelectrics is stable with respect to bi-axial strain and that the electro-optic response can be strongly enhanced through the electrical control of the polarization. We attribute the former to the lack of optical phonon softening and a weak elasto-optic response and the latter to the externally-induced softening of a phonon of symmetry A 1. Our results are readily applicable to other polar materials and show that the electro-optic effect can be efficiently engineered to meet the performance criteria of future technologies. 1