Temperature tunable electro-optic response in strained KNbO₃ thin films

KNbO₃ is a classical ferroelectric extensively studied in bulk single crystal form for its strong ferroelectric, piezoelectric and electro-optic properties. However, thin films of KNbO₃ have been relatively less explored. Following successful growth of epitaxially strained KNbO₃ films via suboxide MBE, we demonstrate strong coupling between strain and ferroelectric structure, owing to a large electrostriction coefficient in KNbO₃. A combination of temperature-dependent second-harmonic generation and electro-optic measurements assisted by synchrotron-based x-ray reciprocal space mapping, ferroelectric measurements, and transmission electron microscopy reveal an enhancement of its remanent polarization, optical second harmonic generation coefficient and linear electro-optic coefficients in its room temperature tetragonal phase relative to bulk orthorhombic values. Additionally, at cryogenic temperatures the KNbO₃ films undergo a symmetry lowering phase transition into a monoclinic phase. A dramatic enhancement in electro-optic response is observed in the monoclinic phase, with significantly nonlinear behavior leading to changes in refractive index superior to other ferroelectric thin films, such as those of BaTiO₃.