Effect of epitaxy on interband transitions in ferroelectric KNbO₃

Very large lattice strain and strain-induced polarization are achieved in KNbO₃ using epitaxial growth of a thin KNbO₃ film onto a (001)-oriented SrTiO₃ single-crystal substrate. We demonstrate experimentally that epitaxy produces dramatic changes of interband transitions in the film compared to those of a reference KNbO₃ crystal: the energies of transitions change, some transitions are substantially suppressed and new ones appear in the film. A comparison of the experimental observations with theoretical calculations points to yet unexplored phenomena. Our results indicate that optical refraction and electro-optical coefficients of ferroelectric films can be controlled by epitaxial growth, which is of importance for emerging photonic and optoelectronic applications. (paper)