Effect of epitaxy on interband transitions in ferroelectric KNbO3

Very large lattice strain and strain-induced polarization are achieved in KNbO _3 using epitaxial growth of a thin KNbO _3 film onto a (001)-oriented SrTiO _3 single-crystal substrate. We demonstrate experimentally that epitaxy produces dramatic changes of interband transitions in the film compared to those of a reference KNbO _3 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.