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.
Figure: (a) HRTEM image along [010] perovskite axes of epitaxial KNbO3 film and (001) STiO3 substrate. (b) Second derivative of the imaginary part of the dielectric function in the KNbO3 crystal and epitaxial film. Arrows indicate energies of band-to-band transitions, which are observed in the film, but not in the crystal.