The interband optical transitions in cube-on-cube-type epitaxial ferroelectric NaNbO3 thin films and a reference antiferroelectric NaNbO3 crystal are studied in the spectral range of 0.74–9 eV using variable-angle spectroscopic ellipsometry. The energies and the types of the transitions in the films are found to differ significantly from those in the crystal and to vary with epitaxial strain. The results are discussed in terms of epitaxial effects, including higher crystal symmetry, anisotropic lattice strain, and ferroelectric polarization in the NaNbO3 films compared to the crystal.Peer reviewe
