Optical Properties of Ferroelectric Epitaxial K(0.5)Na(0.5)NbO(3) Films in Visible to Ultraviolet Range

The complex index of refraction in the spectral range of 0.74 to 4.5 eV is studied by variable-angle spectroscopic ellipsometry in ferroelectric K(0.5)Na(0.5)NbO(3) films. The 20-nm-thick cube-on-cube-type epitaxial films are grown on SrTiO(3)(001) and DyScO(3)(011) single-crystal substrates. The films are transparent and exhibit a significant difference between refractive indices Δn = 0.5 at photon energies below 3 eV. The energies of optical transitions are in the range of 3.15–4.30 eV and differ by 0.2–0.3 eV in these films. The observed behavior is discussed in terms of lattice strain and strain-induced ferroelectric polarization in epitaxial perovskite oxide films.