Dielectric polarization and refractive indices of ultrathin barium titanate films on strontium titanate single crystals

The electrical and optical properties of ultrathin films of tetragonal barium Titanate (BaTiO3) on strontium titanate (SrTiO3) single-crystal substrates are theoretically investigated using a microscopic quantum mechanical model based on the orbital approximation in correlation with the dipole-dipole interaction. First-, second-, and third-order electronic polarizabilities had to be considered in this calculation in order to obtain accurate results for both the dielectric spontaneous polarization and the refractive indices of the BaTiO3 overlayer. The spontaneous polarization is drastically reduced in the film as its thickness decreases. However, an electronic polarization appears within the SrTiO3 substrate in the neighbourhood of the interface. This polarization, which vanishes far away from the interface into the SrTiO3 bulk, is induced by the polarization of the BaTiO3 film. Furthermore, we find the refractive index for both the BaTiO3 film and the SrTiO3 substrate to be deeply reduced for light polarized perpendicular to the surface.