The surface and interface effects on the dielectric polarization and refractive indices of BaTiO3 single crystals and BaTiO3 ultrathin films on SrTiO3 single crystal substrates are investigated theoretically by using a microscopic model based on the orbital approximation in correlation with the dipole-dipole interaction. The spontaneous polarization of BaTiO3 single crystals is drastically reduced near the c-surface. For BaTiO3/SrTiO3 films, the spontaneous polarization is reduced in the film as its thickness decreases. However, an electronic polarization appears within the SrTiO3 substrate in the neighborhood 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 either for BaTiO3 single crystals or for BaTiO3 films and SrTiO3 substrates to be strongly reduced for light polarized perpendicular to the surface.The surface and interface effects on the dielectric polarization and refractive indices of BaTiO3 single crystals and BaTiO3 ultrathin films on SrTiO3 single crystal substrates are investigated theoretically by using a microscopic model based on the orbital approximation in correlation with the dipole-dipole interaction. The spontaneous polarization of BaTiO3 single crystals is drastically reduced near the c-surface. For BaTiO3/SrTiO3 films, the spontaneous polarization is reduced in the film as its thickness decreases. However, an electronic polarization appears within the SrTiO3 substrate in the neighborhood 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 either for BaTiO3 single crystals or for BaTiO3 films and SrTiO3 substrates to be strongly reduced for light polarized perpendicular to the surface
