Optical characterization of ferroelectric Bi_3.25La _0.75Ti₃O₁₂ thin films

Amorphous and crystalline Bis_3.25La_0.75Ti ₃O₁₂ (BLT) thin films on vitreous silica and sapphire substrates are prepared from chemical solutions. Their optical properties are investigated by transmittance measurements at energies from 1.1 to 5.0 eV. A four-phase model consisting of air, surface rough layer, BLT, and substrate is used to simulate the measured transmittance spectra. The inverse synthesis method with a double Tauc-Lorentz (DTL) dispersion function is used to calculate the optical constants and film thicknesses. The dispersion of the refractive index in the transparent region agrees with Sellmeier's dispersion relation. The absorption edges of the BLT films are different in the amorphous and crystalline cases.