Thermooptic Property of Polycrystalline BiFeO3 Film

We have fabricated a polycrystalline BiFeO 3 (BFO) film with a thickness of 650 nm on a Pt/Ti/SiO 2 /Si substrate by a chemical solution deposition method, and its optical and thermooptic properties were evaluated by spectroscopic ellipsometry. The optical band gap energy of the BFO film was 2.95 eV at 50 °C and monotonically decreased with increasing temperature to 2.79 eV at 530 °C. The extinction coefficient was significantly low at a wavelength longer than 600 nm in the temperature range from 50 to 530 °C. The refractive index of the BFO film was estimated to be 2.96 at 600 nm and 2.68 at 1550 nm at 50 °C using a single Gaussian oscillator, and a thermooptic coefficient of $0.8\times 10^{-4}$ K -1 was obtained at a wavelength of 1550 nm. Although the refractive index decreased with increasing temperature at all wavelengths, the variation was larger at shorter wavelengths. This seems to be caused by the combination of broadening in the Gaussian oscillator vibration and thermal expansion.