Thermooptic properties of epitaxial BiFeO3 films with different orientations

Abstract

We have fabricated epitaxial BiFeO3 (BFO) films with various orientations on a SrTiO3 substrate by chemical solution deposition, and evaluated their optical and thermooptic properties by spectroscopic ellipsometry. The refractive index of the epitaxial BFO films was estimated to range from 2.620 to 2.635 at a wavelength of 1550 nm at room temperature using six Gaussian oscillators, and a thermooptic coefficient ranging from −0.31 × 10−4 to −0.81 × 10−4 K−1 was obtained at a wavelength of 1550 nm. The refractive index increased with temperature. In addition, the rate of increase in refractive index changed at around the Neel temperature. It is suggested that this change is caused by the magnetovolume effect, because a similar change, the d-spacing change at around the Neel temperature, was also obseved. These results suggest that the BFO films have a high potential for application as an optical material with a high refractive index and that the effectual refractive index change can be controlled by the thermal expansion coefficient, even in the same material.