  We report experimental and theoretical investigation of anisotropy in optical
properties and their origin in the ferroelectric and paraelectric phases of
bismuth titanate. Room temperature ellipsometric measurements performed on
pulsed laser deposited bismuth titanate thin films of different orientations
show anisotropy in the dielectric and optical constants, Subsequent
first-principles calculations performed on the ground state structures of
ferroelectric and high temperature paraelectric phases of bismuth titanate show
that the material demonstrates anisotropic optical behavior in both
ferroelectric and paraelectric phases. We further show that O 2p to Ti 3d
transition is the primary origin of optical activity of the material while
optical anisotropy results from the asymmetrically oriented Ti-O bonds in TiO6
octehdra in the unit cell.
