Optical properties of tetragonal and nanoscale BiFeO$_3$*

We measured the optical properties of tetragonal thin film and nanoscale rhombohedral BiFeO$_3$ in the range from near infrared to the near ultraviolet. The absorption spectrum in the tetragonal film is overall blue-shifted compared with that of the rhombohedral BiFeO$_3$ film. It shows an absorption onset near $2.25$ eV, a direct $3.1$ eV band gap, and charge transfer excitations that are $\sim$0.4 eV higher than those of the rhombohedral counterpart. In the nanoparticles, the band gap decreases from $2.7$ eV to $\sim$2.3 eV, and the well-known $3.2$ and $4.5$ eV charge transfer excitations split into multiplets. We discuss these results in terms of structural strain, surface strain, and local symmetry breaking.