Optical response of BiFeO3 films subjected to uniaxial strain

The impact of single-axis lattice expansion on the optical response of BiFeO₃ films is examined. Low-energy He implantation is used to tailor morphotropic phases of BiFeO₃ films and study changes in their optical spectra with continuously increasing lattice expansion. He ion implantation of epitaxial rhombohedral (R)- and tetragonal (T)-like BiFeO₃ films induces uniaxial out-of-plane strain that, on R-like films, eventually leads to a complete R-T phase transition. This approach allows us to provide insights into the optical response of BiFeO₃ films. Strain doping of T-like films leads to a significant redshift of the optical absorption spectra that is theoretically explained by a lowering of Fe 3dt_2g states. R-like films, on the other hand, show a less-pronounced sensitivity to uniaxial strain and a blueshift of about 250 meV at the strain-induced R-T transition. Here, the results demonstrate that strain doping allows a deeper examination of the optical properties of epitaxial phases that are otherwise impossible to access by standard epitaxy.