Optics of epitaxial strained strontium titanate films

Diverse effects of lattice strain on the optical properties in the near-infrared to vacuum ultraviolet spectral range are experimentally revealed in cube-on-cube-type epitaxial perovskite SrTiO3 films grown on compressive substrate. Compared to the reference crystal, the tetragonal antiferrodistortive film exhibits spectral blueshifts, which are consistent with the theoretically predicted bandgap widening. In addition to this strain-induced interband effect, the peculiar near-edge absorption and smearing of interband transitions are found. It is shown that Frohlich-type electron–phonon coupling can be enhanced by strain and lead to this behavior. It is suggested that electron–phonon interactions can play an important role in optical properties of ferroelectric films and deserve further studies.