Optically Induced Picosecond Lattice Compression in the Dielectric Component of a Strongly Coupled Ferroelectric/Dielectric Superlattice

Abstract

Above‐bandgap femtosecond optical excitation of a ferroelectric/dielectric BaTiO3/CaTiO3 superlattice leads to structural responses that are a consequence of the screening of the strong electrostatic coupling between the component layers. Time‐resolved X‐ray free‐electron laser diffraction shows that the structural response to optical excitation includes a net lattice expansion of the superlattice consistent with depolarization‐field screening driven by the photoexcited charge carriers. The depolarization‐field‐screening‐driven expansion is separate from a photoacoustic pulse launched from the bottom electrode on which the superlattice is epitaxially grown. The distribution of diffracted intensity of superlattice X‐ray reflections indicates that the depolarization‐field‐screening‐induced strain includes a photoinduced expansion in the ferroelectric BaTiO3 and a contraction in CaTiO3. The magnitude of expansion in BaTiO3 layers is larger than the contraction in CaTiO3. The difference in the magnitude of depolarization‐field‐screening‐driven strain in the BaTiO3 and CaTiO3 components can arise from the contribution of the oxygen octahedral rotation patterns at the BaTiO3/CaTiO3 interfaces to the polarization of CaTiO3. The depolarization‐field‐screening‐driven polarization reduction in the CaTiO3 layers points to a new direction for the manipulation of polarization in the component layers of a strongly coupled ferroelectric/dielectric superlattice.