  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 was 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.
