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

TL;DR

This study demonstrates that femtosecond optical excitation induces a net lattice expansion in a ferroelectric/dielectric superlattice through depolarization-field screening, revealing new insights into polarization manipulation at the atomic scale.

Contribution

The paper presents the first direct observation of optically induced lattice expansion driven by depolarization-field screening in a ferroelectric/dielectric superlattice.

Findings

Net lattice expansion observed in superlattice after optical excitation

Differential strain response: expansion in BaTiO3 and contraction in CaTiO3

Depolarization-field screening reduces polarization, enabling new control methods

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 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.