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

A time-resolved x-ray free-electron laser diffraction study indicates that optical excitation of a strongly coupled 2(BaTiO₃)/4(CaTiO₃) (BT/CT) superlattice (SL) results in a transient overall photoinduced lattice expansion of the SL. There are two origins of the photoinduced distortion: a depolarization screening-driven structural change and an acoustic pulse launched from the bottom electrode on which the SL film was deposited. The acoustic pulse propagates through the film with a longitudinal sound velocity of 6 km/s, reflects from the surface, and propagates into the substrate. The depolarization screening-induced strain persists for a longer time. The intensities of the SL Bragg and satellite reflections were analyzed to determine the photoinduced strain in each component of the SL. The analysis showed the depolarization field screening led to an expansion of 0.04% in BT layers and a contraction of 0.01% in CT layers. The smaller magnitude of compression in CT layers can arise from the contribution of octahedral rotation patterns at BT/CT interfaces to the polarization of the CT layers. The photoinduced lattice compression and corresponding polarization reduction in CT points to a possibility of attaining metastable polarization configurations in strongly coupled SLs.