Structural evidence for ultrafast polarization rotation in ferroelectric/dielectric superlattice nanodomains

TL;DR

This study reveals ultrafast polarization rotation in ferroelectric/dielectric superlattice nanodomains triggered by optical pulses, showing complex nanoscale polarization dynamics and new mechanisms for polarization control.

Contribution

It provides direct experimental evidence of ultrafast polarization rotation in superlattice nanodomains using time-resolved x-ray techniques, a phenomenon previously only observed in bulk materials.

Findings

Ultrafast optical pulses induce rapid polarization changes in superlattice nanodomains.

Two distinct acoustic pulses are generated by optical excitation.

Polarization rotation occurs via reorientation at domain boundaries, not just uniform polarization change.

Abstract

Weakly coupled ferroelectric/dielectric superlattice thin film heterostructures exhibit complex nanoscale polarization configurations that arise from a balance of competing electrostatic, elastic, and domain-wall contributions to the free energy. A key feature of these configurations is that the polarization can locally have a significant component that is not along the thin-film surface normal direction, while maintaining zero net in-plane polarization. PbTiO3/SrTiO3 thin-film superlattice heterostructures on a conducting SrRuO3 bottom electrode on SrTiO3 have a room-temperature stripe nanodomain pattern with nanometer-scale lateral period. Ultrafast time-resolved x-ray free electron laser diffraction and scattering experiments reveal that above-bandgap optical pulses induce rapidly propagating acoustic pulses and a perturbation of the domain diffuse scattering intensity arising from the nanoscale stripe domain configuration. With 400 nm optical excitation, two separate acoustic pulses are observed: a high-amplitude pulse resulting from strong optical absorption in the bottom electrode and a weaker pulse arising from the depolarization field screening effect due to absorption directly within the superlattice. The picosecond scale variation of the nanodomain diffuse scattering intensity is consistent with a larger polarization change than would be expected due to the polarization-tetragonality coupling of uniformly polarized ferroelectrics. The polarization change is consistent instead with polarization rotation facilitated by the reorientation of the in-plane component of the polarization at the domain boundaries of the striped polarization structure. The complex steady-state configuration within these ferroelectric heterostructures leads to polarization rotation phenomena that have been previously available only through the selection of bulk crystal composition.