X-ray Imaging of Gigahertz Ferroelectric Domain Dynamics

Gigahertz Ferroelectric
Left: Schematic illustration of spatially-resolved pump-probe experiment and domain configuration of a BaTiO3 single crystal sample. The inset shows a unit cell of BaTiO3. Right: The measured lattice strain (dots) oscillates at gigahertz frequency that confirms the prediction by the dynamical phase-field theory (solid lines).

A team of researchers has made an important advance in broadening our understanding of light-induced mesoscale dynamics using the U.S. Department of Energy’s Advanced Photon Source (APS) at Argonne National Laboratory. Time-resolved x-ray diffraction microscopy, aided by a newly developed dynamical phase-field method (DPFM) , revealed how lattice waves can be excited by light pulses and the resulting local structural changes among mesoscopic domains in ferroelectrics, a widely utilized material for sensors, nanoscale positioners, and information storage devices.

Light interaction with matter offers a new way of controlling material properties by harnessing energy transport and conversion in functional materials without contact on ultrafast time scales. However, the desired dynamical control is complicated by the inhomogeneous response of real materials. The ultrafast dynamics depend not only on the intrinsic properties of the compound but also, strongly, on mesoscale structures such as surfaces, domains, interfaces, and defects that govern the coupling between various degrees of freedom.

Rich mesoscale phenomena inspire new functions but pose a great challenge to characterize and understand their fundamental processes in the time domain.

The researchers from The Pennsylvania State University and Argonne National Laboratory shone ultrafast laser pulses on a prototypical ferroelectric single crystal, BaTiO3. Due to absorption of the light, electrons are liberated from bonding states and start to move within a block of the material — a ferroelectric domain — whose lengths are on the order of a micrometer.  These free charge carriers can experience very different local forces that pull them in certain directions depending on which domain they occupy.

As a result, the lattice start to respond to light-activated charge carriers on ultrafast time scales. To capture these local dynamics that occur on micrometer length and sub-nanosecond time scales, a focused x-ray beam from the X-ray Science Division 7-ID-C x-ray beamline at the APS, an Office of Science user facility at Argonne, was used to probe within and across domains following optical excitation, with the lattice dynamics manifesting itself as the change of diffraction intensity from the probing x-ray beam. The local structural dynamics, including oscillatory lattice spacing and rotation at gigahertz frequency in individual domains, can be clearly resolved.

Understanding such complex behavior without modeling tools that account for spatial complexity and short time scales is an outstanding challenge.  The authors have newly developed a dynamical phase-field method (DPFM) which was used to successfully predict these rich structural dynamics. This demonstration of the ability to reveal mesoscopic structural changes opens up new opportunities for understanding mesoscale phenomena and optimizing the functionalities of technologically important materials.

The continuous innovation of x-ray instruments at the APS has allowed users to enter a new spatiotemporal regime to solve challenging problems that could not previously be answered. With the APS Upgrade, higher spatial resolution and better signal-to-noise ratio will further empower these cutting-edge x-ray research tools for meeting mesoscopic energy challenges.

Source : Department of Energy’s Advanced Photon Source (APS)