Visualization of Ferroelectric Domains with Picosecond Time and Nanometer Length Resolution

Engineering domains and domain walls in ferroelectric materials is at the heart of realizing novel nanoelectronic and photonic devices. To this end, electric fields, light, strain and doping have been employed to manipulate domains at the atomic scale. Control of the domain structures with light pulses particularly presents strong potential to engineer electronic and optoelectronic functionalities in these materials in an all-optical fashion. Nevertheless, non-equilibrium photoexcited states in such complex materials could not be resolved in space and time with the desired resolution, i.e., picoseconds and nanometers, due to limitations in available characterization techniques. In this work, we develop a novel characterization approach to visualize nanoscale domain structures in photoexcited ferroelectric thin films over picosecond time with nanometer length scales via time-resolved nanoprobe X-ray diffraction imaging (TR-NXRD) at the Advanced Photon Source. Using this approach, we investigate stripe-domain bismuth ferrite (BFO) thin films that exhibit periodic 71 degree domain walls. Surprisingly, we find that the domain pattern transiently melts following an intense above bandgap laser pulse as revealed by strongly bleached domain pattern contrast. The washout of the domain pattern immediately after the photoexcitation arises due to weakening of the in-plane ferroelectric polarization linked with a large out-of-plane photostriction and rhombohedral-to-tetragonal phase transformation which overall destabilize the domain walls. Furthermore, we find that the dynamic domain evolution is fully reversible as the domain pattern reappears after 10 ns following the excitation laser pulse. Reappearance of the domains is consistent with the fact that the domain structure is pinned by the mechanical boundary conditions between the substate and the thin film. Overall, we unveil a novel way to manipulate stripe domains in BFO thin film at sub-nanosecond timescales by visualizing via TR-NXRD. This technique is further applicable to a broad range of materials to visualize non-equilibrium dynamics across nanoscale spatial heterogeneities critical to uncovering hidden structure-property relationships overlooked before due to the absence of methods that can simultaneous access high spatial and temporal resolution.