Light makes special materials move at ultrafast speeds

(Nanowerk News) Ferroelectric materials are unusual because they have an electrically positive side and an electrically negative side, and these sides can be switched with an electric field. Relaxor ferroelectrics are special ferroelectric materials with greatly enhanced electrical and mechanical properties. These properties originate from the materials’ domain structure. These are microscopic areas where the direction of polarization is aligned. Knowing how quickly these material’s properties can change is critical to understanding and using them. However, scientists have not been able to measure how fast these materials can respond. This study shows that light can modulate the electric polarization within the relaxor’s domains in a few trillionths of a second. The researchers measured this speed using ultrafast electron diffraction at the atomic level to obtain snapshots of the evolving domain structure. The team combined these measurements with theory to understand how light modulates the relaxor structure. Depiction of the dynamic atomic and nanoscale evolution of a relaxor ferroelectric following photoexcitation. Light drives ultrafast reconfigurations of the domains and rotation of the ferroelectric polarization in a few trillionths of a second. (Image: SLAC National Accelerator Laboratory) The findings are published in Nano Letters (“Light-Driven Ultrafast Polarization Manipulation in a Relaxor Ferroelectric”). Relaxors already have many applications: energy storage, sensors, transducers, and actuators. Their unique properties originate from their many microscopic polarization domains. Fast control of these domains will unlock many more applications, and understanding how these processes work will advance materials science. Previous studies have shown that electric fields and/or temperature can rotate the polarization in domains, but the time taken has never been measured. This work demonstrates that the rotation happens on a picosecond (a trillionth of a second) timescale. The research also suggests a new way to control relaxor domains at the atomic and nano scales. Three research groups with expertise in material synthesis, time-resolved experiments, and phase-field simulation worked together to investigate light-induced physical phenomena in relaxor ferroelectrics. They studied PMN-0.32PT, which is one of the most notable relaxors. The project used the Ultrafast Electron Diffraction (UED) Facility at the Linac Coherent Light Source (LCLS), a Department of Energy Office of Science user facility. The researchers triggered the light-induced response using a femtosecond (a quadrillionth of a second) 266 nm laser and probed the diffraction pattern with a high energy 100 femtosecond duration electron beam. By changing the delay time of the laser and electron beams, they collected snapshots of the structure with femtosecond time resolution. They found significant intensity changes of particular diffraction peaks which enabled deduction of the atomic scale motion occurring within each unit cell. Phase-field simulations were used to understand the mechanisms and pathways by which light modulated the structure, indicating that a light-induced temperature jump played a key role, and showing that the polarization can be modulated in both magnitude and direction. This study defines new opportunities for dynamic reconfigurable control of the polarization in nanoscale relaxor ferroelectrics.