As materials undergo phase transitions, ultrafast “sonograms” are revealing the dynamic changes in trillionth-of-a-second intervals. Common phase transitions include the melting of candle wax before it burns and the dissolving of sugar into water. They are purely structural alterations that produce dramatic changes in a material’s physical properties, and they play a significant role both in nature and in industrial processes from chip fabrication to steel making. Now, a team of physicists from Vanderbilt University has collaborated with scientists at the Fritz Haber Institute of the Max Planck Society in Berlin to shed new light on how vanadium dioxide (VO2), the fastest phase transition material known, shifts from its transparent to reflective phase. Transitions like these are difficult for scientists to catch because they happen so quickly. A femtosecond pulse of infrared laser light “pumps” the atomic lattice in vanadium dioxide, including a phase transition. (Image: Fritz Haber Institute) “This means that there is a lot that we still don’t know about the dynamics of these critical processes,” said physics professor Richard Haglund, who directed the Vanderbilt researchers. To better understand the phenomenon in VO2 — which shifts from a transparent, semiconducting phase to a reflective, metallic phase in the time it takes a beam of light to travel one-tenth of a millimeter — the physicists developed a new technique to obtain a more complete picture of ultrafast phase changes. The new method, which can track structural changes that take place within materials at intervals of less than one-trillionth of a second, is a variation on a standard technique known as “pump and probe.” It uses an infrared laser that can produce powerful pulses of light that last only femtoseconds. When the pump pulses strike the surfaces of the target material, they generate high-frequency atomic vibrations determined by the material’s composition and phase. These vibrations change during a phase transition, so they can be used to identify and track the transition in time. At the same time, the physicists split off a small fraction of the infrared beam, converting it into white light and illuminating the surface of the target. These lattice vibrations produce changes in the material’s surface reflectivity, which scientists can use to track what is happening inside of the material by mapping the changes taking place at its surface. “With this new technique, we were able to see a lot of details that we’ve never seen before,” Haglund said. Such details included how the electrons in the material rearrange first and then are followed by the movement of much more massive atoms as the material shifts from its semiconductor to metallic-phase orientation. These characteristics provide new information that could be used to design high-speed optical switches using this material. Vanadium dioxide is one of a class of materials now being considered for use in faster computer memory. When mixed with suitable additives, it makes a window coating that blocks infrared transmission on hot days and reduces heat loss during cool weather. It also holds potential applications in optical shutters, sensors and cameras. “The real power of this technique is that it is sensitive to atomic changes inside the material which are usually observed using expensive large-scale x-ray sources,” said Simon Wall, an Alexander von Humbolt fellow at the Fritz Haber Institute. “Now we can do the experiment optically and in the lab on a tabletop.” The project, which was funded by grants from the National Science and Alexander von Humboldt foundations, was reported in the March 6 issue of Nature Communications. For more information, visit: www.vanderbilt.edu