Ultrafast sonograms shed new light on rapid phase transitions: The new method is a variation on a standard method known as 'pump-and-probe.' It uses an infrared laser that can produce powerful pulses of light that only last for femtoseconds (millionths of a trillionth of a second). When these pump pulses strike the surface 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 (the probe), convert it into white light and use it to illuminate the surface of the target. It turns out that these lattice vibrations produce changes in the material's surface reflectivity. As a result, the physicists can track what is happening inside the material by mapping the changes taking place on its surface.