The phase transformation dynamics induced in ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ films by picosecond laser pulses were studied using real-time reflectivity measurements with subnanosecond resolution. Evidence was found that the thermal diffusivity of the substrate plays a crucial role in determining the ability of the films to crystallize and amorphize. A film/substrate configuration with optimized heat flow conditions for ultrafast phase cycling with picosecond laser pulses was designed and produced. In this system, we achieved reversible phase transformations with large optical contrast $\text{(>20\%)}$ using single laser pulses with a duration of 30 ps within well-defined fluence windows. The amorphization (writing) process is completed within less than 1 ns, whereas crystallization (erasing) needs approximately 13 ns to be completed.

Structural switch for fast switching: Phase-change materials are important for computer memory. They can quickly switch from glassy to crystalline using a thermal pulse and then lock in that structure for a long time at lower temperature. Zalden et al. probed the underlying atomic structure of two phase-change materials during this switching using ultrafast x-rays and simulations (see the Perspective by Rao et al. ). A liquid-liquid phase transition in both materials allowed fast switching at high temperatures. The lower-temperature glass locks in the structure, allowing for long-term memory storage. Science , this issue p. 1062 ; see also p. 1032