  A class of chalcogenide alloy materials that shows significant changes in
optical properties upon an amorphous-to-crystalline phase transition has lead
to development of large data capacities in modern optical data storage. Among
chalcogenide phase-change materials, Ge2Sb2Te5 (GST) is most widely used
because of its reliability. We use a pair of femtosecond light pulses to
demonstrate the ultrafast optical manipulation of atomic arrangements from
tetrahedral (amorphous) to octahedral (crystalline) Ge-coordination in GST
superlattices. Depending on the parameters of the second pump-pulse, ultrafast
nonthermal phase-change occurred within only few-cycles (~ 1 ps) of the
coherent motion corresponding to a GeTe4 local vibration. Using the ultrafast
switch in chalcogenide alloy memory could lead to a major paradigm shift in
memory devices beyond the current generation of silicon-based flash-memory.
