  Multi-component chalcogenides, such as quasi-binary GeTe-Sb$_{2}$Te$_{3}$
alloys, are widely used in optical data storage media in the form of rewritable
optical discs. Ge$_{2}$Sb$_{2}$Te$_{5}$ (GST) in particular has proven to be
one of the best-performing materials, whose reliability allows more than
10$^{6}$ write-erase cycles. Despite these industrial applications, the
fundamental kinetics of rapid phase-change in GST remain controversial and
active debate continues over the ultimate speed limit. Here we explore
ultrafast structural transformation in a photo-excited GST superlattice, where
GeTe and Sb$_{2}$Te$_{3}$ are spatially separated, using coherent phonon
spectroscopy with pump-pump-probe sequences. By analysing the coherent phonon
spectra in different time regions, complex structural dynamics upon excitation
are observed in GST superlattice (but not in GST alloys), which can be
described as the mixing of Ge sites from two different coordination
environments. Our results suggest possible applicability of GST superlattice
for ultrafast switching devices.
