  The extreme electro-optical contrast between crystalline and amorphous states
in phase change materials is routinely exploited in optical data storage and
future applications include universal memories, flexible displays,
reconfigurable optical circuits, and logic devices. Optical contrast is
believed to arise due to a change in crystallinity. Here we show that the
connection between optical properties and structure can be broken. Using a
unique combination of single-shot femtosecond electron diffraction and optical
spectroscopy, we simultaneously follow the lattice dynamics and dielectric
function in the phase change material Ge2Sb2Te5 during an irreversible state
transformation. The dielectric function changes by 30% within 100 femtoseconds
due to a rapid depletion of electrons from resonantly-bonded states. This
occurs without perturbing the crystallinity of the lattice, which heats with a
2 ps time constant. The optical changes are an order-of-magnitude larger than
those achievable with silicon and present new routes to manipulate light on an
ultrafast timescale without structural changes.
