Time-domain separation of optical properties from structural transitions in resonantly bonded materials

Femtosecond optical spectroscopy and single-shot electron diffraction measurements during the photoinduced amorphization of the phase-change material Ge 2 Sb 2 Te 5 demonstrate that optical properties can be separated from the structural state. The extreme electro-optical contrast between crystalline and amorphous states in phase-change materials is routinely exploited in optical data storage 1 and future applications include universal memories 2 , flexible displays 3 , reconfigurable optical circuits 4 , 5 , and logic devices 6 . Optical contrast is believed to arise owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase-change material Ge 2 Sb 2 Te 5 during an irreversible state transformation. The dielectric function changes by 30% within 100 fs owing 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.