title: Time-domain separation of optical properties from structural transitions in resonantly bonded materials.
abstract: 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 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 Ge2Sb2Te5 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.
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany.
authors.forename: Lutz
authors.initials: L
authors.lastname: Waldecker
authors.affiliation: ICFO-Institut de Ciències Fotòniques, Mediterranean Technology Park, 08860, Castelldefels, Barcelona, Spain.
authors.forename: Timothy A
authors.initials: TA
authors.lastname: Miller
authors.affiliation: ICFO-Institut de Ciències Fotòniques, Mediterranean Technology Park, 08860, Castelldefels, Barcelona, Spain.
authors.forename: Miquel
authors.initials: M
authors.lastname: Rudé
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany.
authors.forename: Roman
authors.initials: R
authors.lastname: Bertoni
authors.affiliation: ICFO-Institut de Ciències Fotòniques, Mediterranean Technology Park, 08860, Castelldefels, Barcelona, Spain.
authors.forename: Johann
authors.initials: J
authors.lastname: Osmond
authors.affiliation: ICFO-Institut de Ciències Fotòniques, Mediterranean Technology Park, 08860, Castelldefels, Barcelona, Spain.|ICREA-Institució Catalana de Recerca i Estudi Avançats, 08015 Barcelona, Spain.
authors.forename: Valerio
authors.initials: V
authors.lastname: Pruneri
authors.affiliation: SUTD- Singapore University of Technology &Design, 8 Somapah Road 487372 Singapore, Singapore.
authors.forename: Robert E
authors.initials: RE
authors.lastname: Simpson
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany.
authors.forename: Ralph
authors.initials: R
authors.lastname: Ernstorfer
authors.affiliation: ICFO-Institut de Ciències Fotòniques, Mediterranean Technology Park, 08860, Castelldefels, Barcelona, Spain.
authors.forename: Simon
authors.initials: S
authors.lastname: Wall