title: Diffraction of quantum dots reveals nanoscale ultrafast energy localization.
abstract: Unlike in bulk materials, energy transport in low-dimensional and nanoscale systems may be governed by a coherent "ballistic" behavior of lattice vibrations, the phonons. If dominant, such behavior would determine the mechanism for transport and relaxation in various energy-conversion applications. In order to study this coherent limit, both the spatial and temporal resolutions must be sufficient for the length-time scales involved. Here, we report observation of the lattice dynamics in nanoscale quantum dots of gallium arsenide using ultrafast electron diffraction. By varying the dot size from h = 11 to 46 nm, the length scale effect was examined, together with the temporal change. When the dot size is smaller than the inelastic phonon mean-free path, the energy remains localized in high-energy acoustic modes that travel coherently within the dot. As the dot size increases, an energy dissipation toward low-energy phonons takes place, and the transport becomes diffusive. Because ultrafast diffraction provides the atomic-scale resolution and a sufficiently high time resolution, other nanostructured materials can be studied similarly to elucidate the nature of dynamical energy localization.
authors.affiliation: Physical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology , Pasadena, California 91125, United States.
authors.forename: Giovanni M
authors.initials: GM
authors.lastname: Vanacore
authors.forename: Jianbo
authors.initials: J
authors.lastname: Hu
authors.forename: Wenxi
authors.initials: W
authors.lastname: Liang
authors.forename: Sergio
authors.initials: S
authors.lastname: Bietti
authors.forename: Stefano
authors.initials: S
authors.lastname: Sanguinetti
authors.forename: Ahmed H
authors.initials: AH
authors.lastname: Zewail
keywords: Femtosecond diffraction; ballistic transport; energy localization; quantum dots