title: Direct observation of acoustic oscillations in InAs nanowires.
abstract: Time-resolved X-ray diffraction and optical reflectivity are used to directly measure three different acoustic oscillations of InAs nanowires. The oscillations are excited by a femtosecond laser pulse and evolve at three different time scales. We measure the absolute scale of the initial radial expansion of the fundamental breathing eigenmode and determine the frequency by transient optical reflectivity. For the extensional eigenmode we measure the oscillations of the average radial and axial lattice constants and determine the amplitude of oscillations and the average extension. Finally we observe a bending motion of the nanowires. The frequencies of the eigenmodes are in good agreements with predictions made by continuum elasticity theory and we find no difference in the speed of sound between the wurtzite nanowires and cubic bulk crystals, but the measured strain is influenced by the interaction between different modes. The wurtzite crystal structure of the nanowires however has an anisotropic thermal expansion.
authors.affiliation: Centre for Molecular Movies, Niels Bohr Institute, University of Copenhagen, Copenhagen Ø, Denmark. som@fys.ku.dk
authors.forename: Simon O
authors.initials: SO
authors.lastname: Mariager
authors.forename: Dmitry
authors.initials: D
authors.lastname: Khakhulin
authors.forename: Henrik T
authors.initials: HT
authors.lastname: Lemke
authors.forename: Kasper S
authors.initials: KS
authors.lastname: Kjaer
authors.forename: Laurent
authors.initials: L
authors.lastname: Guerin
authors.forename: Laura
authors.initials: L
authors.lastname: Nuccio
authors.forename: Claus B
authors.initials: CB
authors.lastname: Sørensen
authors.forename: Martin M
authors.initials: MM
authors.lastname: Nielsen
authors.forename: Robert
authors.initials: R
authors.lastname: Feidenhans'l