Ultrafast dynamics of electrons and phonons in graphitic materials

Doctor of PhilosophyDepartment of PhysicsItzhak Ben-ItzhakPatrick RichardThis work focuses on the ultrafast dynamics of electrons and phonons in graphitic materials. In particular, we experimentally investigated the factors which influence the transport properties of graphite and carbon nanotubes. In the first part of this dissertation, we used Time-resolved Two Photon photoemission (TR-TPP) spectroscopy to probe the dynamics of optically excited charge carriers above the Fermi energy of double-wall carbon nanotubes (DWNTs). In the second part of this study, time-resolved anti-Stokes Raman (ASR) spectroscopy is applied to investigating in real time the phonon-phonon interactions, and addressing the way the temperature affects the dynamics of single-wall carbon nanotubes (SWNTs) and graphite. With respect to the first part, we aim to deeply understand the dynamics of the charge carriers and electron-phonon interactions, in order to achieve an as complete as possible knowledge of DWNTs. We measured the energy transfer rate from the electronic system to the lattice, and we observed a strong non-linear increase with the temperature of the electrons. In addition, we determined the electron-phonon coupling parameter, and the mean-free path of the electrons. The TR-TPP technique enables us to measure the above quantities without any electrical contacts, with the advantage of reducing the errors introduced by the metallic electrodes. The second investigation uses time-resolved ASR spectroscopy to probe in real time the G-mode non-equilibrium phonon dynamics and the energy relaxation paths towards the lattice by variation of the temperature in SWNTs and graphite. The lifetime range of the optically excited phonons obtained is 1.23 ps to 0.70 ps in the lowest (cryogenic temperatures) and highest temperature limits, respectively. We have also observed an increase in the energy of the G-mode optical phonons in graphite with the transient temperature. The findings of this study are important since the non-equilibrium phonon population has been invoked to explain the negative differential conductance and current saturation in high biased transport phenomena

Time-resolved Raman Spectroscopy of Optical Phonons in Graphite: Phonon Anharmonic Coupling and Anomalous Stiffening

Time-resolved Raman spectroscopy has been applied to probe the anharmonic coupling and electron-phonon interaction of optical phonons in graphite. From the decay of the transient anti-Stokes scattering of the G-mode following ultrafast excitation, we measured a lifetime of 2.2+/-0.1ps for zone-center optical phonons. We also observed a transient stiffening of G-mode phonons, an effect attributed to the reduction of the electron-phonon coupling for high electronic temperatures

One- and two-dimensional photo-imprinted diffraction gratings for manipulating terahertz waves

Emerging technology based on artificial materials containing metallic structures has raised the prospect for unprecedented control of terahertz waves through components like filters, absorbers and polarizers. The functionality of these devices is static by the very nature of their metallic or polaritonic composition, although some degree of tunability can be achieved by incorporating electrically biased semiconductors. Here, we demonstrate a photonic structure by projecting the optical image of a metal mask onto a thin GaAs substrate using a femtosecond pulsed laser source. We show that the resulting high-contrast pattern of photo- excited carriers can create diffractive elements operating in transmission. With the metal mask replaced by a digital micromirror device, our photo-imprinted photonic structures provide a route to terahertz components with reconfigurable functionality.Comment: 12 pages, 5 figure

Ultra-low-loss Polaritons in Isotopically Pure Materials: A New Approach

Conventional optical components are limited to size-scales much larger than the wavelength of light, as changes in the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultra-thin, co-called "flat" optical components that beget abrupt changes in these properties over distances significantly shorter than the free space wavelength. While high optical losses still plague many approaches, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes in comparison to plasmon-polariton-based nanophotonics. We experimentally observe a three-fold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics. Our results provide the foundation for a materials-growth-directed approach towards realizing the loss control necessary for the development of PhP-based nanophotonic devices

La situation juridique en Italie à l’époque du Justinien, observations en occasion de la Novelle "Pragmatica Sanctio pro petitione Vigilii"

 Le règne de Justinien fut marquè par le projet de la restauration de l’Empire Romaine et la reconquête des provinces occidentales, perdues par les inva­sions barbares. L’article donc s’occupe avec la situation juridique en Italie avant et après cet effort. Précisément après la fin de la guerre de Goths (535-553) et la victoire des Byzantins, Justinien avec sa Novelle “Pragmatica Sanctio pro petitione Vigilii” (a.554), organise l’administration du territoire ita­lien et re­donne aux propriétaires les terres qui avaient été aliénées par le rois des Goths Totila. Malgré les efforts de Justinien la reconquête été éphé­mère. Trois ans après sa mort (a.568) les Lombards traversent les Alpes et con­­quièrent les deux tiers de la péninsule