Publications

2012

• La pile à combustible, une solution vers des véhicules propres
  
  • Clochard Marie-Claude
  Flash X - La revue scientifique de l'Ecole polytechnique, Ecole polytechnique, 2012 (14), pp.13-17.
• Second-harmonic generation in silicon waveguides strained by silicon nitride
  
  • Cazzanelli M.
  • Bianco F.
  • Borga E.
  • Pucker G.
  • Ghulinyan M.
  • Degoli E.
  • Luppi E.
  • Veniard V.
  • Ossicini S.
  • Modotto D.
  • Wabnitz S.
  • Pierobon R.
  • Pavesi L.
  Nature Materials, Nature Publishing Group, 2012, 11 (2), pp.148. Silicon photonics meets the electronics requirement of increased speed and bandwidth with on-chip optical networks. All-optical data management requires nonlinear silicon photonics. In silicon only third-order optical nonlinearities are present owing to its crystalline inversion symmetry. Introducing a second-order nonlinearity into silicon photonics by proper material engineering would be highly desirable. It would enable devices for wideband wavelength conversion operating at relatively low optical powers. Here we show that a sizeable second-order nonlinearity at optical wavelengths is induced in a silicon waveguide by using a stressing silicon nitride overlayer. We carried out second-harmonic-generation experiments and first-principle calculations, which both yield large values of strain-induced bulk second-order nonlinear susceptibility, up to 40 pm V-1 at 2,300 nm. We envisage that nonlinear strained silicon could provide a competing platform for a new class of integrated light sources spanning the near-to mid-infrared spectrum from 1.2 to 10μm. (10.1038/NMAT3200)
  DOI : 10.1038/NMAT3200
• Les vortex dans les supraconducteurs : une matière molle au sein de la matière dure
  
  • van Der Beek K.
  • Lévy-Bertrand Florence
  Reflets de la Physique, EDP sciences, 2012, 28, pp.4. (10.1051/refdp/201228004)
  DOI : 10.1051/refdp/201228004
• Isotope effects in lithium hydride and lithium deuteride crystals by molecular dynamics simulations
  
  • Dammak Hichem
  • Antoshchenkova Ekaterina
  • Hayoun Marc
  • Finocchi Fabio R.D.
  Journal of Physics: Condensed Matter, IOP Publishing [1989-....], 2012, 24, pp.435402. Molecular dynamics (MD) simulations have been carried out to study isotope effects in lithium hydride and lithium deuteride crystals. Quantum effects on nuclear motion have been included through a quantum thermal bath (QTB). The interatomic forces were described either within the density-functional theory (DFT) in the generalized gradient approximation (GGA) or by the phenomenological approach using the shell model. For both models, the isotopic shift in the lattice parameter can be successfully predicted by QTB-MD simulations. The slope of the experimental isotopic shift in pressure is satisfactorily reproduced by QTB-MD within DFT-GGA, in contrast to both density-functional perturbation theory and QTB-MD with the shell model. We have analyzed the reasons of these discrepancies through the vibrational densities of states and the isotopic shifts in bulk modulus. The results illustrate the importance of anharmonic contributions to vibrations and to the isotopic pressure shift between LiH and LiD. (10.1088/0953-8984/24/43/435402)
  DOI : 10.1088/0953-8984/24/43/435402
• Thermalization of photoexcited carriers in bismuth investigated by time-resolved terahertz spectroscopy and ab initio calculations
  
  • Timrov I.
  • Kampfrath T.
  • Faure Jérôme
  • Vast N.
  • Ast C.R.
  • Frischkorn C.
  • Wolf M.
  • Gava P.
  • Perfetti L.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2012, 85 (15), pp.NA. The charge carrier dynamics of photoexcited bismuth generates a Drude response that evolves over time. Our data show that the plasma frequency of bismuth displays an initial increase and a subsequent decay. We have performed ab initio calculations on bulk bismuth within the density functional theory and show that this peculiar behavior is due to local extrema in the valence and conduction bands. It follows that most of the carriers first accumulate in these extrema and reach the Fermi level only 0.6 ps after the photoexcitation. © 2012 American Physical Society. (10.1103/PhysRevB.85.155139)
  DOI : 10.1103/PhysRevB.85.155139
• Raman activity of sp(3) carbon allotropes under pressure: A density functional theory study
  
  • Flores-Livas Jose A.
  • Lehtovaara Lauri
  • Amsler Maximilian
  • Goedecker Stefan
  • Pailhes Stephane
  • Botti Silvana
  • San Miguel Alfonso
  • Marques Miguel A. L.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2012, 85 (15), pp.NA. Raman spectroscopy is a powerful tool to study the intrinsic vibrational characteristics of crystals, and, therefore, it is an adequate technique to explore phase transitions of carbon under pressure. However, the diamond-anvil cell, which is used in experiments to apply pressure, appears as a broad intense feature in the spectra. This feature lies, unfortunately, in the same range as the principal modes of recently proposed sp(3) carbon structures. As these modes are hard to distinguish from the diamond cell background, we analyze all Raman-active modes present in the sp(3) carbon structures in order to find detectable fingerprint features for an experimental identification. (10.1103/PhysRevB.85.155428)
  DOI : 10.1103/PhysRevB.85.155428
• Positron annihilation in latex templated macroporous silica films: pore size and ortho-positronium escape
  
  • Liszkay L.
  • Guillemot F.
  • Corbel C.
  • Boilot J.P.
  • Gacoin T.
  • Barthel E.
  • Pérez A.
  • Barthe M.F.
  • Desgardin P.
  • Crivelli P.
  • Gendotti U.
  • Rubbia A.
  New Journal of Physics, Institute of Physics: Open Access Journals, 2012, 14, pp.065009. Depth profling of positron annihilation characteristics has been used to investigate the pore size distribution in macroporous PMMA latex templated SiO2 films deposited on glass or Si and prepared with 11-70% porosity. The correlation between the annihilation characteristics shows that o-Ps escape (re-emission) into vacuum occurs in all films with a porosity threshold that is pore size dependent. For 60 ± 2% porosity, the o-Ps reemission yield decreases from ~ 0:25 to ~ 0:11 as the pore size increases from 32 to 75 nm. The o-Ps reemission yield is shown to vary linearly with the specific surface area per mass unit and the slope is independent of pore size, 9:1±0:4 g cm-1. For 32 nm pores, the o-Ps annihilation lifetimes in the films, 17(2)ns and 106(5) ns, show that o-Ps annihilates from micropores with small effective size (1:4 ± 4 nm) and from macropores with large effective size (~ 32 nm). Above the porosity threshold, the o-Ps-escape model predicts the annihilation lifetime in the films to be 19±2 ns. Our results imply that o-Ps effciently detects the microporosity present in the silica walls. At low porosity, its capture into the micropores competes with its capture into the macropores. At higher porosity (when the distance between micropores and macropores become small), this capture into the micropores assists the capture into the macropores. (10.1088/1367-2630/14/6/065009)
  DOI : 10.1088/1367-2630/14/6/065009
• Clusters dissolution of Yb³⁺ in codoped SiO₂-Al₂O₃-P₂O₅ glass fiber and its relevance to photodarkening
  
  • Deschamps T.
  • Ollier N.
  • Vezin H.
  • Gonnet C.
  The Journal of Chemical Physics, American Institute of Physics, 2012, 136, pp.014503-1 - 014503-4. Using a combination of pulse electron paramagnetic resonance and photoluminescence spectroscopy, we demonstrate the major role of phosphorous rather than aluminium in the rare-earth dissolution process, an essential advance in telecommunication and solid laser fields. Our results also provide new insight into the micro-structural origin of the photodarkening process occurring in Yb doped fiber. (10.1063/1.3673792)
  DOI : 10.1063/1.3673792
• Nucleation of single wall carbon nanotubes of various chiralities
  
  • Beuneu F.
  Solid State Communications, Elsevier, 2012, 152 (13), pp.1155. A simple model for the nucleation and growth of single wall carbon nanotubes from a graphene sheet at the surface of a metallic catalyst saturated in carbon is developed. It enables to predict the geometry and energy of tube embryos of all possible chiralities, as well as the way that they can grow. It is shown that armchair-like chiralities are energy preferred for geometrical reasons. This result is discussed and compared to experimental literature. © 2012 Elsevier Ltd. All rights reserved. (10.1016/j.ssc.2012.03.041)
  DOI : 10.1016/j.ssc.2012.03.041