Kikkawa, James M

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2008 - 20104
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  • Publication
    Two-dimensional photonic crystals with anisotropic unit cells imprinted from poly(dimethylsiloxane) membranes under elastic deformation
    (2008-10-24) Zhu, Xuelian; Zhang, Ying; Chandra, Dinesh; Cheng, Shih-Chieh; Kikkawa, James M; Yang, Shu
    We study structural symmetries of two-dimensional (2D) photonic crystals with anisotropic unit cells, including square- and rectangular-lattices with orientationally modulated elliptic motifs, and a compound structure consisting of circles with sixfold rotational symmetry and elliptical lines with twofold symmetry, which are created through elastic deformation of a single elastomeric membrane with circular pores. We then investigate the photonic bandgap (PBG) properties of the corresponding 2D Si posts and their tolerance to the structural deviation. We find that in the compound structure the overall PBGs are dominated by the sublattice with a higher symmetry, while the total symmetry is determined by the one with a lower symmetry.
  • Publication
    Photoluminescence and Band Gap Modulation in Graphene Oxide
    (2009-03-19) Luo, Zhengtang; Vora, Patrick; Mele, Eugene J; Johnson, A.T. Charlie; Kikkawa, James M
    We report broadband visible photoluminescence from solid graphene oxide, and modifications of the emission spectrum by progressive chemical reduction. The data suggest a gapping of the two-dimensional electronic system by removal of π-electrons. We discuss possible gapping mechanisms, and propose that a Kekule pattern of bond distortions may account for the observed behavior.
  • Publication
    Chirality Dependence of the K-momentum Dark Excitons in Carbon Nanotubes
    (2010-04-27) Vora, Patrick M; Mele, Eugene J; Tu, X; Kikkawa, James M; Zheng, M
    Using a collection of 12 semiconducting carbon-nanotube samples, each highly enriched in a single chirality, we study the chirality dependence of the K-momentum dark singlet exciton using phonon sideband optical spectroscopy. Measurements of bright absorptive and emissive sidebands of this finite momentum exciton identify its energy as 20–38 meV above the bright singlet exciton, a separation that exhibits systematic dependencies on tube diameter, 2n+m family, and chiral index. We present calculations that explain how chiral angle dependence in this energy separation relates to the Coulomb exchange interaction and elaborate the dominance of the KAA; phonon sidebands over the zone-center phonon sidebands over a wide range of chiralities. The Kataura plot arising from these data is qualitatively well described by theory but the energy separation between the sidebands shows a larger chiral dependence than predicted. This latter observation may indicate a larger dispersion for the associated phonon near the K point than expected from finite distance force modeling.
  • Publication
    Energy of K-Momentum Dark Excitons in Carbon Nanotubes by Optical Spectroscopy
    (2008-10-06) Torrens, Omar N.; Zheng, Ming; Kikkawa, James M
    Phonon sideband optical spectroscopy determines the energy of the dark K-momentum exciton for (6,5) carbon nanotubes. One-phonon sidebands appear in absorption and emission, split by two zone-boundary (K-point) phonons. Their average energy locates the E11 K-momentum exciton 36 meV above the E11 bright level, higher than available theoretical estimates. A model for exciton-phonon coupling shows the absorbance sideband depends sensitively on the K-momentum exciton effective mass and has minimal contributions from zone-center phonons, which dominate the Raman spectra of carbon nanotubes.