Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L28: Focus Session: Carbon Nanotube Optics IV |
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Sponsoring Units: DMP Chair: Feng Wang, University of California, Berkeley Room: Colorado Convention Center 302 |
Tuesday, March 6, 2007 2:30PM - 2:42PM |
L28.00001: Probing Non-equilibrium Phonon Dynamics in Carbon Nanotubes by Time-Resolved Raman Spectroscopy Daohua Song, Feng Wang, Tony F. Heinz In this paper we present a direct determination by femtosecond pump-probe laser spectroscopy of the lifetime of zone-center optical phonons in semiconducting single-walled carbon nanotubes. The non-equilibrium phonon population was created by the rapid relaxation following ultrafast optical excitation of the E$_{22}$ transition of a suspension of isolated HipCO nanotubes. As a probe of the phonon population, we made use of antiStokes Raman scattering from G mode. From the variation of the Raman signal with pump-probe delay, we deduced a phonon lifetime around 1 ps. The relation between the measured population lifetime and Raman linewidth will be considered, as will be the implication of this result for the existence of non-equilibrium phonon population in nanotubes carrying high current densities. [Preview Abstract] |
Tuesday, March 6, 2007 2:42PM - 2:54PM |
L28.00002: One Dimensional Exciton Diffusion on Semiconducting Nanotubes using Time Resolved Photoabsorption Spectroscopy R. M. Russo, D.E. Luzzi, E.J. Mele We extend our recently reported analysis of the population relaxation of optically excited states on semiconducting carbon nanotubes to study the spectral shifts of their photoabsorption spectra. Highly excited tubes show a long time $1/\sqrt t $ decay of their photobleaching spectra which is well described by a one dimensional diffusion limited two body population relaxation. We find that the absorption spectra also show time-dependent spectral shifts with respect to the ground state absorption spectra. The spectral shifts are of order 10 nm and history dependent: two tubes prepared from different initial excitation densities but evolving to the same instantaneous excitation density show different lineshapes and spectral shifts. These features are analyzed by a model for the distribution of exciton separations produced in a diffusing population. The model provides an excellent parameter free description of the lineshape, and gives an estimate of the experimental exciton diffusion constant. [Preview Abstract] |
Tuesday, March 6, 2007 2:54PM - 3:06PM |
L28.00003: Tracing exciton formation and relaxation in (6,5)- enriched single walled carbon nanotubes with sub-10 fs resolution Larry L\"uer, Calogero Sciascia, Christoph Gadermaier, Guglielmo Lanzani, Jared Crochet, Tobias Hertel We perform pump and probe spectroscopy on (6,5) enriched single walled carbon nanotubes using broadband visible pulses of 7 fs duration. Apart from the direct photogeneration of the E22 exciton, we find a delayed channel which is operative at higher pump intensities during the first 20 fs after photoexcitation. It results in i) a saturation of the maximum population of the E22 exciton and ii) a strong retardation of the relaxation kinetics of E22 into E11, that cannot be accounted for by considering regeneration of E22 states by annihilation of E11 states. We suggest free carrier recombination as origin of the delayed E22 formation channel. The G mode oscillation of the nanotubes is traced via coherent oscillations as function of probe wavelength. It exhibits an abrupt phase jump at the maximum of the E22 absorption band, clearly demonstrating the oscillation of the E22 transition energy exerted by the G mode vibrational distorsion. Mapping the oscillatory amplitudes against probe wavelength allows us to separate oscillations in the ground state from those in the excited state. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:42PM |
L28.00004: Time-Domain Ab Initio Studies of Phonon-Induced Relaxation of Electronic Excitations in Carbon Nanotubes Invited Speaker: Electron-phonon interactions in carbon nanotubes (CNT) determine response times of optical switches and logic gates, the extent of heating and energy loss in CNT wires and field-effect transistors, and even a mechanism for CNT superconductivity. Numerous time-resolved experiments have revealed intriguing features of the electron-phonon relaxation in CNTs in response to external stimuli. We report the ab initio studies of the relaxation performed in real-time, directly mimicking the experimental data. The results reveal a number of unexpected features of the relaxation processes, including the differences between the intraband relaxation and electron-hole recombination, the photoexcitation energy dependence of the relaxation, the importance of defects, the dependence on the excitation intensity, and a detailed role of active phonon modes. \newline \newline [1] B. F. Habenicht, C. F. Craig, O. V. Prezhdo, ``Electron and hole relaxation dynamics in a semiconducting carbon nanotube'', \textit{Phys. Rev. Lett.} \textbf{96} 187401 (2006); \textit{Virtual J. Nanoscale Sci. {\&} Tech,} May 29, 2006; \textit{Virtual J. of Ultrafast Science}, June 2006 [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L28.00005: Exciton-Polariton Dynamics in Carbon Nanotubes Igor Bondarev This work addresses theoretically the nonlinear response of phonon-coupled excitons[1] in carbon nanotubes to an external electromagnetic field. The photon Green's function approach developed recently to quantize the electromagnetic field in the presence of quasi-1D absorbing bodies[2],[3] is being used to study the dynamics of phonon-coupled excitonic states interacting with the surface photonic modes excited by the external electromagnetic field in semiconductor carbon nanotubes. The formation of the new elementary excitations, exciton-polaritons, representing the eigen states of the full photon-matter Hamiltonian has been studied for small-diameter nanotubes under strong exciton-photon coupling. Time-resolved simulations have been performed of the coherent exciton- polariton dynamics with the exciton-phonon interactions taken into account. The criteria for the coherent control of the excitonic states population in optically excited carbon nanotubes have been formulated. \newline [1]F.Plentz et al, Phys. Rev. Lett. 95, 247401 (2005). \newline [2]I.V.Bondarev and Ph.Lambin, Phys. Rev. B 72, 035451 (2005). \newline [3]I.V.Bondarev and Ph.Lambin, in: Trends in Nanotubes Reasearch (NovaScience, New York, 2006), p.139. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L28.00006: Photoinduced transient mid-infrared absorption in single-walled carbon nanotubes Yoichi Murakami, William Rice, Junichiro Kono We have performed optical pump - mid-infrared (MIR) probe spectroscopy on single-walled carbon nanotubes (SWNTs). The second excitonic absorption band ($E_{22})$ of (6,5) SWNTs was resonantly excited and the resulting photoinduced absorption was monitored in the MIR range (3.5 -- 5.5 $\mu $m) in a time range up to several hundred ps. Carrageenan films containing individualized CoMoCAT SWNTs formed on sapphire substrates were used for the measurement. This sample is optically transparent in the $\sim $3.5 -- 6 $\mu $m region, where the transition of $E_{11}$ excitons from the lowest dark state (1$g)$ to the second bright state (2$u)$ is expected to be observed. Our preliminary data shows the existence of photoinduced absorption in the investigated range. The origin of the observed transient absorption will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L28.00007: The Graphenic Bicontinuum Provides a Unified Analytical Treatment of Lattice Dynamics in Carbon Nanostructures. Cristiano Nisoli, Vincent Crespi, Eric Mockensturm A two-field bi-continuum model for the vibrational dynamics of graphene and carbon nanotubes describes a wealth of phenomena absent in a traditional continuum, including optical phonons, the high wave-vector nonlinearity of the acoustic branches, and even the hexagonal graphenic Brillouin zone. Since it includes all the degrees of freedom of the honeycomb lattice, the model provides a complete description of important electromechanical effects such as strain-induced gap opening or gap-induced phonon softening. The bi-continuum provides a unified framework for understanding and extending a previously disparate accumulation of analytical and computational results on deformations and vibrations in carbon nanostructures. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L28.00008: ARPES study of the electronic dynamics from graphene to graphite Shuyun Zhou, Gey-Hong Gweon, Jeff Graf, David Siegel, Elizabeth Rollings, Alessandra Lanzara We report unique electronic information about the low energy electronic dynamics of atomically-thin graphene and bulk graphite by using high-resolution angle-resolved photoemission spectroscopy (ARPES). I will discuss the evolution of the electronic structure from single layer graphene to graphite and the dynamic of the Dirac quasiparticles as a function of energy, momentum, temperature and sample thickness.~ I will also discuss some very interesting features near the Fermi energy E$_{F}$ and address the effects of disorder on the low energy excitations. These findings from the electronic side can provide insight on the intriguing physics in graphene and graphite, as well as other carbon-based materials. References: [1] S. Y. Zhou \textit{et al.} Nature Phys. \textbf{2}, 595 (2006) [2] S. Y. Zhou \textit{et al.} Annals of Physics \textbf{321}, 1730 (2006) [3] E. Rollings, G.-H. Gweon, S.Y. Zhou \textit{et al}. J. Phys. Chem. Solids \textbf{67}, 2172 (2006) [4] S. Y. Zhou \textit{et al.} Phys. Rev. B \textbf{71}, 161403(R) (2005) [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L28.00009: Electron-phonon renormalization effects in the ARPES spectra of doped graphene: a first principles approach Francesco Mauri, Matteo Calandra Recent experimental investigations of the hole excitations in graphene [1], bilayer graphene [2] and graphite [3] by angular resolved photoemission indicated the occurrence of kink structures in the band dispersions and in the lifetime of hole excitations. These kinks have been attributed to electron-phonon coupling effects. In this work we calculate the effect of electron-phonon scattering on the angular resolved photoemission spectra (ARPES) of graphene as a function of doping. We use electron-phonon coupling parameters derived from density functional theory calculations. We compare our results for the quasiparticle dispersion and for the lifetime of the electrons and holes with those obtained from ARPES.\\ ${\rm [1]}$ A. Bostwick {\it et al.}, cond-mat/0609660 \\ ${\rm [2]}$ T. Ohta {\it et al.}, Science {\bf 313}, 951 (2006) \\ ${\rm [3]}$ S. Zhou {\it et al.} cond-mat/0609028 [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L28.00010: Anisotropic electron-phonon coupling in doped graphene Jessica McChesney, Aaron Bostwick, Taisuke Ohta, Thomas Seyller, Karsten Horn, Eli Rotenber The effects of doping single layer graphene are investigated by mapping the valence band in the vicinity of EF using angle-resolved photoemission spectroscopy (ARPES). The carrier concentration was varied from 0.04 -- 1.05 electrons per unit cell with the deposition of Ca and K at low temperatures. As the doping increases there is an enhancement of the electron-phonon coupling along certain high symmetry directions. Changes in electron-phonon coupling parameter, lambda, shows that the systems goes through a transition from the weak-coupling regime to the strong-coupling regime. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L28.00011: Thermo-Plasma Polariton within Scaling Theory of Single-Layer Graphene Oskar Vafek Electrodynamics of single-layer graphene is studied in the scaling regime. At any finite temperature, there is a weakly damped collective thermo-plasma polariton mode whose dispersion and wavelength dependent damping is determined analytically. The electric and magnetic fields associated with this mode decay exponentially in the direction perpendicular to the graphene layer, but unlike the surface plasma polariton modes of metals, the decay length and the mode frequency are strongly temperature dependent. This may lead to new ways of generation and manipulation of these modes. [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L28.00012: Theory of resonant multiphonon Raman scattering in graphene monolayers Denis Basko, Igor Aleiner The Raman spectrum of graphene consists of distinct narrow peaks corresponding to different optical phonon branches as well as their overtones [1]. We show how the relative intensities of the overtone peaks encode information about relative strengths of different inelastic scattering processes electrons are subject to. In particular, assuming that the most important processes are electron-phonon and electron-electron scattering, it is shown that one can deduce their relative interaction strengths from the Raman spectra. [1] A. C. Ferrari et al., Phys. Rev. Lett. 97, 187401 (2006); A. Gupta et al., cond-mat/0606593; D. Graf et al., cond-mat/0607562. [Preview Abstract] |
Tuesday, March 6, 2007 5:18PM - 5:30PM |
L28.00013: Extracting optical properties of individual or few layers of graphite oxide sheets on surfaces by developing simple optical approaches Inhwa Jung, Richard Piner, Dimitry Dikin, Sasha Stankovich, Supinda Watcharotone, Rodney Ruoff An optical method for extracting optical properties of individual or few layers of graphite oxide sheets is presented. The substrate consists of a dielectric layer of controlled thickness on semiconducting silicon. The intensity ratio between reflected light from the material and the substrate can be optimized through choice of the optical properties and the thickness of the dielectric layer; analysis of the reflection of an incident light beam demonstrates this, and confocal microscopy images obtained on different thickness dielectric layers verifies the analysis. By comparing the measured and predicted intensity ratios of single layers of graphite oxide the optical properties before and after thermal treatment were obtained. The use of a designed substrate in terms of the thickness and optical properties of a dielectric layer on silicon, could find use for optically characterizing exceptionally thin platelets and also thin biological materials which might otherwise not be discerned through ``standard'' optical microscopy. [Preview Abstract] |
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