A. G. Walsh

Publications (6)14.1 Total impact

• Source

  Available from: Anna K Swan

  Article: Electron correlation effects on the femtosecond dephasing dynamics of E22 excitons in (6,5) carbon nanotubes.

  J R Schneck, A G Walsh, A A Green, M C Hersam, L D Ziegler, A K Swan
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  ABSTRACT: Highly nonlinear pump fluence dependence was observed in the ultrafast one-color pump-probe responses excited by 38 fs pulses resonant with the E(22) transition in a room-temperature solution of (6,5) carbon nanotubes. The differential probe transmission (ΔT/T) at the peak of the pump-probe response (τ = 20 fs) was measured for pump fluences from ∼10(13) to 10(17) photons/pulse cm(2). The onset of saturation is observed at ∼2 × 10(15) photons/pulse cm(2) (∼8 × 10(5) excitons/cm). At pump fluences >4 × 10(16) photons/pulse cm(2) (∼1.6 × 10(6) excitons/cm), ΔT/T decreases as the pump fluence increases. Analogous signal saturation behavior was observed for all measured probe delays. Despite the high exciton density at saturation, no change in the E(22) population decay rate was observed at short times (<300 fs). The pump probe signal was modeled by a third-order perturbation theory treatment that includes the effects of inhomogeneous broadening. The observed ΔT/T signal is well-fit by a pump-fluence-dependent dephasing rate linearly dependent on the number of excitons created by the pump pulse. Therefore, the observed nonlinear pump intensity dependence is attributed to the effects of quasi-elastic exciton-exciton interactions on the dephasing rates of single carbon nanotubes. The low fluence total dephasing time is 36 fs, corresponding to a homogeneous width of 36 meV (290 cm(-1)), and the derived E(22) inhomogeneous width is 68 meV (545 cm(-1)). These results are contrasted with photon-echo-derived parameters for the E(11) transition.
  The Journal of Physical Chemistry A 01/2011; 115(16):3917-23. · 2.95 Impact Factor
• Article: Optical determination of electron-phonon coupling in carbon nanotubes.

  Y Yin, A N Vamivakas, A G Walsh, S B Cronin, M S Unlü, B B Goldberg, A K Swan
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  ABSTRACT: We report on an optical method to directly measure electron-phonon coupling in carbon nanotubes by correlating the first and second harmonic of the resonant Raman excitation profile. The method is applicable to 1D and 0D systems and is not limited to materials that exhibit photoluminescence. Experimental results for electron-phonon coupling with the radial breathing mode in 5 different nanotubes show coupling strengths from 3-11 meV. The results are in good agreement with the chirality and diameter dependence of the e-ph coupling calculated by Goupalov et al.
  Physical Review Letters 02/2007; 98(3):037404. · 7.37 Impact Factor
• Article: Tunable Resonant Raman Scattering From Singly Resonant Single Wall Carbon Nanotubes

  Y. Yin, A. G. Walsh, A. N. Vamivakas, S.B. Cronin, A. (Sasha) M. Stolyarov, M. Tinkham, W. Bacsa, M. S. Nl, B.B. Goldberg, A K Swan
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  ABSTRACT: We perform tunable resonant Raman scattering on 17 semiconducting and seven metallic singly resonant single wall carbon nanotubes. The measured scattering cross section as a function of laser energy provides information about a tube's electronic structure, the lifetime of intermediate states involved in the scattering process, and also the energies of zone center optical phonons. Recording the scattered Raman signal as a function of tube location in the microscope focal plane allows us to construct two-dimensional spatial maps of singly resonant tubes. We also describe a spectral nano-scale artifact, which we have termed as the “nano-slit effect.”
  IEEE Journal of Selected Topics in Quantum Electronics 12/2006; 12(6):1083-1090. · 3.78 Impact Factor
• Source

  Available from: Nick Vamivakas

  Article: Chirality dependence of the radial breathing phonon mode density in single wall carbon nanotubes

  A. N. Vamivakas, Y. Yin, A. G. Walsh, M.S. Unlu, B.B. Goldberg, A K Swan
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  ABSTRACT: A mass and spring model is used to calculate the phonon mode dispersion for single wall carbon nanotubes (SWNTs) of arbitrary chirality. The calculated dispersions are used to determine the chirality dependence of the radial breathing phonon mode (RBM) density. Van Hove singularities, usually discussed in the context of the single particle electronic excitation spectrum, are found in the RBM density of states with distinct qualitative differences for zig zag, armchair and chiral SWNTs. The influence the phonon mode density has on the two phonon resonant Raman scattering cross-section is discussed.
  10/2006;
• Source

  Available from: Wolfgang Bacsa

  Article: Tunable Resonant Raman Scattering from Singly Resonant Single Wall Carbon Nanotubes

  Y. Yin, A. G. Walsh, A. N. Vamivakas, S. Cronin, A. M. Stolyarov, M. Tinkham, W. Bacsa, M.S. Unlu, B.B. Goldberg, A K Swan
  [show abstract] [hide abstract]
  ABSTRACT: We perform tunable resonant Raman scattering on 17 semiconducting and 7 metallic singly resonant single wall carbon nanotubes. The measured scattering cross-section as a function laser energy provides information about a tube's electronic structure, the lifetime of intermediate states involved in the scattering process and also energies of zone center optical phonons. Recording the scattered Raman signal as a function of tube location in the microscope focal plane allows us to construct two-dimensional spatial maps of singly resonant tubes. We also describe a spectral nanoscale artifact we have coined the "nano-slit effect".
  07/2006;
• Article: Electron-phonon coupling of G mode and assignment of a combination mode in carbon nanotubes

  Y. Yin (尹 彦, A. G. Walsh, A. N. Vamivakas, S. B. Cronin, D. E. Prober, B. B Goldberg
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  ABSTRACT: We measure the electron-phonon (e-ph) coupling matrix elements of the G band in semiconducting carbon nanotubes (CNTs) in comparison to the radial breathing mode (RBM). The experimental result for the G+ mode [Γ-point longitudinal optical (LO) phonon] at the E22 transition suggests an e-ph coupling strength of approximately 30 meV or 12.5 eV/Å, which is within the expected range of 23 meV to 83 meV for a 1-nm-diameter CNT. This coupling (as the square of the matrix elements) for the G+ mode is an order of magnitude higher than that of the RBM and other measured phonon modes. In addition we assign a Raman feature observed around 1700 cm−1 to be the combination mode of a low energy (∼400 cm−1) and a high energy (∼1300 cm−1) zone-boundary phonon. Both the phonon energies and phonon dispersions of those modes support the assignment.
  Phys. Rev. B. 84(7).