Browsing by Author "Kim, J.-H."

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    Dephasing of G-band phonons in single-wall carbon nanotubes probed via impulsive stimulated Raman scattering
    (American Physical Society, 2012) Kim, J.-H.; Yee, K.-J.; Lim, Y.-S.; Booshehri, L.G.; Haroz, E.H.; Kono, J.
    We study the coherent dynamics of G-band phonons in single-wall carbon nanotubes through impulsive stimulated Stokes and anti-Stokes Raman scattering. The probe energy dependence of the phonon amplitude as well as the preferential occurrence between Stokes and anti-Stokes components in response to chirpedpulse excitation are well explained within our model. The temperature dependence of the observed dephasing rate clearly exhibits a thermally activated component, with an activation energy that coincides with the frequency of the radial breathing mode (RBM). This fact provides a clear picture for the dephasing of G-band phonons by random frequency modulation via interaction with the RBM through anharmonicity.
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    Renormalized energies of superfluorescent bursts from an electron-hole magnetoplasma with high gain in InxGa1−xAs quantum wells
    (American Physical Society, 2013) Kim, J.-H.; Lee, J.; Noe, G.T.; Wang, Y.; Wojcik, A.K.; McGill, S.A.; Reitze, D.H.; Belyanin, A.A.; Kono, J.
    We study light emission properties of a population-inverted 2D electron-hole plasma in a quantizing magnetic field. We observe a series of superfluorescent (SF) bursts, discrete both in time and energy, corresponding to the cooperative recombination of electron-hole pairs from different Landau levels. Emission energies exhibit strong renormalization due to many-body interactions among the photogenerated carriers, showing pronounced red shifts as large as 20 meV at 15 T. However, the lowest Landau level emission line remains stable against renormalization and show excitonic magnetic field dependence. Interestingly, our time-resolved measurements show that this lowest-energy SF burst occurs only after most upper states become empty, suggesting that this excitonic stability is related to the “hidden symmetry” of 2D magnetoexcitons expected in the magnetic quantum Limit.
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    Theory of coherent phonons in carbon nanotubes and graphene nanoribbons
    (IOP Publishing, 2013) Sanders, G.D.; Nugraha, A.R.T.; Sato, K.; Kim, J.-H.; Kono, J.; Saito, R.; Stanton, C.J.; Richard E. Smalley Institute for Nanoscale Science and Technology
    We survey our recent theoretical studies on the generation and detection of coherent radial breathing mode (RBM) phonons in single-walled carbon nanotubes and coherent radial breathing like mode (RBLM) phonons in graphene nanoribbons. We present a microscopic theory for the electronic states, phonon modes, optical matrix elements and electronﾖphonon interaction matrix elements that allows us to calculate the coherent phonon spectrum. An extended tight-binding (ETB) model has been used for the electronic structure and a valence force field (VFF) model has been used for the phonon modes. The coherent phonon amplitudes satisfy a driven oscillator equation with the driving term depending on the photoexcited carrier density. We discuss the dependence of the coherent phonon spectrum on the nanotube chirality and type, and also on the graphene nanoribbon mod number and class (armchair versus zigzag). We compare these results with a simpler effective mass theory where reasonable agreement with the main features of the coherent phonon spectrum is found. In particular, the effective mass theory helps us to understand the initial phase of the coherent phonon oscillations for a given nanotube chirality and type. We compare these results to two different experiments for nanotubes: (i) micelle suspended tubes and (ii) aligned nanotube films. In the case of graphene nanoribbons, there are no experimental observations to date. We also discuss, based on the evaluation of the electronﾖphonon interaction matrix elements, the initial phase of the coherent phonon amplitude and its dependence on the chirality and type. Finally, we discuss previously unpublished results for coherent phonon amplitudes in zigzag nanoribbons obtained using an effective mass theory.