Browsing by Author "Ren, L."

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    Circular polarization dependent cyclotron resonance in large-area graphene in ultrahigh magnetic fields
    (American Physical Society, 2012) Booshehri, L.G.; Mielke, C.H.; Rickel, D.G.; Crooker, S.A.; Zhang, Q.; Ren, L.; Haroz, E.H.; Rustagi, A.; Stanton, C.J.; Jin, Z.; Sun, Z.; Yan, Z.; Tour, J.M.; Kono, J.
    Using ultrahigh magnetic fields up to 170 T and polarized midinfrared radiation with tunable wavelengths from 9.22 to 10.67 μm, we studied cyclotron resonance in large-area graphene grown by chemical vapor deposition. Circular polarization dependent studies reveal strong p-type doping for as-grown graphene, and the dependence of the cyclotron resonance on radiation wavelength allows for a determination of the Fermi energy. Thermal annealing shifts the Fermi energy to near the Dirac point, resulting in the simultaneous appearance of hole and electron cyclotron resonance in the magnetic quantum limit, even though the sample is still p-type, due to graphene's linear dispersion and unique Landau level structure. These high-field studies therefore allow for a clear identification of cyclotron resonance features in large-area, low-mobility graphene samples.
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    Collective antenna effects in the terahertz and infrared response of highly aligned carbon nanotube arrays
    (The American Physical Society, 2013) Ren, L.; Zhang, Q.; Pint, C.L.; Wójcik, A.K.; Bunney, M. Jr; Arikawa, T.; Kawayama, I.; Tonouchi, M.; Hauge, R.H.; Belyanin, A.A.; Kono, J.
    We study macroscopically aligned single-wall carbon nanotube arrays with uniform lengths via polarization-dependent terahertz and infrared transmission spectroscopy. Polarization anisotropy is extreme at frequencies less than ∼100 cm−1 with no sign of attenuation when the polarization is perpendicular to the alignment direction. The attenuation for both parallel and perpendicular polarizations increases with increasing frequency, exhibiting a pronounced and broad peak around 450 cm−1 in the parallel case. We model the electromagnetic response of the sample by taking into account both radiative scattering and absorption losses. We show that our sample acts as an effective antenna due to the high degree of alignment, exhibiting much larger radiative scattering than absorption in the mid/far-infrared range. Our calculated attenuation spectrum clearly shows a non-Drude peak at ∼450 cm−1 in agreement with the experiment.