High-frequency coherent phonons resonantly excited in metallic single-walled carbon nanotubes (M-SWCNTs) were investigated via spectrally resolved pump-probe spectroscopy using 7.5-fs laser pulses. In addition to first-order coherent phonons such as radial breathing mode (RBM) and M and G modes, we clearly observed second-order high-frequency coherent phonons of 2D and 2G modes, which can be regarded as squeezed phonons. We found that the amplitudes of the RBM, G and 2D modes were resonantly enhanced at specific wavelengths: the maximum resides at a wavelength whose energy is smaller than that of the van Hove singularities in M-SWCNTs by an amount corresponding to the phonon energy. Furthermore, the 2D mode has stronger enhancement than the other first-order Raman modes. These results indicate that the enhancement originates from a Stokes-stimulated Raman-scattering process at van Hove singularities and that efficient resonance enhancement occurs for the 2D mode, possibly through double resonance due to the trigonal warping effect and strong electron-phonon coupling due to the Kohn anomaly.

Abstract. Using wavelength-resolved pump-probe spectroscopy with a sub-10-fs laser, we investigated resonant enhancement of radial breathing mode and G-mode coherent phonons in carbon nanotubes (CNTs), and successfully distinguished the electronic states of CNTs with different chiralities.

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