  Coherent phonons in single wall carbon nanotubes (SWNTs) are observed as
oscillations of the differential absorption coefficient as a function of time
by means of pump-probe spectroscopy. For the radial breathing mode (RBM) of a
SWNT, the coherent phonon signal is understood to be a result of the modulated
diameter-dependent energy gaps due to the coherent RBM phonon oscillations.
However, this mechanism might not be the dominant contribution to other phonon
modes in the SWNT. In particular, for the G band phonons, which correspond to
bond-stretching motions, we find that the modulation of the interatomic dipole
matrix element gives rise to a strong coherent G band phonon intensity
comparable to the coherent RBM phonon intensity. We also further discuss the
dependence of coherent G band and RBM phonon amplitudes on the laser excitation
pulse width.
