  We have used a femtosecond pump-probe impulsive Raman technique to explore
the polarization dependence of coherent optical phonons in highly-purified and
aligned semiconducting single-wall carbon nanotubes (SWCNTs). Coherent phonon
spectra for the radial breathing modes (RBMs) exhibit a different monochromatic
frequency between the film and solution samples, indicating the presence of
differing exciton excitation processes. By varying the polarization of the
incident pump beam on the aligned SWCNT film, we found that the anisotropy of
the coherent RBM excitation depends on the laser wavelength, which we consider
to be associated with the resonant and off-resonant behavior of RBM excitation.
