  A single-wall carbon nanotube possesses two different types of plasmons
specified by the wavenumbers in the azimuthal and axial directions. The
azimuthal plasmon that is caused by interband transitions has been studied,
while the effect of charge doping is unknown. In this paper, we show that when
nanotubes are heavily doped, intraband transitions cause the azimuthal plasmons
to appear as a plasmon resonance in the near-infrared region of the absorption
spectra, which is absent for light doping due to the screening effect. The
axial plasmons that are inherent in the cylindrical waveguide structures of
nanotubes, account for the absorption peak of the metallic nanotube observed in
the terahertz region. The excitation of axial (azimuthal) plasmons requires a
linearly polarized light parallel (perpendicular) to the tube's axis.
