  We theoretically investigate intersubband plasmon excitations in doped single
wall carbon nanotubes (SWNTs) by examining the dependence of plasmon frequency
on the nanotube diameter, chirality, and Fermi energy. The intersubband
plasmons can be excited by light with polarization perpendicular to the
nanotube axis and thus the plasmon excitations corresponds to optical
transitions between the two different subbands, which are sensitive to the
Fermi energy. In every SWNT, this mechanism leads to the emergence of the
optical absorption peak at the plasmon frequency for a given Fermi energy,
$E_F$. The plasmon frequencies calculated for many SWNTs with diameter $d_t <
2$ nm exhibit a dependence on $(1/d_t)^{0.7}$ and the frequencies are further
affected by Fermi energy as $E_F^{0.25}$. With this knowledge, it is possible
to develop a map of intersubband plasmon excitations in doped SWNTs that could
be useful to quickly estimate the doping level and also be an alternative way
to characterize nanotube chirality.
