  Optical-absorption cross-sections and energy-loss spectra of aligned
multishell carbon nanotubes are investigated, on the basis of photonic
band-structure calculations. A local graphite-like dielectric tensor is
assigned to every point of the tubules, and the effective transverse dielectric
function of the composite is computed by solving Maxwell's equations in media
with tensor-like dielectric functions. A Maxwell-Garnett-like approach
appropriate to the case of infinitely long anisotropic tubules is also
developed. Our full calculations indicate that the experimentally measured
macroscopic dielectric function of carbon nanotube materials is the result of a
strong electromagnetic coupling between the tubes. An analysis of the
electric-field pattern associated with this coupling is presented, showing that
in the close-packed regime the incident radiation excites a very localized
tangential surface plasmon.
