  We present results for the optical absorption spectra of small-diameter
single-wall carbon and boron nitride nanotubes obtained by {\it ab initio}
calculations in the framework of time-dependent density functional theory. We
compare the results with those obtained for the corresponding layered
structures, i.e. the graphene and hexagonal BN sheets. In particular, we focus
on the role of depolarization effects, anisotropies and interactions in the
excited states. We show that already the random phase approximation reproduces
well the main features of the spectra when crystal local field effects are
correctly included, and discuss to which extent the calculations can be further
simplified by extrapolating results obtained for the layered systems to results
expected for the tubes. The present results are relevant for the interpretation
of data obtained by recent experimental tools for nanotube characterization
such as optical and fluorescence spectroscopies as well as polarized resonant
Raman scattering spectroscopy. We also address electron energy loss spectra in
the small-q momentum transfer limit. In this case, the interlayer and intertube
interactions play an enhanced role with respect to optical spectroscopy.

Materials Science