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We present results for the optical absorption spectra of small-diametersingle-wall carbon and boron nitride nanotubes obtained by {\it ab initio}calculations in the framework of time-dependent density functional theory. Wecompare the results with those obtained for the corresponding layeredstructures, i.e. the graphene and hexagonal BN sheets. In particular, we focuson the role of depolarization effects, anisotropies and interactions in theexcited states. We show that already the random phase approximation reproduceswell the main features of the spectra when crystal local field effects arecorrectly included, and discuss to which extent the calculations can be furthersimplified by extrapolating results obtained for the layered systems to resultsexpected for the tubes. The present results are relevant for the interpretationof data obtained by recent experimental tools for nanotube characterizationsuch as optical and fluorescence spectroscopies as well as polarized resonantRaman scattering spectroscopy. We also address electron energy loss spectra inthe small-q momentum transfer limit. In this case, the interlayer and intertubeinteractions play an enhanced role with respect to optical spectroscopy.Comment: 13 pages, 9 embedded eps figures. Submitted to Applied Physics

Optical absorption and electron energy loss spectra of carbon and boron nitride nanotubes: a first-principles approach