  Excitonic spectra of armchair graphene nanoribbons (AGNRs) obtained from a
full many-body exact diagonalization of the Hubbard model are reported for both
longitudinally and transversely polarized photons, thus providing a complete
survey of low-energy may-body optical excitations in these systems. The
resulting one-photon allowed eigenstates turn out to be well separated in
energy from each other but both couple to the same set of two-photon allowed
states. The magnitude of the calculated optical oscillator strengths for
perpendicular polarization suggest that these optical features can be indeed
observed in polarized absorption measurements.
