Optical spectra of single-wall carbon nanotube bundles

The low-frequency optical properties of single-wall carbon nanotube bundles are studied within the gradient approximation. The nanotube geometry (diameter and chiral angle) and the polarization direction strongly affect the optical absorption function, the dielectric function, the loss function, and the reflectance. The low-frequency absorption spectra in the ${\mathbf{E}}_{\parallel }$ case clearly exhibit three absorption bands. The semiconducting (metallic) nanotubes induce the first and the second absorption bands (the third absorption band). Subpeaks, which come from the single-particle excitations of the band-edge states, exist in each absorption band. They could be used to determine the preferred nanotube geometry in a single-walled nanotube bundle. Similar results are obtained for the dielectric function and the reflectance spectra. However, the loss spectra could exhibit the prominent peaks due to the inter-π-band plasmons. They are not useful in determining the distribution of diameter and chiral angle. The inter-π-band plasmons also lead to clear plasmon edges in the reflectance spectra. The calculated results could essentially explain the measured absorption spectra and loss spectra.