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 Ei 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-p-band plasmons. They are not useful in determining the distribution of diameter and chiral angle. The inter-p-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.