The Effect of Intraband Transitions on the Optical Spectra of Metallic Carbon Nanotubes

First-principles calculations of the electronic structure and linear optical properties of metallic carbon nanotubes are carried out using the full-potential linear-augmented plane-wave method. The effect of intraband transition and electric-field polarization on the optical spectra of (5,2) chiral, (15,0) zigzag, and (8,8) armchair metallic carbon nanotubes are investigated. The optical spectra are calculated for both electric-field polarizations, parallel and perpendicular to the axis of the tube. It is revealed that the optical spectra are anisotropic along these two polarizations. For the parallel polarization to the tube axis, adding the intraband transition contributions shows that the dielectric function has singularity at zero frequency due to the screening effect in metallic carbon nanotubes.