The effect of light polarization on the interband transition spectra of zigzag carbon nanotubes and its diameter dependence

Abstract

Although the optical absorption spectra of carbon nanotubes with perpendicularly polarized light are known to be suppressed due to the depolarization effect, peaks with significant magnitudes have been reported in photoluminescence absorption spectra of single-walled nanotubes under perpendicular polarization. We study the effect of light polarization on the interband optical transition spectra of nanotubes, independently from the depolarization effect. We employ the density functional theory to calculate the absorption spectra of periodic zigzag carbon nanotubes for parallel and perpendicular polarization of light in a wide, infrared-visible-ultraviolet range (0–11eV). We have chosen (4,0), (8,0), and (16,0) nanotubes with diameters of ~0.34, ~0.63, and ~1.27nm, respectively, to also investigate the dependence of the polarization effect on nanotube diameter. Although the overall spectrum for perpendicular polarization is sparse compared to the one for parallel polarization, the transition probability can be quite significant at certain photon energies for perpendicular polarization. For an (8,0) nanotube, for example, we observe sharp peaks at 1.2eV, 1.8eV and 10.6eV. As the nanotube diameter decreases, parallel polarization seems to become progressively more favorable for absorption than perpendicular polarization in the infrared/visible range. Surprisingly, in the ultraviolet region, this trend is reversed; for nanotubes with small diameter, we notice a higher probability of absorption for ultraviolet light with perpendicular polarization compared to the one with parallel polarization. This can be important in optoelectronic applications of carbon nanotubes for ultraviolet absorption and emission.