Fig. 1.

Illustration of optical transitions and tube structures studied here. (a) Schematics of optical transitions in SWNTs corresponding to collinear, parallel-polarized (red and green color) and perpendicular, cross-polarized (blue color) excitations as illustrated by directions of the respective transition dipole moments μ_ij. (b) Molecular structures of carbon nanotubes considered in this study. Optimal geometries are presented in the perspective depth view for visual comparison of tube diameters and chiralities, labeled according to the tube index (n and m). All tubes are finite and chosen to have approximately the same length of ≈9–12 nm. Dangling bonds are terminated with hydrogen atoms.