Masaru Tachibana, 2013, proposed the "characterization of laser-induced defects and modification in carbon nanotubes by Raman spectroscopy" (open access, distributed under the terms of the Creative Commons Attribution 3.0 License). "The G band in SWCNTs is a more complex spectral feature. Due to the folding of graphene sheet into the SWCNT and the symmetry breaking effects associated with the nanotube curvature, G band splits into G + and G -, which are related to atomic vibrations preferentially along (LO) and perpendicular (TO) to the tube (folding) axis, respectively, for semiconducting SWCNT" . "Perpendicular" TO means tangential optical mode. "For metallic tubes, electron-phonon coupling softens the LO modes, so that G + and G -are actually with TO and LO modes, respectively. The G -peak for metallic tubes is fitted by asymmetric and broad BWF. … Actually, due to the symmetry breaking effects associated with the nanotube curvature, G band in SWCNT generates up to six Raman-allowed G-band peaks corresponding to two totally symmetric A1 modes, two E1 modes and two E2 symmetry modes. Three of each exhibit LO or TO-like vibration. Due to the delocalization effect and special resonance conditions, the A1 modes usually dominate the G-band spectra" .