Optical Properties of Single-Wall Carbon Nanotubes

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Raman scattering spectra of single-wall nanotubes with mean radii 0.55,0.65 and 1.0 nm show size-dependent multiple splittings of the optical phonon peak corresponding to E_<2g> mode in graphite. These splittings constitute the first experimental evidence for the unique feature of nanotubes that they exhibit discrete and diameter-dependent dispersions arising from their cylindrical symmetry. The observed dispersion is well explained on the basis of graphite, and shows possibilities of predicting and controlling the basic property of nanotubes in the zone-folding scheme.Resonant Raman scattering from optical phonons in single-wall nanotubes of mean diamater 1.1 nm has been measured in the wavelength of incident laser beam between 450 nm and 800 nm. The scattering intensities exhibit anomalies in the vicinity of 690 nm, being a minimum in the longitudinal components and a maximum in the transverse. Our optical transmission measurement also exhibits a dip in the same photon energy range. The anomalies are caused by the incident laser beam resonant with the zone-folded electronic structures in nanotubes. These results constitute a direct experimental evidence of the critical point in the density of ststes produced by the cylindrical symmetry of nanotubes.