  Current methods for producing single-walled carbon nanotubes (SWNTs) lead to
heterogeneous samples containing mixtures of metallic and semiconducting
species with a variety of lengths and defects. Optical detection at the single
nanotube level should thus offer the possibility to examine these
heterogeneities provided that both SWNT species are equally well detected.
Here, we used photothermal heterodyne detection to record absorption images and
spectra of individual SWNTs. Because this photothermal method relies only on
light absorption, it readily detects metallic nanotubes as well as the emissive
semiconducting species. The first and second optical transitions in individual
semicontucting nanotubes have been probed. Comparison between the emission and
absorption spectra of the lowest-lying optical transition reveal mainly small
Stokes shifts. Side bands in the near-infrared absorption spectra are observed
and assigned to exciton-phonon bound states. No such sidebands are detected
around the lowest transition of metallic nanotubes.
