  We show that optical second-harmonic generation (SHG) can be observed from
individual single-walled carbon nanotubes (SWCNTs) and, furthermore, allows
imaging of individual tubes. Detailed analysis of our results suggests that the
structural noncentrosymmetry, as required for SHG, arises from the non-zero
chiral angle of the SWCNT. SHG thus has potential as a fast, non-destructive,
and simple method for imaging of individual nanomolecules and for probing their
chiral properties. Even more, it opens the possibility to optically determine
the handedness of individual SWCNTs.
