  Light emission from nanostructures exhibits rich quantum effects and has
broad applications. Single-walled carbon nanotubes (SWNTs) are one-dimensional
(1D) metals or semiconductors, in which large number of electronic states in a
narrow range of energies, known as van Hove singularities, can lead to strong
spectral transitions. Photoluminescence and electroluminescence involving
interband transitions and excitons have been observed in semiconducting SWNTs,
but are not expected in metallic tubes due to non-radiative relaxations. Here,
we show that in the negative differential conductance regime, a suspended
quasi-metallic SWNT (QM-SWNT) emits light due to joule-heating, displaying
strong peaks in the visible and infrared corresponding to interband
transitions. This is a result of thermal light emission in 1D, in stark
contrast with featureless blackbody-like emission observed in large bundles of
SWNTs or multi-walled nanotubes. This allows for probing of the electronic
temperature and non-equilibrium hot optical phonons in joule-heated QM-SWNTs.
