  The bright exciton emission of semiconducting carbon nanotubes is appealing
for optoelectronic and quantum photonic devices as well as fundamental studies
of light-matter interaction in one-dimensional nanostructures. The photophysics
of excitons in carbon nanotubes till date is however largely affected by
extrinsic effects of the dielectric environment. Here we carried out
time-resolved photoluminescence measurements over 14 orders of magnitude for
ultra-clean carbon nanotubes bridging an airgap over pillar posts. Our
measurements demonstrate a new regime of intrinsic exciton photophysics with
prolonged spontaneous emission times up to T1=18 ns, between one to two orders
of magnitude better than prior measurements and in agreement with values first
hypothesized by theorists about a decade ago. Further we establish for the
first time the exciton decoherence times of individual nanotubes in the
time-domain and found four-fold prolonged values up to T2=2.1 ps compared to
ensemble measurements. These first observations motivate new discussions about
the magnitude of the intrinsic dephasing mechanism while the prolonged exciton
dynamics is promising for device applications.
