  Single air-suspended carbon nanotubes (length 2 - 5 microns) exhibit high
optical quantum efficiency (7 - 20%) for resonant pumping at low intensities.
Under ultrafast excitation, the photoluminescence dramatically saturates for
very low injected exciton numbers (2 to 6 excitons per pulse per SWCNT). This
PL clamping is attributed to highly efficient exciton-exciton annihilation over
micron length scales. Stochastic modeling of exciton dynamics and femtosecond
excitation correlation spectroscopy allow determination of nanotube absorption
(2 - 6%) and exciton lifetime (85 +- 20 ps).
