Saturation of the Photoluminescence at Few-Exciton Levels in a Single-Walled Carbon Nanotube under Ultrafast Excitation

Single air-suspended carbon nanotubes (length 2-5 mu m) exhibit high optical quantum efficiency (7-20%) for low intensity resonant pumping. Under ultrafast excitation (150 fs), emission dramatically saturates at very low exciton numbers (2-6), which is attributed to highly efficient exciton-exciton annihilation over micron-length scales. Similar saturation behavior for 4 ps pulse excitation shows nonlinear absorption is not a contributing factor. The absorption cross sections (0.6-1.8 X 10(-17) cm(2)/atom) are determined by fitting to a stochastic model for exciton dynamics.