Transient grating measurements of excitonic dynamics in single-walled carbon nanotubes: The dark excitonic bottleneck.

Transient grating measurements affirm the excitonic model for single-walled carbon nanotubes (SWNT) by identifying the dark exciton (D) as the population relaxation bottleneck in semiconducting-SWNT (S-SWNT). The data allow the reconstruction of the kinetics of excitonic cascade and cooling, from band continuum to vibrational cooling in the ground electronic state. In S-SWNT, the intraband relaxation occurs in 40 fs, localization into the 2g exciton occurs in 50 fs, followed by the excitonic cascade: 2g --> 1u --> D --> 1g with time constants of 175 fs, 3 ps, 300 ps, respectively. Fluorescence from the 1u state is quenched by efficient population transfer to 1D dark exciton. In metallic tubes, cooling is completed on the time scale of 1 ps.