Highly nonlinear pump fluence dependence was observed in the ultrafast one-color pump - probe responses excited by 38 fs pulses resonant with the E22 transition in a room-temperature solution of (6,5) carbon nanotubes. The differential probe transmission (Î"T/T) at the peak of the pump - probe response (π = 20 fs) was measured for pump fluences from ̃1013 to 1017 photons/pulse cm2. The onset of saturation is observed at ̃2 - 1015 photons/pulse cm 2 (̃8 - 105 excitons/cm). At pump fluences >4 - 1016 photons/pulse cm2 (̃1.6 - 106 excitons/cm), Î"T/T decreases as the pump fluence increases. Analogous signal saturation behavior was observed for all measured probe delays. Despite the high exciton density at saturation, no change in the E22 population decay rate was observed at short times (<300 fs). The pump probe signal was modeled by a third-order perturbation theory treatment that includes the effects of inhomogeneous broadening. The observed Î"T/T signal is well-fit by a pump-fluence-dependent dephasing rate linearly dependent on the number of excitons created by the pump pulse. Therefore, the observed nonlinear pump intensity dependence is attributed to the effects of quasi-elastic exciton - exciton interactions on the dephasing rates of single carbon nanotubes. The low fluence total dephasing time is 36 fs, corresponding to a homogeneous width of 36 meV (290 cm- 1), and the derived E22 inhomogeneous width is 68 meV (545 cm- 1). These results are contrasted with photon-echo-derived parameters for the E11 transition.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry