Ultrafast Mid-Infrared Intra-Excitonic Response of Individualized Single-Walled Carbon Nanotubes*

We report a femtosecond mid-infrared study of the broadband low-energy response of individualized (6,5) and (7,5) single-walled carbon nanotubes. Strong photoinduced absorption is observed around 200 meV, whose transition energy, oscillator strength, chirality enhancement and dynamics manifest the observation of quasi-1D intra-excitonic transitions. A model of the nanotube 1$s$-2$p$ intra-excitonic cross section yields excellent agreement. Our study further reveals a saturation of the photoinduced absorption with increasing phase-space filling of the correlated $e-h$ pairs.