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