We report on the dynamics of the dielectric function of single-wall carbon nanotubes from 10 to 30 THz after ultrafast excitation. The absence of a distinct free-carrier response is attributed to the photogeneration of strongly bound excitons in large-gap tubes. Instead, we find a feature of enhanced transmission caused by the blocking of optical transitions in small-gap tubes. A rapidly decaying flat background with pronounced dichroism is due to spatially localized nonthermal charge carriers with increased hopping rate.
The publication is available at link DOI: 10.1103/PhysRevLett.96.027401