Ultrafast dynamics in metallic and semiconducting carbon nanotubes

Transient polarized pump/probe transmission changes upon pulse photoexcitation at 1.6 eV were measured in chiral enriched carbon nanotubes ensembles of both metallic and semiconductor characters, in a very broad probe spectral range from mid-IR to visible, combining several ultrafast laser systems. We identify a photoinduced absorption band at ~0.7 eV as due to hot Dirac fermions in the metallic tubes and measure its dynamics. The hot carriers equilibrate within the electronic system in ~400 fs and with the lattice within ~2 ps; in agreement with hot Dirac fermions thermalization in graphene. We also assign excitonic transitions in the semiconducting tubes, mainly of (6,5) chirality; in particular, an intraexciton transition at ~0.4 eV represents a lower limit for the exciton binding energy.