High energetic excitons in carbon nanotubes directly probe   charge-carriers
  Theory predicts peculiar features for excited-state dynamics in one dimension (1D) that are difficult to be observed experimentally. Single-walled carbon nanotubes (SWNTs) are an excellent approximation to 1D quantum confinement, due to their very high aspect ratio and low density of defects. Here we use ultrafast optical spectroscopy to probe photogenerated charge-carriers in (6,5) semiconducting SWNTs. We identify the transient energy shift of the highly polarizable S33 transition as a sensitive fingerprint of charge-carriers in SWNTs. By measuring the coherent phonon amplitude profile we obtain a precise estimate of the Stark-shift and discuss the binding energy of the S33 excitonic transition. From this, we infer that charge-carriers are formed instantaneously with sizable quantum yield even upon pumping the first exciton, S11. The decay of the photogenerated charge-carrier population is well described by a model for geminate recombination in 1D, suggesting an initial charge carrier separation of the same order of the exciton correlation length. 
Giancarlo Soavi, Francesco Scotognella, Daniele Viola, Timo Hefner,   Tobias Hertel, Giulio Cerullo, Guglielmo Lanzani
cond-mat.mes-hall