Exciton dynamics in (6,5) single-walled carbon nanotubes studied with sub-20 fs pump-probe spectroscopy

Using femtosecond pump probe spectroscopy with sub-20 fs resolution, we probe fundamental properties of the En exciton in (6,5) single walled carbon nanotubes, prepared by density gradient ultracentrifugation. From the initial photobleaching (PB) signal, measured faster than any relaxation process, we obtain the one- dimensional electron-hole correlation length ("exciton size") of the excitonic wavefunction along the nanotube. Anisotropy measurements in highly purified samples show that there is virtually no depolarisation of the E₁₁ bleach over 40 ps. A photoinduced absorption (PA) band, blueshifted against the E₁₁ bleach, shows only weak anisotropy. We find that exciton decay is pump-intensity dependent only at elevated pump intensities. Our high time resolution allows us to demonstrate that exciton motion is diffusive. We find a very short exciton diffusion length of below 10 nm. In consequence, many excitons can co-exist on a single nanotube without annihilation, which is an important aspect for applications in high-intensity environments (Lasing, switching etc.). We discuss the role of traps in exciton relaxation anisotropy.