Exciton collision broadening in single-wall carbon nanotubes

The one-dimensionality of carriers in single-wall carbon nanotubes results in strong Coulomb interactions. Besides the non-perturbative binding of an electron-hole pair into an exciton, several time-resolved experiments have shown evidence for efficient exciton-exciton interactions. These studies have revealed that the exciton recombination is driven by an exciton-exciton annihilation mechanism. In fact such an Auger process is known to play a key role in the population relaxation of Frenkel excitons in organic materials, and especially J-aggregates and conjugated polymers. Here we show that the similarity with organic molecules breaks down for the dephasing induced by exciton-exciton interactions. We have studied the exciton dephasing by means of the non-linear optical technique of spectral-hole burning. We show that the collision broadening is surprisingly not limited by exciton-exciton Auger annihilation. We demonstrate that the coherence relaxation is determined by quasi-elastic exciton-exciton scattering within the fundamental excitonic band. This process is typical of Wannier excitons in inorganic semiconductors, while exciton annihilation is usually observed for Frenkel excitons in organic compounds. Our results reveal the unique non-linear properties of carbon nanotubes in between inorganic and organic materials