Résumé
Single-walled carbon nanotubes (SWNT) are one-dimensional nanostructures where the Coulomb interactions between charge carriers are strongly enhanced compared to systems of higher dimensionality. This results in an electron-hole bound state -the so-called exciton- with a binding energy of the order of one third of the bandgap, which controls the SWNT's optical properties. As a matter of fact, the exciton-exciton interactions are particularly efficient and drive the exciton recombination and dephasing dynamics. The investigation of excitonic complexes in semiconducting SWNTs is currently a topic of intense debate. The biexciton and the trion are expected to have a binding energy of about a hundred meVs. Whereas the trion has been recently observed, first in doped nanotubes and then by means of all-optical generation, there is no experimental evidence for the biexciton. Here we present the first observation of the biexciton in semiconducting single-wall carbon nanotubes using nonlinear optical spectroscopy. Our experiments consist in a spectrally resolved pump-probe technique in SWNTs embedded in a gelatine at cryogenic temperature. Our measurements of the differential transmission spectrum reveal the universal asymmetric line shape of the Fano resonance intrinsic to the biexciton transition.