Dynamics of the Lowest-Energy Excitons in Single-Walled Carbon Nanotubes under Resonant and Nonresonant Optical Excitation

The dynamics of lowest-energy E₁₁ excitons in undoped and hole-doped single-walled carbon nanotubes (SWCNTs) are investigated using transient absorption spectroscopy and theoretical calculations. In undoped SWCNT samples, E₁₁ excitons under resonant E₁₁ photoexcitation show faster decay than those under high-energy nonresonant photoexcitation. Doping SWCNTs with holes accelerates the E₁₁ exciton decay through exciton-hole interactions; moreover, both resonant and nonresonant photoexcitation in hole-doped SWCNTs lead to nearly identical decay profiles. Theoretical calculations, which are based on a simple model that accounts for bright, dark, and charged exciton states, agree well with the experimental results. We ascribe the fast E₁₁ exciton decay under the resonant E₁₁ photoexcitation to the redistribution of exciton populations in the E₁₁ bright exciton band, which includes optically accessible and inaccessible states; the rapid exciton redistribution is caused by exciton-hole scattering. Our study provides a clear understanding of the global features of exciton dynamics in SWCNTs.