Abstract
We report femtosecond transient absorption spectroscopic study on the (6, 5)
single-walled carbon nanotubes and the (7, 5) inner tubes of a dominant double-walled
carbon nanotube species. We found that the dynamics of exciton relaxation probed at the
first transition-allowed state (E11) of a given tube type exhibits a markedly slower decay
when the second transition-allowed state (E22) is excited than that measured by exciting
its first transition-allowed state (E11). A linear intensity dependence of the maximal
amplitude of the transient absorption signal is found for the E22 excitation, whereas the
corresponding amplitude scales linearly with the square root of the E11 excitation
intensity. Theoretical modeling of these experimental findings was performed by
developing a continuum model and a stochastic model with explicit consideration of the
annihilation of coherent excitons. Our detailed numerical simulations show that both
models can reproduce reasonably well the initial portion of decay kinetics measured upon
the E22 and E11 excitation of the chosen tube species, but the stochastic model gives
qualitatively better agreement with the intensity dependence observed experimentally
than those obtained with the continuum model.
