  We studied the excitation density dependence of photoluminescence (PL)
spectra of excitons and trions (charged excitons) in hole-doped single-walled
carbon nanotubes. We found that the PL intensity of trions exhibited a strong
nonlinear saturation behavior as the excitation density increased, whereas that
of excitons exhibited a weak sublinear behavior. The strong PL saturation of
trions is attributed to depletion of doped holes that are captured by excitons
in the formation processes. Moreover, the effective radiative lifetime of a
trion was evaluated to be approximately 20 ns.
