The photophysics of trions in single-walled carbon nanotubes (SWCNTs) is reviewed briefly. The trion state is observed energetically below the exciton state in hole-doped SWCNTs, and shows a simple single-exponential decay. The decay dynamics of trions depends on the photoexcitation condition. The optical responses of trions can be discussed by considering the energy relaxation of the optically accessible trion state. Detailed studies of excitons, trions, and biexcitons provide a deep understanding of the material properties of SWCNTs and are required for the design of photonic devices based on SWCNTs.
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