Ultra-fast optical spectroscopy of micelle-suspended single-walled carbon nanotubes

We present results of wavelength-dependent ultrafast pump-probe experiments on micelle-suspended single-walled carbon nanotubes. The linear absorption and photoluminescence spectra of the samples show a number of chirality-dependent peaks and, consequently, the pump-probe results sensitively depend on the wavelength. In the wavelength range corresponding to the second van Hove singularities (VHSs) we observe subpicosecond decays, as has been seen in previous pump-probe studies. We ascribe these ultra-fast decays to intraband carrier relaxation. On the other hand, in the wavelength range corresponding to the first VHSs, we observe two distinct regimes in ultra-fast carrier relaxation: Fast (0.3-1.2 ps) and slow (5-20 ps). The slow component, which has not been observed previously, is resonantly enhanced whenever the pump photon energy resonates with an interband absorption peak, and we attribute it to interband carrier recombination. Finally, the slow component is dependent on the pH of the solution, which suggests an important role played by H ions surrounding the nanotubes.