Wavelength-dependent, near-infrared pump-probe study of micelle-suspended Single-Walled Carbon Nanotubes (SWCNTs) whose linear absorption spectra show chirality-assigned peaks is presented. Two distinct relaxation regimes were observed: fast (0.3--1.2 ps) and slow (5--20 ps). The slow component, which has previously been unobserved in pump-probe measurements of bundled tubes, was resonantly enhanced whenever the pump photon energy matched with an interband absorption peak, and it is attributed to interband carrier recombination. It represents the lower limit of the intrinsic radiative recombination time of photoexcited carriers in SWCNTs since the exact value of this parameter depends on the presence of possible nonradiative recombination channels.
The slow decay component was highly dependent on the pH of the solution, suggesting that the surrounding H+ ions strongly affect electronic states in nanotubes through the Burnstein-Moss effect. The effect was bandgap energy dependent, affecting the smaller bandgap tubes more significantly.
To elucidate carrier dynamics in more detail, nondegenerate pump-probe experiments with wide and continuum probing throughout the lowest and second lowest energy transition ranges of SWCNTs were used. Complex signals were revealed with photoinduced absorption and bleaching, both of which were strongly wavelength dependent. Due to the high optical quality of unbundled SWCNT samples, clear signs of band filling and broadening of the exciton absorption peaks were found to be the main nonlinear mechanisms. The identification of these nonlinear mechanisms presents a novel explanation of the observed nonlinear behavior of nanotubes in general and helps clarify the controversial issues presented in previously published work. This explanation is also consistent with the previously observed pump-probe signals in bundled nanotube samples.
Another novel and important conclusion drawn from the nondegenerate pump-probe experiments is that the position of the exciton absorption peaks is unchanged in the presence of high density electron-hole pairs, even when their density is comparable to the Mott density. The stability of the excitons observed for the first time in nanotubes is similar to what has been seen in the studies on the emission properties of GaAs-based semiconductor quantum wires. Although binding energies of these two 1D material systems are very different, the exciton stability seems to be a mark of their unique 1D nature.