Excitation energy dependence of the ultrafast transient absorption response of single-wall carbon nanotubes

We use sub-picosecond transient absorption spectroscopy to demonstrate selective excitation in a polydisperse aqueous sample of single-wall carbon nanotubes. Tuning the excitation energy above and below the bandgap of a specific semiconducting nanotube produces significant variation in the photoinduced absorption change at the bandgap. Comparing the recovery of the ultrafast photoinduced absorption for metallic and semiconducting nanotubes for excitation above and below the lowest interband transition, we find that metallic SWNTs show a faster recovery. We attribute the faster recovery to the absence of long-lived exciton states in metallic SWNTs.