Femtosecond two-pulse cross correlation in single-walled carbon nanotubes: evedence of intersubband transitions and role of G-band-assisted transitions.

By femtosecond pump-probe absorption spectroscopy we demonstrated intersubband and phonon-assisted transitions in single-walled carbon nanotubes (SWNTs). After resonant excitations of specific tube types, electrons are promoted from the ground state to the second excitonic state (EX2) in PLV-prepared SWNTs or to vibrational excited levels of the first excitonic state (EX1) in HiPco-prepared SWNTs. At time-zero delay a [1 + 1] resonant cross-correlation due to short lifetimes of higher excited states or phonon modes can be observed. These results suggest that intersubband transitions between the excitonic states in SWNTs are allowed. The interpretation of the temporal profiles at pump/probe combinations implies that the G-band mode, which is known to have a high Raman intensity, plays an important role for the interpretation of both femtosecond cross-correlation measurements and photoluminescence spectra.