Real-Time Spectroscopy of Single-Walled Carbon Nanotubes for Negative Time Delays by Using a Few-Cycle Pulse Laser

Several investigations using the femtosecond pump-probe technique have been made for coherent phonon dynamics in single-walled carbon nanotubes (SWNTs), which are promising candidates as devices for nanoelectronics and photonics due to their unique properties. However, the most relevant spectroscopic analyses by far have been performed in the positive time range, with the pump pulse coming before the probe. We studied the real-time coherent phonon in SWNTs using 7.1-fs pump-probe measurements for negative time delays in which vibronic coherence between the ground state and exciton state can be observed. Coherent vibrations corresponding to the radial breathing mode caused by the wave packet generated in the exciton state were detected, in contrast to the ground-state wave packet dynamics for positive delays previously reported. The present results show that excited-state wave packets contribute greatly to radial breathing modes in four, (6,5), (6,4), (7,5), and (8,3), chiral systems, but very little to G modes, even though the oscillation period of the latter is 20 fs, which is much shorter than the 100 fs of the exciton life. We discussed the differences in dephasing times in the four different chiral systems.