Origin of coherent G band phonon spectra in single wall carbon nanotubes

TL;DR

This paper investigates the origin of coherent G band phonon spectra in single wall carbon nanotubes, revealing that modulation of interatomic dipole matrix elements significantly contributes to the observed signals, alongside the known diameter-dependent mechanisms.

Contribution

It introduces the idea that G band phonons are strongly influenced by dipole matrix element modulation, expanding understanding beyond the diameter-dependent energy gap mechanism.

Findings

Coherent G band phonon intensity is comparable to RBM phonon intensity.

Dipole matrix element modulation significantly contributes to G band phonon signals.

Coherent phonon amplitudes depend on laser pulse width.

Abstract

Coherent phonons in single wall carbon nanotubes (SWNTs) are observed as oscillations of the differential absorption coefficient as a function of time by means of pump-probe spectroscopy. For the radial breathing mode (RBM) of a SWNT, the coherent phonon signal is understood to be a result of the modulated diameter-dependent energy gaps due to the coherent RBM phonon oscillations. However, this mechanism might not be the dominant contribution to other phonon modes in the SWNT. In particular, for the G band phonons, which correspond to bond-stretching motions, we find that the modulation of the interatomic dipole matrix element gives rise to a strong coherent G band phonon intensity comparable to the coherent RBM phonon intensity. We also further discuss the dependence of coherent G band and RBM phonon amplitudes on the laser excitation pulse width.