Real-time observation of nonlinear coherent phonon dynamics in single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are pi-conjugated, quasi-one-dimensional structures consisting of rolled-up graphene sheets that, depending on their chirality, behave as semiconductors or metals(1); owing to their unique properties, they enable groundbreaking applications in mechanics, nanoelectronics and photonics(2,3). In semiconducting SWNTs, medium-sized excitons (3-5 nm) with large binding energy and oscillator strength are the fundamental excitations(4-8); exciton wavefunction localization and one-dimensionality give rise to a strong electron-phonon coupling(9-11), the study of which is crucial for the understanding of their electronic and optical properties. Here we report on the use of resonant sub-10-fs visible pulses(12) to generate and detect, in the time domain, coherent phonons in SWNT ensembles. We observe vibrational wavepackets for the radial breathing mode (RBM) and the G mode, and in particular their anharmonic coupling, resulting in a frequency modulation of the G mode by the RBM. Quantum-chemical modelling(13) shows that this effect is due to a corrugation of the SWNT surface on photoexcitation, leading to a coupling between longitudinal and radial vibrations.