Resonant Coherent Phonon Spectroscopy of Single-Walled Carbon Nanotubes

TL;DR

This study combines femtosecond pump-probe spectroscopy and microscopic theory to analyze coherent phonons in single-walled carbon nanotubes, revealing chirality-dependent light absorption and phonon dynamics.

Contribution

It introduces a microscopic model explaining coherent phonon generation and detection, matching experimental data across different nanotube chiralities.

Findings

Resonant enhancement occurs at exciton resonances.

Coherent phonon amplitudes follow a driven oscillator model.

Mod 2 nanotubes exhibit stronger phonon signals than mod 1.

Abstract

Using femtosecond pump-probe spectroscopy with pulse shaping techniques, one can generate and detect coherent phonons in chirality-specific semiconducting single-walled carbon nanotubes. The signals are resonantly enhanced when the pump photon energy coincides with an interband exciton resonance, and analysis of such data provides a wealth of information on the chirality-dependence of light absorption, phonon generation, and phonon-induced band structure modulations. To explain our experimental results, we have developed a microscopic theory for the generation and detection of coherent phonons in single-walled carbon nanotubes using a tight-binding model for the electronic states and a valence force field model for the phonons. We find that the coherent phonon amplitudes satisfy a driven oscillator equation with the driving term depending on photoexcited carrier density. We compared our theoretical results with experimental results on mod 2 nanotubes and found that our model provides satisfactory overall trends in the relative strengths of the coherent phonon signal both within and between different mod 2 families. We also find that the coherent phonon intensities are considerably weaker in mod 1 nanotubes in comparison with mod~2 nanotubes, which is also in excellent agreement with experiment.