Polarization and driving force analysis of coherent optical shear phonons in graphite

TL;DR

This study investigates how femtosecond laser pulses induce polarized coherent optical shear phonons in graphite, revealing their polarization dependence and short driving force lifetime through ultrafast electron diffraction.

Contribution

It demonstrates the polarization dependence of shear phonons in graphite and quantifies the driving force lifetime using a driven-oscillator model.

Findings

Phonon polarization aligns with laser polarization.

Driving force lifetime is approximately 37 fs.

The phonon lifetime is shorter than excited carrier lifetime.

Abstract

Coherent optical phonons in the degenerate inter-layer shear mode of graphite launched by femtosecond laser pulses were investigated using ultrafast electron diffraction. The collective atomic motion is shown to be polarized in a direction related to the linear polarization of the incoming laser pulse. Using a fit with a generic driven-oscillator model, the lifetime of the oscillator's driving force is determined to 37(30) fs. This is much shorter than the lifetime of excited carriers in graphite but similar to the time scale of the loss of the hot carrier's k-space anisotropy.