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

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.