Coherent lattice vibration of interlayer shearing mode of graphite

We have observed a coherent oscillation signal of the interlayer shearing phonon mode of highly oriented pyrolytic graphite by means of femtosecond pump-probe experiment. The optical polarization dependence reveals a nontrivial phase shift between Rayleigh and Raman components. The detailed analysis of the phase shift clarifies that the Raman component mainly comes from the real part of the dielectric function. The excitation density dependence of the coherent phonon shows the same saturation behavior as that of the electronic response and the generation process of the coherent phonon is attributed to the real excitation of the pi-pi* transition.