  Using ultrafast optical pump-probe spectroscopy, we study the relaxation
dynamics of hot optical phonons in few-layer and multi-layer graphene films
grown by epitaxy on silicon carbide substrates and by chemical vapor deposition
on nickel substrates. In the first few hundred femtoseconds after
photoexcitation, the hot carriers lose most of their energy to the generation
of hot optical phonons which then present the main bottleneck to subsequent
carrier cooling. Optical phonon cooling on short time scales is found to be
independent of the graphene growth technique, the number of layers, and the
type of the substrate. We find average phonon lifetimes in the 2.5-2.55 ps
range. We model the relaxation dynamics of the coupled carrier-phonon system
with rate equations and find a good agreement between the experimental data and
the theory. The extracted optical phonon lifetimes agree very well with the
theory based on anharmonic phonon interactions.
