Using ultrafast optical pump-probe spectroscopy, we study the relaxationdynamics of hot optical phonons in few-layer and multi-layer graphene filmsgrown by epitaxy on silicon carbide substrates and by chemical vapor depositionon nickel substrates. In the first few hundred femtoseconds afterphotoexcitation, the hot carriers lose most of their energy to the generationof hot optical phonons which then present the main bottleneck to subsequentcarrier cooling. Optical phonon cooling on short time scales is found to beindependent of the graphene growth technique, the number of layers, and thetype of the substrate. We find average phonon lifetimes in the 2.5-2.55 psrange. We model the relaxation dynamics of the coupled carrier-phonon systemwith rate equations and find a good agreement between the experimental data andthe theory. The extracted optical phonon lifetimes agree very well with thetheory based on anharmonic phonon interactions.
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