Ultrafast Relaxation Dynamics of Hot Optical Phonons in Graphene

TL;DR

This study investigates the ultrafast relaxation of hot optical phonons in graphene using pump-probe spectroscopy, revealing phonon lifetimes of about 2.5 ps and showing that relaxation dynamics are consistent across different growth methods and substrates.

Contribution

It provides the first detailed measurement and modeling of hot optical phonon relaxation times in graphene, confirming theoretical predictions with experimental data.

Findings

Optical phonon lifetimes are approximately 2.5 ps.

Relaxation dynamics are independent of growth technique, layer number, and substrate.

Experimental results agree with anharmonic phonon interaction theory.

Abstract

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.