Optical generation and detection of local non-equilibrium phonons in suspended graphene

TL;DR

This study demonstrates that optical and acoustic phonons in suspended graphene can be driven out of local equilibrium under laser illumination, revealing weak coupling of flexural phonons with hot electrons and optical phonons, impacting thermal transport understanding.

Contribution

It introduces a first principles-based model linking Raman spectra to local phonon temperatures, highlighting non-equilibrium phonon behavior in suspended graphene.

Findings

Flexural phonons have a lower local temperature than other phonons.

Weak coupling between flexural modes and hot electrons/optical phonons.

Implications for energy dissipation and thermal measurements in graphene.

Abstract

The measured frequencies and intensities of different first- and second- order Raman peaks of suspended graphene are used to show that optical phonons and different acoustic phonon polarizations are driven out of local equilibrium inside a sub-micron laser spot. The experimental results are correlated with a first principles-based multiple temperature model to suggest a considerably lower equivalent local temperature of the flexural phonons than those of other phonon polarizations. The finding reveals weak coupling between the flexural modes with hot electrons and optical phonons. Since the ultrahigh intrinsic thermal conductivity of graphene has been largely attributed to contributions from the flexural phonons, the observed local non-equilibrium phenomena have important implications for understanding energy dissipation processes in graphene-based electronic and optoelectronic devices, as well as in Raman measurements of thermal transport in graphene and other two-dimensional materials.