  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.
