Transient Hydrodynamic Lattice Cooling by Picosecond Laser Irradiation of Graphite

TL;DR

This study demonstrates transient lattice cooling in graphite at high temperatures using picosecond laser pulses, providing evidence for hydrodynamic phonon transport phenomena in the material.

Contribution

It presents the first direct observation of transient lattice cooling in graphite via pump-probe thermal reflectance, supporting hydrodynamic phonon transport theory.

Findings

Transient lattice cooling observed at 80-120 K

Cooling occurs near the adiabatic center of a ring-shaped pump beam

Results align with hydrodynamic phonon transport and second sound theories

Abstract

Recent theories and experiments have suggested hydrodynamic phonon transport features in graphite at unusually high temperatures. Here, we report a pico-second pump-probe thermal reflectance measurement of heat pulse propagation in graphite. The measurement results reveal transient lattice cooling near the adiabatic center of a 15 $\mu$m diameter ring-shape pump beam at temperatures between 80 and 120 K. While such lattice cooling has not been reported in recent diffraction measurements of second sound in graphite, the observation here is consistent with both hydrodynamic phonon transport theory and prior heat pulse measurements of second sound in bulk sodium fluoride.