Transient Hydrodynamic Lattice Cooling by Picosecond Laser Irradiation of Graphite

Recent theories and experiments have suggested hydrodynamic phonon transport features in graphite at unusually high temperatures. Here, we report a picosecond pump-probe thermal reflectance measurement of heat-pulse propagation in graphite. The measurement results reveal transient lattice cooling near the adiabatic center of a $15\text{−}\mu \mathrm{m}$-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.

Figure 1

Experimental setup of optical heat-pulse measurement. (a) Schematic diagram of optical heat-pulse measurement setup with a pair of axicon lenses to form a ring-shape pump of the 1064 nm picosecond pulse laser. A continuous-wave laser at 553 nm wavelength is focused on the center of the ring-shape pump as a probe. (b) Schematic of the ring-shape pump and probe at the center on the sample surface. Red arrows indicate heat-propagating directions. (c) An example image of the laser beam on the sample surface.

Figure 2

Normalized reflectance data measured on the LG-HOPG at different temperatures indicated in the figure. The radius of the ring-shape pump is $15\mu \mathrm{m}$. The arrow indicates lattice cooling due to second sound propagation. Because $dR/dT$ is negative for graphite, the normalized reflectance (R) data shown in Figs. 2, 3, 4 are the original data multiplied with a negative value so that the normalized signal has the same sign as the temperature change.

Figure 3

Normalized reflectance data measured on the SG-HOPG at (a) 294 and (b) 80 K. The measurement was conducted at three different radii ($r$) of the ring-shape pump, including 11, 15, and $18\mu \mathrm{m}$. The horizontal dashed line indicates zero temperature rise.

Figure 4

(a)–(e) Snapshots of the calculated transient temperature profiles on the top surface of a graphite sample irradiated by a $15\mu \mathrm{m}$ radius pump beam at an ambient temperature of 80 K. The heat pulse propagates inward in (a)–(c) and then outward in (d), leaving a negative temperature rise at the adiabatic center in (e) due to the hydrodynamic thermal fluctuation. (f) Evolution of the temperature profile along the radial direction at each time. (g) Comparison between the experiment and the calculation results for a pump beam radius of $15\mu \mathrm{m}$ at three different temperatures of 80, 100, and 150 K.