Near-critical nanosecond laser-induced phase explosion on graphite surface

Optical reflectivity, removal rate and ablative recoil pressure magnitudes were measured as a function of laser fluence during high-power UV nanosecond laser ablation of graphite. At low fluences only melting and weak surface vaporization of molten carbon were observed. At moderate fluences there is a very narrow fluence interval where the reflected fluence starts to saturate, while the removal rate and ablative recoil pressure rise drastically in a correlated manner, indicating the onset of a near-critical surface phase explosion. Then, at higher fluences the reflected fluence, removal rate and recoil pressure saturate with an appearance of a luminous plume, altogether indicating negligible specular reflectance and absorbance on the target surface due to its complete screening by the highly-absorbing laser plume. The overall strong correlation between the removal rate and recoil pressure magnitudes may indicate rather quasi-continuous removal of the near-critical superheated molten carbon layer by a propagating unloading wave in the absence of a crucial sub-surface temperature maximum in the layer.