Possibility of rarefaction shock wave under short pulse laser ablation of solids

Attention is drawn to a phenomenon that may give a radically different explanation for the recent observations of the system of dark rings above a solid surface vaporized by a short laser pulse. If a substance is heated to near-critical temperature, the existence of the compression shock wave becomes impossible, whereas the rarefaction wave takes a form of shock. The rarefaction shock can be considered as an interface in the expanding near-critical substance to form Newton rings in time-resolved optical microscopy experiments. The qualitative picture of the laser-ablated material expansion in vacuum with the generation of the rarefaction shock is discussed.