Thermal and fluid processes of a thin melt zone during femtosecond laser ablation of glass: the formation of rims by single laser pulses

We study the formation mechanism of rims created around femtosecond laser ablated craters on glass. Experimental studies of the surface morphology reveal that a thin rim is formed around the smooth craters and is raised above the undamaged surface by about 50-100 nm. To investigate the mechanism of rim formation following a single ultrafast laser pulse, we perform a one-dimensional theoretical analysis of the thermal and fluid processes involved in the ablation process. The results indicate the existence of a very thin melted zone below the surface and suggest that the rim is formed by the high pressure plasma producing a pressure-driven fluid motion of the molten material outwards from the centre of the crater. The numerical solutions of pressure-driven fluid motion of the thin melt demonstrate that the melt can flow to the crater edge and form a rim within the first nanoseconds of the ablation process. The possibility that a tall rim can be formed during the initial stages of the plasma is suggestive that the rim may tilt outwards towards the low pressure region creating a resolidified melt splash as observed in the experiments. The possibility of controlling or suppressing the rim formation is discussed also.