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 center of the crater. The heat transfer calculations show a melt thickness of the order of 1 μm and a melt lifetime of the order of 1 μs. The numerical solution of fluid motion of the thin melt, on the other hand, demonstrate that the time for the melt to flow to the crater edge and to form a rim is about 1 μs, which is of the same order of magnitude as the melt lifetime. The possibility of controlling or suppressing the rim formation is also discussed. ∗email: [email protected]; tel: (650) 723-0161, fax: (650) 723-2666