Excimer laser ablation: energy or power density? A different approach

In all excimer laser irradiation data, the induced laser ablation process is constantly refered to an energy density (fluence) and, eventually, to a fluence threshold that sets the onset of matter emission. In the work presented here, ablation experiments have been performed on a specific class of ceramic (sialon) with two excimer sources A and B that differ in their respective time-profiles, with a full pulse width of either (A) 23 or (B) 44 ns. Working with these sources at identical fluences and number of pulses, products of irradiation may differ drastically. In particular, it is observed that an irreversible surface decomposition may be achieved with laser A at a given threshold fluence, not with laser B that requires a higher fluence. Results are analyzed and interpreted taking into account the time dependence of the laser emission and its consequences in terms of photon-electron interactions at the material surface. Differing strongly from existing models of ablation, a two-step model is presented that incorporates i) creation of a free-electron gas at the material surface and ii) its subsequent photoemission giving rise to surface ionization and structural instability. Results are discussed in the light of that model. In particular, a distinct advantage is given to short pulse excimer sources in their ability to modify surface properties at lesser fluences and with reduced heat generation, as compared to longer pulse sources.