SHOCK-WAVES GENERATED BY CONFINED XECL EXCIMER LASER ABLATION OF POLYIMIDE

We investigate shock waves generated by excimer laser ablation of sheet polyimide confined in water. The velocities of the ablation-induced pressure waves in the water are determined by an optical probe system. We measure supersonic velocities up to a few hundred microns away from the irradiated surface, indicating the formation of shock waves. We use these velocities to calculate the corresponding pressures. They are already in the kbar range at fluences comparable to the threshold of ablation. The shock pressure varies as the square root of the incident laser fluence, a behavior that is explained by the rapid heating of the confined gaseous products of ablation. The initially planar shock waves propagate, become spherical, and decay within a few hundred microns in the surrounding water to acoustic waves. During spherical expansion the shock pressure drops as the inverse of the square of the propagation distance. The shock waves generated may be relevant in explaining photoacoustic damage observed in biological tissue after excimer-ablation at corresponding irradiances. They may also be important in material processing applications of excimer laser ablation of polymers as they can lead to plastic deformation.