UV-laser ablation of absorbing dielectrics by ultra-short laser pulses

UV-laser material ablation by ultra-short laser pulses (USLP) reveals the features of laser-matter interactions on the picosecond and subpicosecond time scales. In the present communication, the effect of two successive USLP on absorbing dielectrics is considered and the dependence of the ablated depth on the delay time between USLP (delay curve) is investigated within the framework of a surface evaporation model. A possible role of stepwise ionization and impact ionization in such experiments is discussed. The condition is derived when both the impact ionization and the stepwise ionization are insufficient in the case of UV USLP ablation. Material ablation is considered for various models of electronic structure such as two- and three-level systems with different relations between relaxation times and absorption cross-sections for corresponding transitions. These models apply to polymers. Main factors which influence qualitative behavior of the delay curve are determined. It is shown that even in such a relatively simple model of laser ablation its behavior is not governed solely by the effective light induced bleaching or darkening. (C) 1998 Elsevier Science B.V.