Merging Spot Size and Pulse Number Dependence of Femtosecond Laser Ablation Thresholds: Modeling and Demonstration with High Impact Polystyrene

The spot size dependence of pulse laser-induced ablation thresholds of solid materials cannot be satisfactory described by the two existing quantitative models based on defect densities and on heat accumulation. In the present study, the heat accumulation model was amended but still yielded results in contradiction to experimental observations. The existing defect model was extended to account for incubation where optically active high-density defects with a separation below the radiation wavelength are generated. The reduction of the threshold for large beam radii could be ascribed to the laser spot covering a finite number of optically active low-density defects (LDD) embedded in the matrix material. This new generic model combining the spot size and pulse number dependence of femtosecond pulse laser-induced ablation thresholds was demonstrated with high-impact polystyrene. The average distance of the optically active LDD obtained from the generic model could be confirmed by scanning electron microscopy.