We have investigated the damage for ZrO<sub>2</sub>/SiO<sub>2</sub> 800 nm 45° high-reflection mirror and MgF<sub>2</sub>/ZnS 800 nm interference filter with femtosecond pulses. The damage morphologies and evolution of ablation crater depths with laser fluences are dramatically different from that with pulse longer than a few tens of picoseconds. We also report their single-short damage thresholds for pulse durations ranging from 50 fs to 900 fs, which depart from the diffusion-dominated τ<sup>1/2</sup> scaling. A developed avalanche model, including the production of conduction band electrons (CBE) and laser energy deposition, is applied to study the damage mechanisms. The theoretical results agree well with our measurements.
Based on the avalanche model, the mechanism of femtosecond laser-induced ablation in fused silica was investigated. The three microscopic processes, including the production of conduction band electrons (CBE), the deposition of laser energy, and the diffusion of CBE and energy, were solved by a finite element method (FEM) of two-dimension cylinder coordinate. The conduction band electrons (CBE) were produced through photoionization and impact ionization, which were calculated via Keldysh theory and Double-flux model, respectively. The accumulated charge and the electrostatic field were also calculated, and the evolution of microexplosion was discussed based on this model. The results indicate that the CBE and energy diffusion plays an important role in the ablation of dielectrics by femtosecond laser pulse.