We study single-shot damage morphology of MgF2 irradiated under 800 nm laser using scanning electronic microscope (SEM). The dependence of damage threshold on laser pulse width is measured by the linear relationship between the area of damage spot and the logarithm of laser energy. The pulse duration ranges from 55 fs to 750 fs. We develop the multiple-equation model (Phys. Rev. Lett. Vol.92, pp.187401, 2004), in which two-photon absorption in the conduction band is considered. The experimental results agree well with our model.
We have investigated the damage for ZrO2/SiO2 800 nm 45° high-reflection mirror and MgF2/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 τ1/2 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.
The ablation process in sapphire and fused silica are studied with laser at 800 nm and 400 nm respectively. Comparing with the features of the ablated craters induced by different laser, we find that lasers with short wavelength and pulse duration can produce more exquisite ablation crater with small area and steep gradient. By means of determining the Fth with detection of the scattered light, the developments of the threshold fluence of dielectrics as a function of pulse duration are presented. While interpreting our results with existent model of optical breakdown, we discuss the excitation mechanism of conduction band electrons (CBE) in transparent dielectrics.