Chengbin Li

Ultrafast electronic excitation in CaF2 crystals

A new pump and probe experimental system was developed, the pump pulse duration of which is stretched and is much longer than that of the probe pulse. Using this system, time-resolved electronic excitation processes and damage mechanisms in CaF2 crystals were studied. The measured reflectivity of the probe pulse begins to increase at the peak of the pump pulse and increases rapidly in the latter half of the pump pulse, when the pump pulse duration is stretched to 580 fs. Our experimental results indicate that both multiphoton ionization and impact ionization play important roles in the generation of conduction band electrons, at least they do so when the pump pulse durations are equal to or longer than 580 fs.

Femtosecond laser pulse induced damage in thin films

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.

Ablation mechanism of fused silica under femtosecond laser pulse

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.