Modeling of moving breakdown by femtosecond laser pulses in dielectrics

Laser ablation is becoming increasingly important in the fields of micromaching, thin film formation, and bioengineering applications. In laser ablation, the ablation rates and feature quality strongly depend on the size of the breakdown region in the material. This region is characterized by a high density of free electrons, which absorb a large fraction of energy from the laser pulse that results in material vaporization in solids or liquids. For nanosecond- and picosecond pulses, the breakdown region tends to form near the beam focus and then expand back along the beam path toward the laser; this phenomenon is called moving breakdown. For femtosecond pulses, however, breakdown begins up the beam path and then propagates toward the focal point. A moving breakdown model presented by Docchio et al. (1988a) successfully explains and predicts the timedependent breakdown region in the nanosecond regime, however it does not adequately describe propagation of the breakdown region at pico- and femtosecond time scales. In the present work, a modified moving breakdown model is proposed that includes the pulse propagation and small spatial extent of ultrafast laser pulses. This revised model shows that pulse propagation becomes significant for pulsewidths less than 10 picoseconds. The new model characterizes the pulse behavior as it interacts with a material within the focal volume in both solids and liquids. The model may also be useful in estimating the time- and space-resolved electron density in the interaction volume, the breakdown threshold of a material, shielding effectiveness, energy deposition, and the temperature increase in the material.