Ultrashort pulse laser material removal

Laser material interaction is a very complicated problem. Depending on the pulse length, the mechanism could be thermal or electronic. There are three regimes of pulse length: long (> ns), short (> ps and <ns), and ultrashort (< ps) pulses. In the long pulse regime, the laser energy in most cases can be modeled as a surface heat source. The deposited energy melts and vaporizes the substrate. The material is removed in the form of vapor and liquid. Depending in the material, if the pulse length is short enough, the laser energy first absorbed by the electrons does not have enough time to equilibrate with the lattice. In this case, the electron and lattice temperatures are different. Consequently, the ususal local thermal equilibrium does not apply. Two-temperature models have been used by a number of researchers to model the interaction. In the ultrashort regime, the laser material interaction is through electron excitation, such as avalanche, impact, multiphoton, and strong optical field ionizations. These hot electrons equilibrate with themselves very rapidly, in the order of femtoseconds. As the electrons are excited from their ground states, the bonding between the nuclei are weakened. The Coulomb force causes the material to expand. In this paper, we postulate that the material removal is due to Coulomb explosion. A simple model is developed to model the Coulomb explosion. The model can then be used to predict the material removal rate.