Material removal during ultra-short laser ablation takes place
primarily due to critical point phase separation (CPSS) mechanism.
During this process, no appreciable change in material density
Occurs until it reaches the maximum temperature. After reaching
the maximum temperature the density begins to decrease and
ablation will take place for the material cells whose expansion
trajectories have entered the unstable zone near the critical point
whereas the mass below it will condense back on the target. The
separation temperature serves as the threshold value and its
location can be found by the heat diffusion equations. During
the laser ablation, the laser energy is first absorbed by the free
electrons in the metal and due to electron phonon coupling this
energy is transferred to the lattice. At the surface there is direct
solid– vapor or solid–plasma transition but deep inside the metal
liquid phase is present.