Steps and thresholds in laser ablation

Laser ablation involves two main steps, namely laser-solid interaction and removal of ablated material. 

There are at least two borderline situations as regards the interaction of a laser pulse with a surface in an ablation process. These situations are usually described quantitatively 

The ablation threshold is the irradiance value at which the material starts to separate from the sample. As a rule, the material separates through vaporization to a depth where the energy supplied by the laser beam equals the energy of vaporization. The material can separate in other forms including solid or liquid fragments and products of the photochemical decomposition of the sample. Because the energy deposited over the sample must exceed its latent heat of vaporization (//, J/lcg), the threshold fluence J/m-) is given by 

The sudden increase in crater depth observed during high irradiance ( 10 W/cm ) laser ablation of silicon [17], which has been ascribed to phase explosion, can be used to establish a new threshold the threshold irradiance for phase explosion. This threshold depends on two laser parameters, viz. beam spot size and wavelength. The larger the beam size and the longer the incident wavelength are, the higher is the laser irradiance required to cause phase explosion. 

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