Short pulse laser ablation is photomechanical, not thermal or chemical

Proposed mechanisms for pulsed laser ablation of biological tissue include photochemical, photothermal and photomechanical models. The principal observed effects which the correct model must explain include the high efficiency of the process, typically an order of magnitude less energy is required than for long pulse or cw ablation, and the minimal thermal damage to surrounding tissue. The photomechanical model postulates that ablation is initiated when the laser-induced stress exceeds the tensile strength of the material. In a version of the photomechanical model, called photospall, one assumes the target occupies an infinite half-space and performs a one dimensional analysis. The required tensile stress is then created by the reflection of the initial compressive stress from the free surface. The predictions of this model are in good agreement with observed spallation in materials like metals where the absorption depth is so small that the one dimensional approximation holds. This model was also suggested as a mechanism for the ablation of biological tissue, however there are serious discrepancies between predictions and experiments.