Above a certain fluence threshold, laser light incident on a target induces a rapid removal of material, a process called ablation. A survey of results from the literature shows that, if the laser energy deposition time is short compared with the time required to accelerate matter, the ablation threshold energy is an order of magnitude lower than the energy required to evaporate material. This is true for many different lasers and wavelengths and occurs because, in this regime, photomechanical effects dominate the ablation process. Using the theory of thermoelectricity, we calculated the full three-dimensional time-dependent variation of the axial, radial, circumferential, and shear stresses induced in a target irradiated by an axially symmetric laser beam, with an exponential absorption depth comparable to the beam radius. The theory reveals that very large transient tensile stresses are generated at the surface. I postulate that when these stresses exceed the tensile strength of the material, ablation occurs.

© 1996 Optical Society of America

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