Laser Desorption from Brittle Solids

Scientific Achievement

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The mechanisms by which material ablates from brittle solids can lead to removal of far more material than would be expected considering only the total energy deposition. When lasers interact with absorptive materials such as concrete, the rate at which energy is deposited is crucial to the efficiency of the process. Because the laser heats a small volume (several mm in diameter) in an otherwise cold material, thermal stress can build up. If the deposition rate is low compared to thermal diffusion the material expands but does not ablate. Depositing the energy on a time scale at which thermal expansion cannot be accommodated by elastic deformation leads to ablation via a thermal shock mechanism. Shear stress is induced by differential expansion, leading to cracking and spalling. If the substrate is an aggregate of inhomogeneous materials, such as concrete, the mechanism is enhanced by the fact that each phase has a different coefficient of thermal expansion.

In a transparent substrate damage is dependent entirely on the generation of initiation sites. These may be physical or chemical. For example, fused silica with embedded gold nanoparticles (20 nm) experiences severe damage when irradiated at high fluence, but only after several tens of laser pulses. The amount of energy deposited in such a small particle is insufficient in itself to cause bulk damage, however as the site is repeatedly irradiated, the physical nature of the defect changes and the substrate itself becomes absorptive. When the site reaches a threshold size, bulk damage occurs explosively. Again, the mechanism is differential thermal expansion on a fast time scale, but in this case damage is dependent on the generation of a localized initiation site. The role of nanoparticles in laser damage is thus demonstrated.

Significance

High power laser ablation can be an effective means of stripping surface layers from brittle absorptive materials such as concrete, and we have elucidated the mechanism by which it occurs. We have shown how to use the laser efficiently, and how to avoid vaporization. We have also shown that ablation of an absorptive brittle solid exhibits significantly different processes than metallic solids.

Nanoparticles, such as those used to polish laser optics, can cause catastrophic failure when irradiated at high power with pulsed lasers. This is important in designing better polishing techniques for optics at the National Ignition Facility, which will be the worldÕs most powerful laser when fully operational. It was previously believed that nanoparticulate contaminants could not absorb enough energy to affect damage, however we have shown that they can serve as initiators even when buried deep in the material.

Performers

M. R. Savina, M. J. Pellin; Argonne National Laboratory