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Environmental particles are ubiquitous in all but the cleanest laboratory environments. Whether these particles are suspended in air or deposited on surfaces, they can greatly degrade the performance of practical laser systems. However, they also create radically different physical situations than what are reported in traditional laser damage testing, which uses ultra-short pulse lasers focused to microscopic spots on pristine dielectric surfaces.
When a surface contaminated with absorbing particles is exposed to a high power laser, the particles heat to their vaporization temperatures, many migrate over the surface, some evaporate away, and some coalesce over time periods on the order of a millisecond. During this time, the particles act as both a conductive heat source and an absorption point for the optical material underneath. If sufficient heat energy is transferred to the optic, then free carriers will be thermally generated across bandgap, and catastrophic breakdown will result if a critical concentration is reached. Thus there is a strong bandgap dependence to contamination-induced breakdown, and despite the common misconceptions about “random failures”, resistance to dirt can be built into an optical system at the design stage.
Particles in air can also nucleate breakdown. The details of this process are still obscure but it appears to have much in common with particles on surfaces. One major difference is the process of laser acceleration of absorbing particles. When the particle is laser-heated to extreme temperature, it begins to evaporate, and each evaporated atom contributes a small amount of momentum to the parent particle. There is a small temperature gradient across the particle due to the directionality of laser heating; therefore one side of the particle evaporates faster than the other. The sum of all of the evaporation events gives the particle a substantial velocity. Interestingly, the heat transfer within the particle cannot be explained by conduction, convection, or radiation, but rather appears to be driven by photon diffusion, a process normally dominant in the photospheres of stars.
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