Light intensity modulations caused by opaque obstacles (e.g., dust) on silica lenses in high-power lasers often enhance the potential for laser-induced damage. To study this effect, particles (10-250 mum) with various shapes were sputter deposited on the input surface and irradiated with a 3-ns laser beam at 355 nm. Although a clean silica surface damages at fluences above 15 J/cm(2), a surface contaminated with particles can damage below 11.5 J/cm(2). A pattern that conforms to the shape of the input surface particle is printed on the output surface. Repetitive illumination resulted in catastrophic drilling of the optic. The damage pattern correlated with an interference image of the particle before irradiation. The image shows that the incident beam undergoes phase (and amplitude) modulations after it passes around the particle. We modeled the experiments by calculating the light intensity distribution behind an obscuration by use of Fresnel diffraction theory. The comparison between calculated light intensity distribution and the output surface damage pattern showed good agreement. The model was then used to predict the increased damage vulnerability that results from intensity modulations as a function of particle size, shape, and lens thickness. The predictions provide the basis for optics cleanliness specifications on the National Ignition Facility to reduce the likelihood of optical damage.
We have used vapor etching of ion tracks to create high aspect ratio (i.e., length much greater than diameter), isolated cylindrical holes through ∼600-nm-thick films of thermally fused silica on silicon. Samples were exposed to the vapor from water-based liquids with various HF and HF+HCl concentrations. Independent control of the temperatures of the vapor and the samples provided the means to vary separately the etching rates for the tracks and the track-free material. The very rapid etching of the small latent track zone can be explained by preferential capillary condensation. Holes with diameters of ∼24 to ∼80 nm have been documented with length/diameter ratios of up to 22. Although we have restricted this study to thin-film silica, we have evidence that such holes are also formed in bulk fused silica.
SummaryStarting from the absorption of laser energy at a subsurface nanoparticle in fused silica, we simulate the consequent buildup of stresses and resulting mechanical material damage . The simulation indicates the formation of micropits with size comparable to a wavelength, similar to experimental observation. Possible mechanisms for enhanced local light absorbtion are discussed.
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