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A new insight into defect-induced laser damage in UV multilayer coatings

: Kaiser, N.; Reichling, M.; Bodemann, A.

Bennett, H.E.; Guenther, A.H. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Laser-Induced Damage in Optical Materials 1994. Proceedings
Bellingham, WA: SPIE, 1995 (Proceedings of SPIE 2428)
ISBN: 0-8194-1776-9
Annual Boulder Damage Symposium <26, 1994, Boulder/Colo.>
Annual Symposium on Optical Materials for High-Power Lasers <26, 1994, Boulder/Colo.>
Fraunhofer IOF ()
Dünne optische Schicht; excimer laser optics; Excimer-Laser-Optik; fluoride thin films; Fluoridschicht; laser induced damage threshold; Laserzerstörschwelle; optical coating; optical thin films; oxide thin films; Oxidschicht; ultraviolet spectral region; ultravioletter Spektralbereich; UV

High performance Al2O3/SiO2 mirror coatings for 248 nm have been investigated with respect to their excimer laser damage resistivity. Global damage thresholds (in the range of 10-20 J/cm2) averaged over large areas were determined with the pulsed photoacoustic mirage detection method. With a raster scanning technique utilizing the same detection scheme, the local damage behaviour was studied with 100 mu m spatial resolution. It was found that the local damage threshold at specific sites was lower than the global damage threshold and it was assumed that this phenomenon was associated with micrometer-scale defects in the multilayer coating. To test this hypothesis photothermal displacement microscopy with mu m lageral resolution was performed on the investigated regions prior to excimer laser light irradiation. Photothermal images revealed an extremely small background absorption and a small number of absorbing defect sites. For a number of such sites a clear correlation between the loca l absorption and the onset of laser damage at that specific location was found. We conclude that the crucial factor determining the damage resistivity of the high quality coating systems are defects and contaminants and that it will be possible to predict their damage thresholds by a complete microscopic photothermal inspection.