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Influence of thermal substrate properties on the damage threshold of UV coatings

: Blaschke, H.; Martin, S.; Morak, A.; Königsdörfer, C.; Roth, M.; Bincheng, L.; Welsch, E.; Thielsch, R.; Kaiser, N.

Exarhos, G.J. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Laser-Induced Damage in Optical Materials 1998
Bellingham, Wash.: SPIE, 1999 (SPIE Proceedings Series 3578)
ISBN: 0-8194-3045-5
Annual Boulder Damage Symposium <30, 1998, Boulder/Colo.>
Conference Paper
Fraunhofer IOF ()

The thermal decay of surface temperature has been calculated and measured on Al2O3/SiO2 coatings by using photothermal technique near damage threshold fluence. It could be shown that under certain conditions the decay times t has been found to be in the order of some microseconds. This fact gives no explanation of the measured decrease of threshold fluence in some cases for higher repetition rates and shot numbers on Al2O3/SiO2 and HfO2/SiO2 multilayers for l = 248nm. Furthermore, it could not be found any influence of the substrate materials. Thus, other than thermal accumulation is responsible for the lowered damage threshold by increasing repetition rate. Additionally, performed calculations of the thermal decay using 20ns, 248nm laser excitation confirm the experimental results. Even in the case of Al2O3/SiO2 coatings on copper no effect of the substrate as a heat sink could be measured. For HfO2/SiO2 coatings the behavior is in accordance with the fact that the thermal conductivit y of HfO2 films is markedly lowered compared to the bulk value.