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Title: Impact of laser-contaminant interaction on the performance of the protective capping layer of 1w high-reflection mirror coatings

In this paper, high dielectric constant multilayer coatings are commonly used on high-reflection mirrors for high-peak-power laser systems because of their high laser-damage resistance. However, surface contaminants often lead to damage upon laser exposure, thus limiting the mirror’s lifetime and performance. One plausible approach to improve the overall mirror resistance against laser damage, including that induced by laser-contaminant coupling, is to coat the multilayers with a thin protective capping (absentee) layer on top of the multilayer coatings. An understanding of the underlying mechanism by which laser-particle interaction leads to capping layer damage is important for the rational design and selection of capping materials of high-reflection multilayer coatings. In this paper, we examine the responses of two candidate capping layer materials, made of SiO2 and Al2O3, over silica-hafnia multilayer coatings. These are exposed to a single oblique shot of a 1053 nm laser beam (fluence ~10 J/cm2, pulse length 14 ns), in the presence of Ti particles on the surface. We find that the two capping layers show markedly different responses to the laser-particle interaction. The Al2O3 cap layer exhibits severe damage, with the capping layer becoming completely delaminated at the particle locations. The SiO2 capping layer, on the other hand,more » is only mildly modified by a shallow depression. Combining the observations with optical modeling and thermal/mechanical calculations, we argue that a high-temperature thermal field from plasma generated by the laser-particle interaction above a critical fluence is responsible for the surface modification of each capping layer. The great difference in damage behavior is mainly attributed to the large disparity in the thermal expansion coefficient of the two capping materials, with that of Al2O3 layer being about 15 times greater than that of SiO2.« less
Authors:
 [1] ;  [1] ;  [1] ;  [1] ;  [1] ;  [2] ;  [1] ;  [1] ;  [1]
  1. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  2. Univ. of Rochester, Rochester, NY (United States)
Publication Date:
OSTI Identifier:
1251046
Report Number(s):
LLNL-JRNL--673678
Journal ID: ISSN 0003-6935; APOPAI
Grant/Contract Number:
AC52-07NA27344
Type:
Accepted Manuscript
Journal Name:
Applied Optics
Additional Journal Information:
Journal Volume: 54; Journal Issue: 29; Journal ID: ISSN 0003-6935
Publisher:
Optical Society of America (OSA)
Research Org:
Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
42 ENGINEERING; 36 MATERIALS SCIENCE protective coatings; laser damage; plasmas