Ion Irradiation Smoothing and Film Bonding for Laser Mirrors
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ION IRRADIATION SMOOTHING AND FILM BONDING FOR LASER MIRRORS
P. P. PRONKO,* A. W. MCCORMICK,* D. C. INGRAM,* A. K. RAI,* J. A. WOOLLAM,** B. R. APPLETON+. AND D. B. POKER+ *Universal Energy Systems, 4401 Dayton-Xenia Road, Dayton, OH 45432, **University of Nebraska, Lincoln, NB, +Solid State Division, Oak Ridge National Lab, Oak Ridge, TN 37830
ABSTRACT Irradiation with high energy heavy ion beams has been investigated as a technique for improving the quality of highly reflecting metallic surfaces to be used as laser mirrors. Properties such as reflectivity, corrosion resistance, film bonding, and threshold to laser surface damage have been examined. Modifications of composition and microstructure of the material associated with the heavy ion irradiation have been measured with RBS, TEM, SEM, Auger, and ESCA. Reflectivity and extinction ellipsometry measurements were made using coefficient keV heavy1 6 ion indicate that Observations techniques. the fluence range of 1015 to 10 cm-2 irradiations in Additionally, MeV produce significant surface smoothing. implants of heavy ions into films of Cu, Ag, Au and Al deposited on molybdenum substrates resulted in improvements to both tarnish resistance and structural bonding integrity. INTRODUCTION In the present study we have directed our attention to the application of energetic (keV and MeV) ion beam technology to the useful modification and improvement of metallurgical surfaces designed for application in laser mirror technology. In order to understand and appreciate the potential of such a processing technique it is necessary to first have a grasp of where the limitation of existing laser mirrors lie, and which features need to be improved. Molybdenum mirrors, because of their attractive high temperature thermal properties, are widely used in applications involving high energy lasers. As a refractory metal, however, molybdenum has disadvantages in that it is difficult to machine as well as having a very modest surface reflectivity for optical radiation. Current technology is unsuccessful at precision machining molybdenum by point diamond turning thereby limiting the ultimate smoothness that can be achieved on complex surfaces. Additionally, the optical reflectivity of Mo for wavelengths below 1 pm is on the order of 60%, compared to much higher reflectivities for such metals as Cu, Ag, Au, or Al. In laser applications, therefore, one often finds high reflectivity coatings being applied to molybdenum mirrors. These coatings, however, are subject to mechanical degradation, tarnishing, corrosion, and laser damage in normal use under atmospheric environments resulting in serious loss of reflective efficiency. Objectives Ultimately, the most important aspect of a treated optical surface is the way in which it interacts with incident light. In the present work, we address the problems of improving surface finish, homogeneity , and film bonding. Laser mirrors must meet the standards required for adequate
Mat. as. soc.Symp. Proc. Vol. 27 (1984) QElsevier science Publishing co
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