Effect of Silica overlayers on laser damage of HfO{sub 2}-SiO{sub 2}56{sup degree} incidence high reflectors Page: 4 of 12
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Effect of Silica Overlayers on Laser Damage
of HfO2-SiO2 560 Incidence High Reflectors
C. C. Walton', F. Y. Gdnin2, R. Chow, M. R. Kozlowski, G. E. Loomis, and E. Pierce
Lawrence Livermore National Laboratory
P.O. Box 808, L-476
Livermore, California 94550 USA.
ABSTRACT
A series of hafnia/silica, oblique incidence (560), 1064 nm high reflectors (HRs) were
prepared and coated with silica overlayers of varying optical thickness from aJ2 to 4X in
order to determine the effect of an overlayer on the laser-damage resistance of the HRs.
The stress and laser damage thresholds for S and P polarization of the HRs were measured,
and the damage sites for P polarization examined by Atomic Force Microscopy (AFM).
All the multilayers were found to be in compression, with an intrinsic stress increasing with
overlayer thickness. The presence of an overlayer and its thickness did not affect the
damage threshold significantly. However, the presence of an overlayer greatly influenced
the size and morphology of the damage. First, the overlayer prevented catastrophic
"burns" of the hafnia top layer. Second, as the overlayer thickness increased, two distinct
damage morphologies were found: jagged pits and round craters. The diameter of these
pits and craters then increased somewhat with thicker overlayers. The depths of the pits
and craters also increased with overlayer thickness, and the depths showed failure
occurring at the interfaces below the hafnia layers. The side-wall angles of the craters were
shallower with thicker overlayers, but there was no angle dependence for the pits. The
craters showed fracture-like features and a small hillock or pit on their bottom surfaces. No
correlation of damage morphology to conditioning or fluence was found.
1. INTRODUCTION
The addition of a silica overcoat has long been thought1,2 to improve laser damage
performance of quarter-wave high reflectors (HRs) of various types, though little
systematic work has been done to document the improvement or to relate it to the thickness
of the overlayer. Wu et al.3,4 reported that for normal incidence tests on single silica layers
with 10 ns-1.06 gm laser pulses, the layers showed damage with no dependence on film
thickness. On the other hand, the damage threshold of ZrO2/ Si02 and TiO2/ Si02
reflectors showed a very strong dependence on overlayer thickness (see Fig. 1). By
measuring the scattering loss and observing the microstructure, Wu et al. concluded that
'Now at Department of Materials Science and Mineral Engineering, University of California, Berkeley CA
94720.
2 To whom mail can be addressed; email: fgenin@llnl.gov Sd
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Walton, C. C.; Genin, F. Y.; Chow, R.; Kozlowski, M. R.; Loomis, G. E. & Pierce, E. Effect of Silica overlayers on laser damage of HfO{sub 2}-SiO{sub 2}56{sup degree} incidence high reflectors, article, January 5, 1995; California. (https://digital.library.unt.edu/ark:/67531/metadc667569/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.