The dislocation substructure associated with the surface of bare copper mirrors is significantly altered by surface preparation techniques and found to have a negligible effect on reflectivity at 10.6 μm. These microstructures, examined by direct observation in the transmission electron microscope, are also observed to have no influence on the residual microstructure of 1350 A thick silver vapor‐deposited overgrowths. By systematically altering the grain size of silver overgrowths on mechanically‐polished copper substrates, it is shown that 10.6 μm reflectivity increases with an increase in grain size. It is demonstrated that porosity in 1350 Å thick silver films is very sensitive to substrate temperature during vapor deposition in 10−7 Torr vacuum, and that porosity has a significant effect on the degradation of residual reflectivity. This feature seems to be able to explain, in large part, the great discrepancies observed in commercial laser mirror reflectivities where the mirror surface is a vapor‐deposited, bare metallic coating.
The attainment of high power density lasers is, in many respects, limited by the properties of windows and mirrors; and is therefore materials limited. Aside from microcracks, voids, and other structural features contributing to laser-induced degradation, there has been some indication that other microstructural phenomena such as the presence of crystal defects might also contribute. While the subject of short-pulse laser damage at the surfaces of both coated and uncoated dielectric materials has received considerable attention in recent years, little attention has been paid to the systematic evaluation of mirror surface and surface coating microstructure on laserinduced damage (or damage threshold). Although some preliminary work has been done on determining the relationship between the optical properties of metal films and departures from crystalline perfection of the films, no efforts have been made to determine the effects of metal coating microstructure on laser damage thresholds.
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