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.
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Szilva et al. (1977) studied this question.
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