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The vector theory of light scattering from slightly rough surfaces allows a prediction in the whole space of the scattering distribution from a high-reflecting multilayer coating. A comparison with experimental results concerning classical quarterwave multidielectric coatings enables us to point out the influence of the initial substrate roughness and that of the fine microstructure of the materials used for the coating. Some results are presented for two materials (TiO2, SiO2), and we specify the consequences of the microstructure “grain size” for each one.
In order to reduce scattering losses, it is interesting to know if it is possible to use non-quarterwave multilayer stacks. A theoretical search for solutions leads us to study the consequences of a protective layer deposited on a classical mirror. The influence of the optical thickness of this layer is first studied. Then we examine the case where the top layers of the stack are non-quarterwave layers, so that the electric field associated with the stationary wave is reduced. We can show that, in some particular cases, the total integrated scattering is perceptibly reduced. Nevertheless, this result is only obtained when the roughnesses of the successive interfaces are not correlated. From an experimental point of view, it is not obvious that we can meet this condition whatever the spatial frequency of the defects, and this leads us to study the effect of the cross-correlation coefficient between interfaces.
absorption, anti-scattering, cross-correlation functions, microstructure, multilayer mirror, roughness, scattering.
Laboratoire d'Optique des Surfaces et des Couches MincesAssocié au C.N.R.S.- U.A.Ecole Nationale Supérieure de Physique de Marseille Domaine Universitaire, de St JérômeMarseille,