Surface roughness and scattering losses in plasmonic waveguide using CST

I am working on multilayer hybrid plasmonic waveguide which involves silver, silica and silicon. I have to calculate surface roughness and scattering losses at 1550nm wavelength. There is one macro defined in CST for surface roughness but that is useful for lossy metal at GHz range. While defining the material also, there comes one option such as: new material->surface roughness impedance at different frequency.
I define drude model for silver which means complex dielectric constant. I am not understanding basically how to incorporate surface roughness into waveguide?. If any body is having literature available for impedance at 1550nm wavelength, that also please share.

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