High-absorbing gradient multilayer coatings with silver nanoparticles

We suggest an efficient way to enhance broadband visible light absorption in multilayer nanostructured systems formed by silver nanoparticle arrays. This approach is based on the explicit use of a gradient change in the parameters of the arrays (such as nanoparticle size or concentration). We analytically derive general conditions of the spectral characteristics of individual arrays which must be satisfied for the realization of highly-absorptive gradient multilayer nanostructured coatings. Two specific types of gradient coatings, (i) with the gradient of the size of the silver nanoparticles in the arrays and (ii) with the gradient of their surface concentration, are numerically studied in detail. The obvious advantages of the size-gradient coatings are revealed, in particular their total thickness of less than a wavelength. Multilayer coatings with a concurrent gradient of both concentration and mean particle size are fabricated from silver island films and their high and broadband absorption is demonstrated experimentally. PACS 78.67.Pt; 78.67.-n; 73.20.Mf; 42.25.Bs