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Gold Sponges

GOLD SPONGES AND GOLD BLACK, The optical properties of gold nanostructures are exceedingly flexible; it is even possible to prepare gold sponges or gold “blacks” which have very high absorptance [86,87]. The essential attribute of such materials is that they possess very well-developed mesoscale porosity and surface roughness so that they trap and absorb incident light.

Because the structure is at a size scale that is well below the wavelength of the incident light, it becomes possible to model the optical response using Effective Medium theory. In this scheme the performance of the complex composite structure is modeled as if it were a simple monolithic material, using “effective” values of the refractive indices n and k derived from numerical analysis of experimental data. In a sense the sponge becomes an example of a “metamaterial” with engineered physical properties.