Toward engineering the color of metals by carving rings on the surface

The electrons in the metal are nimble and mobile and able to conduct electricity. The behaviors are controlled by two things: the band structure and the coulomb interactions. The elementary excitations of this sea of electrons may not be simply fermionic quasi-particles that partially resemble the original electrons. They can also be bosonic, for example, plasma. Such plasmons are hardly excitable by low energy probes, such as visible light. But they can indeed be created by X-ray. What controls the colors are basically visible light. To understand the colors of a particular metal, one needs know how the visible light interacts with which kind of elementary excitations of the similar energy scales. To describe this interaction, one may assume quantum mechanics, but the usual Maxwell equations will suffice, because the visible light has a wave length between 400nm to 760nm, which are indeed very long in comparison with the metallic band gaps of the order of nm (and hence, only the single partially filled band needs be considered). Basically, one has to treat an entangled system of light and electrons, the exact solution of which is a considerable problem. Usually, one treats the metal as a medium that is characterized by a complex dielectric function of frequency. This function determines which photon will be absorbed and which can be transmitted and which will be reflected. The reflected light decides the color. Most naturally occurring metals bear silver color. This is because, the spectrum encoded in the imaginary part of the dielectric function is a continuum in the visible light energy window, rather than a discrete set of resonances. Is it possible to tune the color of a metal without affecting its conductivity? The answer is yes. Due to the complex part of the dielectric function, visible light can hardly enter the bulk metal and can penetrate only a very thin layer near the surface, an effect called "skinning effect". Thus, the colors are actually controlled by the skin. By manipulating the surface electron spectrum, one should be able to tune the color. This has been achieved in a latest work by Jianfa Zhang at the University of Southampton and a few pals [arXiv:1011.1977v1 ]. See a review from Arxiv Blog [http://www.technologyreview.com/blog/arxiv/]:

Their idea is to carve a different type of repeating pattern on to the surface of a metal.

These patterns are smaller than the wavelength of visible light. Instead of causing the light to interfere, they work by changing the properties of the sea of electrons in the metal--in particular its resonant frequency. This alters the frequency of light it absorbs and reflects.

This is the same technique that researchers have been using for some time to build invisibility cloaks . The idea is that by carefully building repeating patterns of subwavelength structures, researchers can tailor the way a "metamaterial" can steer light.

But instead of creating 3D structures that steer light as it passes through the material, Zhang and co carve the relevant structures onto the surface to control the way light is absorbed and reflected.

The structures that do the trick are tiny rings carved into the surface. The team calculate that they can make gold or aluminium appear almost any colour simply by varying the size and depth of these rings. They've even demonstrated the technique on a thin layer of gold.