When the thickness of any conventional material layer which covers a metal surface becomes too small compared to the wavelength, electromagnetic waves do not actually see this layer. That is why it is so difficult to design thin absorbing coverings and some other compact antenna and waveguide components. To solve this problem, the only possibility is to make something resonate in the layer. For example, magnetic molecules in ferrites, or long metal needles incorporated in the layer.
We have recently found that there is another possibility to make a very thin layer resonate, which exploits electromagnetic interactions between certain inclusions in the layer and the metal ground plane. It is found that extremely thin (in wavelengths) coverings with resonant properties can be realized in this way. This offers a possibility to realize, for example, an artificial magnetic conductor as a thin covering over conducting bodies.
Depending on the parameters, the resonant curve can be made extremely narrow, which means very high equivalent quality factors.
In the figure above is seen that the frequency can be in principle shifted to very low frequencies, theoretically to zero frequency. In other words, this means that the thickness of the resonant cover can made extremely thin in wavelengths.