Researchers create small and thin ‘invisibility cloak’

Researchers have succeeded in making a small object ‘disappear’ under an ultra-thin metamaterial. By processing the reflective surface of the material so that it consists of small, light-scattering antennas, an object measuring 36 by 36 micrometers could be made ‘invisible’.

The material would also have another property that other metamaterials in the light-bending category do not, namely the ability to hide objects with sharp edges. The authors of the article in Science also say their material is likely to be scalable. The Berkeley Lab researchers’ ‘invisibility cloak’ manages to remove an object’s features from the light reflected from a certain angle. So it doesn’t make an object disappear from the environment. This is possible due to the way in which the small golden antennae, or rather golden rectangles of different sizes, on the thin gold foil of 80 nanometers thick compensate for the scattering of the light. The red light used, with a wavelength of 730 nanometers, is in the visible spectrum. The latter is special: many other experiments with metamaterials, which often have to bend light around objects, work with wavelengths that are invisible to us.

Normally, an object under a thin foil would be visible because the light hits the covering foil and bounces back in different directions. Because the light reflects in all directions, the object is visible. With this ‘cloak’ or under this foil with its gold antennas, the scattering is counteracted. As a result, the light waves reflect in the same direction as the surface on which the object lies. Because the tiny gold antennae can reflect light in different ways, an object can not only ‘disappear’ or appear flat, but also completely change shape, as if the surface is wrinkled or puckered.

The antennas work by changing the phase of the light waves. This is because each antenna is just too small or too large to fully reflect the incident light. By adjusting the size of the antennas, the phase change allows the light to bounce back as if normal reflection were occurring. The latter already shows an important limitation: the cloak must be designed precisely for a specific object that it has to cover, whereby the cloak can appear slightly flat or have a different shape. Something does not disappear in the environment. Despite the fact that this ‘cloak’ could be made relatively easily on a larger scale, the object under or behind the foil can never move. In addition, the mantle only works with a limited number of wavelengths and it is also not possible to cover very sharp or large objects because shadows will then occur. Those shadows cannot be lifted for the time being.