At first glance, most metamaterials look rather strange. Because they visually resemble ordinary crystals or smooth surfaces. Their composition should not be strange either: some are made of metal, others are made of silicone or even plastic. However, they cause light and other electromagnetic radiation to behave in ways that seem impossible.
For example, some metamaterials can change the direction, phase, and polarization of a light beam in such a way that it effectively drives the light into reverse gear. The beam is refracted by the material in exactly the opposite way to what is usual with ordinary material. This leads to the paradoxical effect that a converging-concave lens made of this metamaterial does not focus but scatter the light. On the contrary, the scattering lens will collect the light – it seems that the laws of physics have been turned upside down.
This paradoxical effect is possible because these metamaterials have a negative refractive index. As a result, the radiation does not refract in the direction of the vertical when it enters this material, but in the opposite direction. Russian physicist Victor Veselago predicted in 1968 that such materials could exist and could be manufactured. However, since negative refractive indices do not appear in nature, it has long been thought that this is impossible. Meanwhile, scientists have developed a myriad of different metamaterials.
Lenses, adapters and holograms
The ability of metamaterials to manipulate radiation, especially light, in ways previously thought impossible, opens up entirely new avenues of application. These materials are now used in the field of optics to develop new types of lenses and displays for cameras, microscopes, and 3D projectors. US researchers recently developed a camera lens made of a metamaterial that is only half a millimeter in size, but can keep up with a classic camera lens that is 500,000 times larger in resolution and light intensity.
Some epitopes can also act as a kind of light transducer: they convert low-energy long-wave radiation into shorter-wave radiation – which is actually impossible without a power source. This is made possible by the resonance effect that doubles the frequency of the radiation. Holograms and 3D videos can also be created using special meta materials.
It all depends on the structure
But what is the secret of these capabilities? The most important characteristic of metamaterials is their structure: they contain repeating small basic units that affect the transmission of light and other radiation in a manner similar to an ordinary crystal. However, the small size and special shape of these entities allow metamaterials to manipulate radiation in physically unusual ways.
The size of the metamaterial structure depends on the wavelength of the radiation: odd refraction occurs only when the repeating base units are smaller than a quarter of the wavelength of the incident radiation. This means that if the metamaterial deals with long-wave radiation such as radar or radio waves, the cells can be several centimeters in size. With visible light, on the other hand, they move in the nanometer range.
Material: from silicone to copper
What the metamaterial is made of and what its structure looks like can also be completely different. Some of these structures consist of small tubes, sheets, or columns embedded in silicon wafers. The regular arrangement of slots, holes, or a similar structure of small stacked logs can also become a supernatural material. Other variants carry small plumes of metals or metal compounds on their surface, whose geometry and spacing produce strange effects of refraction.
The breakthrough lens that researchers from the Massachusetts Institute of Technology (MIT) use to manipulate radio waves is almost a work of art: the flat, concave structure consists of more than 4,000 S-shaped copper hooks, each only a few millimeters in size. These basic units are connected to each other in such a way as to form a lens four centimeters thick and 25 centimeters wide that is transparent to microwaves and radio waves. With a negative refractive index, this chain-mail-like metamaterial can refract and focus radiation as far as rays just meters long.
Supermaterials as a camouflage cloak
Supernatural materials can make the old dream of an invisibility cloak or invisibility cloak a reality. To some extent, making people invisible actually works: Scientists have developed invisibility cloaks for microwaves, infrared, and even individual regions of visible light. However, it is completely impractical and can only hide things much smaller than it. “It resembles Harry Potter’s scales more than Harry Potter’s cloak,” explains John Pendry of Imperial College London.
But the real Harry Potter camouflage cloak is slowly getting closer: In 2015, researchers at the University of California, Berkeley, introduced for the first time a super-thin material that can hide larger, irregularly shaped objects. The new ‘camouflage cloth’ is made of a super-material just 80 nanometers thick that can cling to things underneath like thinner skin. There is a nanostructure of tiny gold nuggets on its surface, which manipulate the incident light in a way that hides the flaws.
However: So far, the camouflage of this overhang has only worked with a certain wavelength of light – in this case red light with a wavelength of 730 nanometers. So it will likely be a long time before there are metamaterials that can make something or someone invisible in the full wavelength range of light.