At first glance, most metamaterials look rather unspectacular. Because to the naked eye they resemble ordinary crystals or smooth surfaces. Their composition does not have to be exotic either: some are made of metal, others of silicon or even plastic. Nevertheless, they cause light and other electromagnetic radiation to behave in seemingly impossible ways.
For example, some metamaterials can change the direction, phase, and polarization of a beam of light in a way that effectively forces the light into reverse gear. The beam is refracted by the material in exactly the opposite way than is usual with a normal material. This leads to the paradoxical effect that a concave converging lens made of this metamaterial does not focus the light but scatters it. Conversely, a diffusing lens would bundle the light – physical laws seem to be turned on their head.
This paradoxical effect is possible because such metametamaterials have a negative refractive index. As a result, the radiation is not refracted towards the perpendicular when it enters this material, but in the opposite direction. The Russian physicist Viktor Veselago predicted in 1968 that such materials could exist and be manufacturable. But because negative refractive indices do not appear to occur in nature, this was long thought to be impossible. In the meantime, however, scientists have developed countless different metamaterials.
Lenses, transformers and holograms
The ability of metamaterials to manipulate radiation, and especially light, in ways previously thought impossible opens up whole new avenues of application. Especially in optics, these materials are now used to develop new types of lenses and displays for cameras, microscopes and 3D projections. US researchers recently developed a camera lens made of 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 terms of resolution and light intensity.
Some meta-lenses can also act as a kind of light transformer: they convert low-energy long-wave radiation into shorter-wave radiation – something that is actually impossible without an energy supply. This is made possible by a resonance effect that doubles the frequency of the radiation. And holograms and hologram videos can also be created using special meta materials.
It all depends on the structure
But what is the secret of these abilities? The highlight of the metamaterials is their structure: They have tiny, repeating basic units that influence the transmission of light and other radiation in a similar way to a normal crystal. However, the small size and special shape of these entities allows the metamaterials to manipulate radiation in physically unusual ways.
How large the structure of a metamaterial can be depends on the wavelength of the radiation: the exotic refraction only occurs when the repeating basic units are smaller than a quarter wavelength of the incident radiation. This means that if the metamaterial is to manipulate 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.
The material: from silicon to copper
What a metamaterial consists of and what its structure is like can also be very different. Some of these constructs consist of tiny tubes, plates, or pillars embedded in silicon chips. A regular arrangement of slots or holes or a structure similar to tiny stacked logs can also become metamaterial. Other variants bear small pillars of metal or metal compounds on their surface, whose geometry and spacing produce the exotic effects of refraction.
A metamaterial lens that researchers from the Massachusetts Institute of Technology (MIT) use to manipulate radio waves is almost a work of art: The flat, concave construct consists of more than 4,000 S-shaped copper hooks, each only a few millimeters in size. These basic units are hooked together in such a way that they form a lens four centimeters thick and 25 centimeters wide that is transparent to microwave and radio waves. Thanks to its negative refractive index, this chainmail-like metamaterial can break and focus the radiation as much as rays that are only meters long.
Metamaterial als Tarnmantel
Metamaterials can even make the old dream of the invisibility cloak or invisibility cloak a reality. To a limited extent, making people invisible is already working: scientists have developed invisibility cloaks for microwaves, infrared light and even individual areas of visible light. However, they are quite unwieldy and can only conceal objects much smaller than themselves. “They resemble Harry Potter’s scales more than Harry Potter’s cloak,” explains John Pendry of Imperial College London.
But a real Harry Potter camouflage cloak is slowly approaching: in 2015, researchers at the University of California in Berkeley presented a metamaterial for the first time that is extremely thin and can even conceal larger, irregularly shaped objects. The novel “camouflage cloth” consists of a metamaterial just 80 nanometers thick that can cling to objects underneath like a thin skin. A nanostructure of tiny gold blocks sits on its surface, which manipulates the incident light in such a way that imperfections are concealed.
However: So far, the camouflage of this meta-cloak only works for a certain wavelength of light – in this case red light with a wavelength of 730 nanometers. It will therefore probably be a long time before there are metamaterials that can make an object or person invisible in the entire wavelength range of light.