Metamaterials – light manipulators

Metamaterial icon image

Metamaterials have a structure that can refract radiation in a seemingly “impossible” way.

images, Getty images

At first glance, most metamaterials look a bit unassuming. Because with the naked eye they resemble ordinary crystals or smooth surfaces. Even their composition does not have to be exotic: some are made of metal, others of silicon or even plastic. Yet they cause light and other electromagnetic radiation to behave in a seemingly impossible way.

Impossible refraction

For example, some metamaterials can change the direction, phase, and polarization of a beam of light in a way that effectively forces the light to reverse. The beam with the material refracts exactly the opposite of the usual material. This leads to the paradoxical effect that a concave converging lens made of this metamaterial does not focus light, but scatters it. Conversely, a scattering lens would bind light – the laws of physics seem to be upside down.

This paradoxical effect is possible because such metametamaterials have a negative refractive index. As a result, the radiation does not break towards the perpendicular, but in the opposite direction when entering this material. In 1968, the Russian physicist Viktor Veselago predicted that such materials could exist and be manufacturable. But because negative refractive indices do not appear to occur in nature, it has long been considered 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 that were previously considered impossible opens up completely new possibilities for use. Especially in optics, these materials are used today to develop new types of lenses and displays for cameras, microscopes and 3D projections. U.S. researchers have recently developed a camera lens made from 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 terms of resolution and light intensity.

Some sweepers can also act as a kind of light transformer: they convert low-energy long-wave radiation into short-wave radiation – something that is actually impossible without energy. This is made possible by the resonant effect, which 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 metamaterials is their structure: They have tiny, repetitive basic units that affect the transmission of light and other radiation much like a normal crystal. However, the small size and special shape of these entities allow metamaterials to manipulate radiation in physically unusual ways.

How large a metamaterial structure can be depends on the wavelength of the radiation: Exotic refraction occurs only when the repeating base units are less than a quarter of the 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 range in the nanometer range.

Meta lens for radio waves

Laboratory craftsmanship: This radio wave sweeper consists of 4,000 S-shaped copper hooks.

Material: from silicon to copper

What a metamaterial consists of and what its structure is can also be very different. Some of these designs consist of small tubes, plates or posts embedded in silicon chips. The regular arrangement of slots or openings or a structure similar to small stacked trunks can also become metamaterial. Other variants carry small columns of metal or metal compounds on their surface, the geometry and spacing of which cause exotic fracture effects.

The metamaterial lens used by researchers at the Massachusetts Institute of Technology (MIT) 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 just a few millimeters in size. These base units are joined together in such a way that they form a lens four centimeters thick and 25 centimeters wide, which is permeable to microwave and radio waves. Due to its negative refractive index, this chain-like metamaterial can break down and concentrate radiation just like rays only meters long.

Metamaterial as Tarnmantel

Metamaterials can even turn an old dream of an invisible cloak or an invisible cloak into reality. Human invisibility is already working to a limited extent: scientists have developed invisible jackets for microwaves, infrared light and even individual areas of visible light. However, they are quite clumsy and can only hide objects much smaller than themselves: “Rather than a Harry Potter coat, they resemble Harry Potter scales,” explains John Pendry of Imperial College London.

Camouflage coat icon image

The ultra-thin camouflage metamaterial developed at the University of California, Berkeley, is covered with golden blocks that manipulate incoming light.

Xiang Zhang / UC Berkeley Group

But Harry Potter’s real camouflage cloak is slowly approaching: in 2015, researchers at the University of California, Berkeley, introduced for the first time a metamaterial that is extremely thin and can hide even larger, irregularly shaped objects. The new “camouflage fabric” consists of a metamaterial just 80 nanometers thick that can adhere to objects beneath it like thin skin. On its surface is a nanostructure of tiny golden blocks that manipulates the incident light to hide imperfections.

However: The camouflage of this metaplane so far only works for a certain wavelength of light – in this case red light with a wavelength of 730 nanometers. Thus, it is likely to take a long time for metamaterials to emerge that can make an object or person invisible over the entire wavelength range of light.

Leave a Reply

Your email address will not be published.