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Metamaterials – Manipulators of Light

Metamaterials possess a structure that can refract radiation in apparently “impossible” ways.

imagenavi, Getty Images

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. However, they cause light and other electromagnetic radiation to behave in seemingly impossible ways.

Impossible refraction

For example, some metamaterials can change the direction, phase and polarization of a light beam in such a way as to effectively force the light into reverse. The ray is refracted by the material in exactly the opposite way to what normally happens with a normal material. This leads to the paradoxical effect that a converging concave lens made with this metamaterial does not focus the light but scatters it. Conversely, a diffusing lens packs the 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 is refracted not towards the perpendicular when it enters this material, but in the opposite direction. Russian physicist Viktor Veselago predicted in 1968 that such materials could exist and be manufacturable. But since negative refractive indices do not appear to exist in nature, it has long been thought that this was impossible. In the meantime, however, scientists have developed countless different metamaterials.

Lenses, transformers and holograms

The ability of metamaterials to manipulate radiation, and in particular light, in ways previously thought to be impossible, opens up entirely new avenues of application. Especially in optics, these materials are now being 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 measures only half a millimeter, but can keep up with a classic camera lens that is 500,000 times larger in resolution and light intensity.

Some meta-lenses can also act as a kind of light transformer: they convert low-energy long-wave radiation into short-wave radiation, which is actually impossible without an energy supply. This is made possible by a resonance effect which doubles the frequency of the radiation. And even holograms and hologram videos can be created using special metamaterials.

It all depends on the structure

But what is the secret of these skills? The highlight of metamaterials is their structure: they have tiny, repeating base units that affect 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 allow the metamaterials to manipulate the radiation in physically unusual ways.

How large the structure of a metamaterial can be depends on the wavelength of the radiation: exotic refraction occurs only when the repeating base units are less than a quarter wavelength of the incident radiation. This means that if the metamaterial has to manipulate long-wave radiation such as radar or radio waves, the cells can be several centimeters in size. With visible light, however, they move in the order of nanometers.

Radio wave meta lens

Craft from the Lab: This radio wave metal lens is made up of 4,000 S-shaped copper hooks.

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 are made up of tiny tubes, plates, or pillars embedded in silicon chips. Even a regular arrangement of slots or holes or a structure resembling tiny stacked logs can become metamaterials. Other variants bring on their surface small pillars of metal or metallic compounds, the geometry and spacing of which produce the exotic effects of refraction.

A metamaterial lens that researchers at the Massachusetts Institute of Technology (MIT) use to manipulate radio waves is almost a work of art: the flat, concave structure is made up of more than 4,000 S-shaped copper hooks, each of a few millimeters in size. These base units are clipped together to form a 4cm thick and 25cm wide lens that is transparent to microwaves and radio waves. Thanks to its negative refractive index, this chainmail-like metamaterial can break up and focus radiation as much as rays only a few meters long.

Metamaterial such as Tarnmantel

Metamaterials can even make the old invisibility cloak or invisibility cloak dream come true. To a limited extent, making people invisible is already working: Scientists have developed invisibility cloaks for microwaves, infrared light, and even single areas of visible light. However, they are quite bulky and can only hide objects that are much smaller than themselves. “They look like Harry Potter scales rather than Harry Potter cloak,” explains John Pendry of Imperial College London.

Camouflage coat icon image

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

Xiang Zhang Group / UC Berkeley

But a true Harry Potter camouflage cloak is slowly approaching: in 2015 researchers from the University of California at Berkeley unveiled for the first time an extremely thin metamaterial capable of concealing even larger and irregularly shaped objects. The new “camouflage fabric” consists of a metamaterial only 80 nanometers thick that can cling to underlying objects like thin skin. On its surface is a nanostructure of tiny blocks of gold, which manipulates the incident light in such a way as to hide imperfections.

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. Therefore, it will likely be a long time before there are metamaterials capable of making an object or person invisible across the full wavelength range of light.

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