So we have lived to see a time when the invisible hat, a familiar attribute of folk tales, does not seem fantastic. Current technologies allow you to hide objects without any magic, relying only on knowledge of the laws of physics.
The history of invisible materials goes back to the period of the formation of the Soviet state, when many scientific projects were launched, sometimes the most fantastic. In 1936, the press wrote about an airplane made of transparent plexiglass covered with amalgam. It was allegedly designed by Robert Bartini, an Italian engineer who fled to the USSR. However, neither photographs nor drawings of that wonderful aircraft have survived, so the secret of its invisibility can be considered lost. Materials that were inaccessible to the eye had to be reinvented.
We see those objects that reflect light. They scatter it at different angles depending on color, material, position relative to the light source. Reflection is captured by the retina and transmitted to the brain, where an image is formed. Accordingly, if the light reflected from the object does not reach the retina, we will not see it. But how can such a technology be put into practice?
To date, scientists have come up with three methods. For example, they suggest making light bend around an object without colliding with it. For this, the thing must be covered with a material with a special structure in the form of a lattice of inclusions-bricks, the size of which is less than a certain wavelength of light.
This is how the artist imagined the invisibility nanocap / Xiang Zhang group, Berkeley Lab / UC Berkeley.
Suppose the spectrum visible to the human eye covers wavelengths from 400 to 700 nanometers, therefore, the grating inclusions should be of the order of 100-200 nanometers. It is no coincidence that they are called meta-atoms. The light will bend around the object covered with meta-atoms, like a pedestrian pit on the road. A similar idea was implemented by physicists from the United States in 2015, creating a material from silicon with a thickness of only 80 nanometers. With its help, it was possible to hide a tiny particle of living cells from the researcher observing it through a microscope.
“You can also make the light pass through the material without being distorted. In physics, a quantity called transmittance is used - it shows the ratio of the radiation flux that has passed through a substance to the flux that has fallen on its surface. For example, light passes through a vacuum without hindrance, so its transmittance is unity. But the metal reflects all electromagnetic waves incident on it. It turns out that for the material to be invisible, the light must pass through it completely, without scattering, like through a vacuum,”says Alexei Basharin, an employee of the NUST MISIS Laboratory of Superconducting Metamaterials.
To do this, the researchers came up with a combination of two materials so that the waves reflected from them extinguish each other and simply pass through without scattering - this state is called anapole. And structures that exhibit unusual properties due to their architecture, and not the characteristics of their constituent substances, are called metamaterials.
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The third method relies on the material's ability to absorb all light without reflecting anything. But it is not very popular, since it will not be possible to completely hide the object behind it - it will cast a shadow.
“The hardest thing is to make a material that is transparent to a wide range of light. Fortunately, this is not necessary, because usually the invisibility function is required for a specific task. For example, to make sure that a certain radiation destroys only cancer cells, and simply does not notice healthy ones. As for invisibility cloaks as entertainment for people, they are unlikely to hit the market anytime soon. It is enough for physicists to prove that a particular metamaterial works, which requires a piece a few micrometers in size. It is simply not interesting and very expensive to produce huge “rags”, at least for now,”Basharin concludes.
Olga Kolentsova