What can you make an invisible hat from?

In early 2001, at the house of Victor Veselago, head of the Laboratory of Magnetic Materials of the Department of Strong Magnetic Fields of the Institute of General Physics RAS (IOFAN) named after A.M. Prokhorov, the phone rang. “Professor, ” said the voice in the receiver in English. - You are worried from New Scientist magazine. We would like to receive your comment on the experiments of American scientists Smith and Schultz described in the journal Science, which has just been published. It seems they managed to create the material that you theoretically predicted more than thirty years ago. "

“Right” and “left” What we will see if we look at material with a negative refractive index Light refraction is a common phenomenon, well known to everyone who has at least once looked at the game of sunlight on a smooth surface of the pond. But it’s hard to imagine the effects that occur in a medium with a negative refractive index (“left” medium) - they are so contrary to generally accepted concepts about the behavior of light. Here are some of them.

Mind exercise

After graduating from school, where he was seriously interested in radio engineering, Veselago entered the newly created Physics and Technology Department of Moscow State University (later becoming an independent university - MIPT). He defended his diploma at the Lebedev Physical Institute under the direction of A.M. Prokhorov (future winner of the 1964 Nobel Prize in Physics), and then continued to work at the institute as a researcher.

In the 1960s, he became interested in magnetic semiconductors - materials that exhibit the properties of both ferromagnets and semiconductors (their conductivity changes with a change in the magnetic field). “There is such an electrovacuum device - a traveling wave lamp, it is used as an amplifier for microwave signals, ” Professor Veselago explains. - The amplification here occurs due to the interaction of electrons with an electromagnetic wave. I thought that it would be possible to try to create such a device in the solid-state version, and for this we needed material that greatly slowed down the speed of propagation of the electromagnetic wave, that is, with a very high refractive index. The refractive index (n) is defined as the square root of the product of dielectric constant (ε) and magnetic constant (μ). The idea with a magnetic semiconductor did not justify itself (it was not possible to select the desired operating mode), but it forced me to carefully consider substances with different ε and μ, both positive and negative. Substances with both positives are well-known ordinary dielectrics. With positive ε and negative μ - ferromagnets. With negative ε and positive μ - plasma. But substances with negative ε and μ were not known then. A sheet with the formula of the refractive index was lying on my table, I looked at it and suddenly thought: but in this case, the refractive index itself may be negative. And not only from the point of view of mathematics! ”

Invisible houses The principle of "invisibility", implemented using metamaterials with a negative refractive index, is applicable not only in optics and radiophysics, but also in acoustics. Scientists have high hopes for creating structures that could “mask” important structures from seismic waves during earthquakes. The experiments of a group of researchers from the Fresnel Institute and Aix-Marseille University showed that the placement of several regular rows of hollow cylinders in the ground refracts and reflects seismic waves.

Against common sense

This conclusion was extremely unusual, since in all textbooks it was implied that the refractive index of any medium is always a positive value. His thoughts, which seemed to him very important, Victor shared with colleagues. But scientists, loaded with work, took his conclusions as a “warm-up for the mind, ” an abstraction that had no physical meaning. And then Veselago wrote a short article entitled “Electrodynamics of substances with simultaneously negative values ​​of ε and μ”, where he showed that the presence of such substances does not contradict any laws of physics, and described their unusual properties. Materials with a negative refractive index in the article were called “left”, and ordinary, with positive, “right” (in the orientation of the vectors characterizing the electromagnetic wave). “Actually, this idea wasn’t the first to come to my mind, ” says Professor Veselago. “Academician S. L. Man talked about this "delshtam, but exclusively in mathematical terms. D. Sivukhin also considered them, but he did not include these considerations in his famous textbook."

Veselago lenses Material with ε = -1, µ = -1 and a negative refractive index of n = -1 can be used to create the so-called Veselago lens. The flat plate of the “left” medium completely transfers the optical field from one side to the other, creating an accurate image without any distortions. Such a lens, unlike a conventional one, lacks an optical axis. It is not able to focus a parallel beam of light, but due to focusing of the near field through it, one can consider details smaller in size than the wavelength of light (diffraction limit).

In the journal Uspekhi Fizicheskikh Nauk (Physics Uspekhi) (Uspekhi Fizicheskikh Nauk) (UFN), the article was also perceived as hypothetical reasoning, but nevertheless published. “I presented my report at an international conference in Moscow, ” recalls Viktor Georgievich, “and received an invitation to speak on this topic at several other conferences - in Italy, France and the USA. After my reports at these conferences, they became interested in the topic, and I prepared another article in the collection. But the high authorities of the Lebedev Physical Institute, upon learning of this, strongly advised me not to engage in any nonsense and not to be distracted from the main work in the department of strong magnetic fields, where we created a large experimental installation “Solenoid”. As a result, this topic was almost forgotten for three long decades. ”

If we illuminate an object comparable in size to the wavelength with a microwave beam, we will see it by reflecting part of the radiation. But if you surround the object with an “invisible cap” made of metamaterial with a negative refractive index, the beam will bend around the object and it will become completely invisible. Such an experiment was demonstrated by researchers from the University of California at San Diego.

From theory to experiment

The article “Experimental Confirmation of a Negative Refractive Index” by David Smith, Sheldon Schulz and Richard Shelby, researchers from the University of California, San Diego, was published in 2001 in the authoritative journal Science and produced the effect of an exploding bomb. In the article, the researchers showed how, using conductive paths and open coil-resonators to control the electrical and magnetic properties of the medium, it is possible to construct a composite material with a negative refractive index for a wavelength of about 3 cm. Such composite materials whose properties are not determined by their chemical composition, and structure, called metamaterials. And since the experimenters already in the second paragraph of the article referred to that very article of 1967, this finally fixed the priority of the invention of such materials to Victor Veselago. The interest in the topic was so high that this article became the most cited in the entire history of the Physics – Uspekhi journal, which has been published since 1918.

And in 2006, one of the authors of the first article, David Smith, who had transferred to Duke University by that time, together with his university colleague David Shurig and physicist John Pendry from Imperial College in London, published an article in Science entitled “Electromagnetic Field Management”. In it, they showed a ring structure made of metamaterial with a negative refractive index, which causes electromagnetic waves to bend around an object located in the center. And this is nothing more than an invisible hat in the most literal sense of the word. True, so far for the centimeter range, but this is only the beginning.

Light, radio, sound, surf

The technology of metamaterials with a negative refractive index is now very actively developing. And not only because it is an interesting science, but also because the results in this area can lead to interesting solutions in applied fields. “The audience is waiting for the invisible hat, the military - the perfect camouflage and invisible radar equipment, ” says Victor Veselago. - But I do not think that in the near future we should expect something like that. But there are already a number of very interesting developments in the field of materials with a negative refractive index not for electromagnetic, but for seismic waves. It is enough to build such a structure around the building, and the waves generated by the earthquake will go around it. And scientists from the Fresnel Institute in Marseille and the University of Liverpool have been developing metamaterials of this type for several years that can protect coastal structures from destructive tidal, storm waves and tsunamis. And of course, my dream is materials with a negative refractive index for the optical range, which will make super lenses for ultra-high resolution optical instruments a reality. ”

The article “The History of the Invisible Hats” was published in the magazine Popular Mechanics (No. 3, March 2015).

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